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4- REPORT FROM SG 1.2:

USE OF 3-D SEISMIC DATA IN EXPLORATION,UNDERGROUND STORAGE

Table of contents

4.1- Objective of the study

4.2- Scope of the study group

4.3- Definition of seismic methods4.3.1- 3D Seismic4.3.2- 4D Seismic4.3.3- The microseismic survey

4.4- Summary of main results4.4.1- 3D Seismic4.4.2- 4D Seismic4.4.3- Expectations

4.5- General recommendations4.5.1- Recommendations in storage4.5.2- Recommendations in production

4.6- Technical recommendations4.6.1- 3D Seismic4.6.2- 4D Seismic4.6.3- Microseismic

4.7- Results from questionnaires – State of the art4.7.1- 3D Seismic applications4.7.2- 3D Seismic techniques4.7.3- 4D Seismic applications4.7.4- 4D Seismic techniques4.7.5- Microseismic applications4.7.6- Microseismic techniques

4.8- Expectation in seismic monitoring4.8.1- Producer’s needs4.8.2- Economic4.8.3- Techniques

PRODUCTION AND

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4.9- Bibliography Synthesis

REFERENCES

DISCLAIMER

Portions of this document may be illegiblein electronic imageproduced from thedocument.

products. Images are

best available original

4- REPORT FROM SG 1.2:

USE OF 3-D SEISMIC DATA IN EXPLORATION, PRODUCTION ANDUNDERGROUND STORAGE

4.1- Objective of the study

The objective of this study was to investigate the experience gained from using 3D and 4D techniquesin exploration, production and underground storage. The use of 3D seismic data is increasing andconsiderable progress in the application of such data has been achieved in recent years. 3D is now inextensive use in exploration, field and storage development planning and reservoir management. Byusing 4D (or timelapse) seismic data from a given producing area, it is also possible to monitor gasmovement as a function of time in a gas field or storage. This emerging technique is therefore veryuseful in reservoir management, in order to obtain increased recovery, higher production, and toreduce the risk of infill wells. These techniques can also be used for monitoring underground gasstorage.

The study gives recommendations on the use of 3D and 4D seismic in the gas industry. For thispurpose, three specific questionnaires were proposed : the first one dedicated to exploration,development and production of gas fields (Production questionnaire), the second one dedicated togas storages (Storage questionnaire) and the third one dedicated to the servicing companies. Themain results are:

- The benefit from 3D is clear for both producing and storage operators in improving structuralshape, fault pattern and reservoir knowledge. The method usually saves wells and improvegas volume management.

- 4D seismic is an emerging technique with high potential benefits for producers. Research in4D must focus on the integration of seismic methodology and interpretation of results withproduction measurements in reservoir models.

4.2- Scope of the study group

The participants are identified in Table 4.1.

Table 4.1: Participants of the study group.

PARTICIPANT COUNTRY COMPANYDaniel Bourjas (Chairman) Gaz de FranceChristian Hubans (Study leader) France ELF-EPFrederic Huguet (Secretary) Gaz de FranceFrangois Verdier Gaz de FrancePaul Fink Austria OMVHarald GranserMaciej Gorski Poland GEOFIZYKAKnut Georg ROssland Norway STATOILLothar Geselle Germany RUHRGASJ@rgen Knudsen Denmark DANSK OLIE & NATURGASEduardo Trinchero Argentina TECPETROLTaska Vasiljevic Yugoslavia NIS Naftagas

The Participants are specialists in different techniques in exploration, production, gas storage andgeophysics, so discussions in which participants shared useful experiences were very profitable forthe study.

The method of work was based on responses to questionnaires which were sent to variouscompanies, members of the IGU and to service companies.

Three kinds of questionnaires were sent :1) A questionnaire on storage applications sent to 52 representative countries2) Another dedicated to gas production sent to the same 52 countries3) And the third one sent to 14 service companies.

Responses were statistically interpreted if the number of responses was large enough. Comments andqualitative responses are integrated in the report. The study was completed with information frombibliography and personal experience.

We received 22 responses to the production questionnaire, 12 to the storage questionnaire and 3 tothe service questionnaire. The companies from 16 out of 52 countries responded. Statistical methodcould be applied to 3D techniques for production and storage questionnaires. But there were only 7partial responses for 4D application.

The number of gas fields involved in the study is at least 349 including 51 oil fields with gas cap, 61gas storage reservoir including 14 aquifers, 38 depleted fields and 9 salt caverns. Statistics arecomputed using available information in the answers.

Source of information : questionnaires, responses and bibliography.

Thanks to the following companies for their collaboration, and in particular to the questionnaires.

Table 4.2: Overview of responses to the questionnaires, listed by companies and activities.

Country Company PRODUCTION STORAGE SERVICES22 12 3

Argentina Capex S.A. xArgentina Perez Company S.A. xArgentina Roch S. A. xAraqfl~na Tecpetrol S.A. x

retina IYPF I x,.;-. lnNA\/ x I x IL 7 Energy i x

Canada IUnion GCroatia IIN) KI.4

as Ltd xm-lwaltaplin x x

Czech Republic MND xDenmark DaFrance Elf r

msk Olie og Natur gas MS ! x

France Gaz de France xGermany BEB Ferdgas und Erdol Gubls x xGermany RUHRGAS AG x

—-—-XK5ermanv I I Ew

Germany lWinterstJaDan lJ?=m ~

Tall x~P=,, ~nergy development Co., Ltd x x

Japan IJapex x xI-ail,al,.1..;1 v

I A I IDevelopment Corporation x x

I Y I Y I,“” ,,., .,., B ,. I .. I

Drway lStatoil xnrl I x I I

?etroleum Limited x& Gas Comp. x x

SC Limited xlCompagnie Generale de Geophysique xINE Naftaaas x

4.3- Definition of seismic methods

4.3.1- 3D Seismic

Seismic reflections enable the production of a 3D image of the subsurface. The source (explosives,vibrations) emits a wave into the ground. The reflection from each sedimentary layer arriving at thesurface, is received by sensors (geophones or hydrophores) and digitally recorded, processed andstored. Initially the recordings were performed along lines (2D seismic), nowadays the acquisition ofdata on regular grids or patterns at the surface (3D seismic) is preferred. So the 3D seismic spans acontinuous 3 dimensional volume from surface or sea level, down to below the targets, and contains adetailed image of the subsurface below (structures, stratigraphic traps, reservoir quality, fluidcontent).

Figure 4.1: 2D Seismic principle.

Figure 4.2: 3D Seismic principle.

4.3.2- 4D Seismic

This general term covers all standard repetitive seismic methods which allow monitoring of changesin fluid content of the reservoir or storage, during production or injection. Two families can bedistinguished :

. Repetition of standard 2D or 3D Surface Seismic or/and Borehole seismic to monitor fluidproduction (preparation of development scheme, enhanced recovery ...).

. Time-lame Seismic allowing the observation of the reservoir production at regular time intervalsand inferring decisions on seismic investments and development scenarios. Such surveys usededicated devices, permanent or temporary, in 2D or 3D configurations, or/and borehole seismicreflection principles.

4.3.3- The microseismic survey

This method also requires equipment decision to monitor the influence of the production bymicroseismic methods. Dedicated devices (surface or well sensors) are permanently installed. Thewave is no longer generated by artificial sources as for the seismic reflection method, but through themechanical movement generated, in the ground, by changes of fluid pressure in the reservoir.

Seismic wave emissions usually have very low intensity (-3 -2 on Richter scale). So borehole sensorsare preferred as they are further away from the natural surface noise, and closer to the sources layersin the vicinity of the reservoir.

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4.4- Summary of main results

4.4.1- 3D Seismic

Companies are generally satisfied by structural results but less for Iithoseismic. Since the beginningof the nineties, there has not been any major technical improvement, but rather a strong decrease ofthe cost of offshore 3D seismic. Onshore, the cost of 3D seismic acquisition is decreasing slowly, butdue to environmental constraints, the overall cost has increased. These environmental constraintsbecome a real problem in the future for industrial countries. Data acquisition is the major part of thecost. Gas producing companies are ahead of storage companies’ in using Iithostratigraphic methods.

4.4.2- 4D Seismic

Repetitive 3D seismic is more common offshore than onshore, due to the environmental problemsand to the expense of acquisition. Onshore, 4D methods are limited to repetitive 2D in small areas.Most 4D projects for underground gas storage (UGS) are funded as research projects. The 4Dtechnique is mature but not yet applied. It is not integrated with reservoir modeling. Favorableconditions for 4D are: soft porous rocks, a compliant matrix or fractured reservoir. The detectability isrelated to the wavelength limit, and improves with signal to noise ratio. In the best cases, the limit ofdetectability in thickness can be one twentieth of the wavelength.

4.4.3- Expectations

For many companies, the emphasis/expectation in 3D seismic is towards accurate structuralmapping. The detection of gas extension, mainly toward the spill point, is the primary need in order toassess the gas volume. The most useful measurement for production remains reservoir pressure.

Benefits ex~ected from monitoring bv !xoducers are :

. Optimisation of the number of production wells and the positioning of infill wells● Minimisation of economic risks● Localisation of unswept areas in order to increase production

The possible benefit may be important (1O to 100 106$) relative to the costs of an 4D operation (lessthan 1 106$ per year). For storage purpose, the expense of seal failure can be as large as the storagecost.

4.5- General recommendations

In this chapter, we will give some recommendations extracted from the answers to the questionnaireand from the study of working group.

The improvement in structural shape, fault pattern and reservoir knowledge is a clear benefit from 3Dseismic for producer and storage operators. This method usually saves wells and improvesmanagement of gas volume. Research in 4D must focus on seismic application and interpretation ofresults, in order to integrate them with production measurements in updating reservoir models.Research must aim to develop reliable equipment for onshore acquisition to reduce costs andimprove the quality and repeatability of measurements. A feasibility study is considered to beessential to inform project decision.

We have to integrate monitoring design very early in the development project of field. The basesurvey should be recorded before starting production for a gas field or gas injection in a storage. Thisis the most favorable condition to obtain good 4D results.

1

4.5.1- Recommendations in storage

A) 3D Seismic

. For structural rmrmoses

Most storage cases are onshore. First 3D seismic surveys has been recorded early nineties.The main problems in 2D land seismic are statics’ corrections, misties and fault interpretations.Usually companies shoot several 2D surveys at different times to solve structural problems. As aresult, the total cost of 2D survey is generally greater than a 3D for the same area. It is recommendedto start exploration with a sparse 2D program in order to reduce the risk of encountering inappropriatestructural shape and to delimit the most appropriate 3D survey area.

Because structures are usually relatively flat and shallow, there are 2 problems: long wavestatics and velocity. A major effort should be made to solve these problems using uphole surveys forstatic calibration and deep wells for velocity measurements.

In this case, pre-stack depth migration is not usually needed.

● For lithostratiara~hic mmwoses

The increasing accuracy of 3D seismic data makes it the tool for reservoir characterisationaway from well positions. The 3D seismic for reservoir analysis should be recorded before starting thedevelopment phase in order to help build the reservoir model and identify the best positions forproduction and control wells. A feasibility study is necessary to know if reservoir characteristics canbe identified with this method. Lithoseismic results are usually good for simple reservoirs, but in morecomplex cases, such as gas bearing shaly sand, Iithology is not easy to identify.This method needs very accurate preserved amplitude processing.

B) 4D Seismic

● Monitoring durinu UGS development Dhase

Considering first monitoring as an insurance to reduce development risk, the monitoringproject must be designed before first gas injection.At this stage, prediction of problems which may arise during the development and production phasesis necessary. Benefits from seismic monitoring are saving control on producing wells, and mostsignificantly saving gas volume. Profitability is achieved with only 1 or 2% of gas volume saved. Thepotential benefits can easily reach ten times the project cost. An appropriate monitoring survey canpartially substitute control wells in complex spill point areas. The survey recorded along structuraltops can detect area with high gas value. Producing wells may be drilled in accordance withmonitoring results, in order to optimise production and reduce the risk of infill wells.The primary seismic method for monitoring is surface seismic. 3D seismic is ideal but not veryrealistic in populated land areas.

If injection can start with a minimum of production on control well, the use of seismic monitoringresults help producers to optimise storage development and equipment. Production wells, can bedrilled in the maximum gas anomaly, and control wells in the direction of the maximum gas bubbleextension.The monitoring program can be extended as a function of gas bubble development and especially inthe spill point area.The monitoring project can be terminated at the end of the development phase, after drillingappropriate control wells.

. Monitoring durinu UGS txoduction ~hase (without seismic reference before gas injection)

In some cases, seismic monitoring can help to solve production problems.4D works for monitoring gas linkage in shallow aquifers or related neighboring structures if the gasanomaly is growing. A feasibility study is necessary to be sure that the gas movement can be seen byseismic with the accuracy needed by the producer.In some cases, a seismic monitoring project for the main reservoir can start during the UGSproduction phase, if gas saturation changes are detectable by seismic. Usually, the seismic anomalyis very weak comparing to the anomaly during the development phase. Therefore, gas movement canbe seen in the case of high variations of gas saturation in a very porous sandstone. These variationsof density of reservoir are due to an important gas to water substitution.

4.5.2- Recommendations in production

A) 3D Seismic for production

First 3D seismic surveys were recorded in the early eighties. Lots of research works on equipmentand processing algorithms have increased the accuracy and reliability of 3D seismic imaging. Theexplosion of computing power and enhancements in image processing have reduced the cost ofobtaining, visualizing and interpreting good quality data.

. For structural mmoses

It is very clear that 3D seismic is THE best method of obtaining an accurate image of reservoir. Agood image is more or less difficult to get depending to the complexity of structure. But currentalgorithms, such as 3D pre-stack depth migration, can cope with the structural complexity.Reservoirs were very simple before 3D observation. Now, they are very complex. Did they change?Certainly not. But way of getting a good geometric description of reservoir is now available. Economicconsideration do not leave any doubt about the necessity to record a 3D survey. One million dollarsfor the survey compare to several millions dollars to drill a side track or a new well if the first ismispositioned.All the service companies can provide similar services. Prices, delays and contracts can differ. This isrelated to the area localisation. A call for tender will ever give information for choosing servicecompany.

The main difficulty in getting accurate structural image in depth is the determination of velocity field.For all the processing algorithms, we can apply the GIGO law: “Garbage In Garbage Out” : a wrongvelocity field will give you a wrong depth image. For this reason, it is necessary to record velocity logand vertical seismic profile (VSP) at least on one well from top to bottom. Walkaway and full wavesonic can be useful in case of anisotropic layers like shaly overburden layers. Anisotropy (TIVmedium) is the first factor which destroys the quality of image. Recent results show that it is possibleto estimate it and currently take it into account. The wrong depth image will give wrong resourcesvolume estimation.

. For Iithostratiurauhic m.moses

The possibility of computing rock physic parameters, like porosity, shaliness, fluid content ... isobvious. The physical phenomena during wave propagation are well-known. The wave behavior at aninterface between two media are fully described by the equations of reflectivity (Zoeppritz equations).These equations explain the relation between any change in rock (porosity, mineralogy, fluid, grainsize ...) and the seismic parameters (amplitude, velocities, etc...). In spite of this physics, it is difficultto get reliable and accurate information from seismic amplitudes. This is related to the great numberof rock parameters which appear equivalent in seismic domain. So we recommend :

1) To use the maximum amount of information to explain seismic amplitudes (logs, rockmeasurements ... on 2 or 3 wells including 200 m in the overburden).

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2)

3)

4)

To use well seismic to study the upscaling between logs and seismic data.

To use amplitude versus offset (AVO) to get shear wave information. This is one way torecognize fluid effects. This requires truly amplitude preserved processing and great carein interpretation.

The use of AVO without any well information can be very unsuccessful.

By taking these precautions, you will enjoy your amplitude studies and get some reliable results. Weshould be able to understand your results, understand the validity limits of your results and evaluateuncertainties.

B) 4D Seismic for production

4D seismic or time-lapse seismic is more recent than 3D. The first experiments took place at the endof the eighties. At that time, the experiments compared 2D to 3D, and results were qualitative. Someevaluations of fluid contacts movements were attempted.

Since that time, 3D seismic has been developed. Acquisition and processing techniques too.Repeatability and processing sequences, to improve comparability between two (or more) surveys,are now applied. During processing a lot of quality controls (QC) are used to check this improvement.So time-lapse seismic is arriving on the market and is being pushed by the service companies.

The main recommendation for 4D seismic is to perform a feasibility study. This study has to answersome of the following questions:

1)

2)

3)

4)

5)

6)

Will the production affect rock physical parameters (saturation change, pressure change...) ?

Is there an effect on elastic parameters (P velocity, S velocity, density) ?

Is this effect strong enough to be detectable by seismic ?

Is the seismic quality of the base survey good enough to detect it ? If not, can I record a newbase survey ?

Is the expected repeatability good enough to detect it? If not, can I record a new basesurvey ?

Do available data give me the keys to interpret the measured differences ? If not, can Iacquire them (cores, logs ...).

If the feasibility study is positive, the following questions must be asked :

1)

2)

3)

4)

What are the risks during production ?

What information can 4D seismic give me?Should I use a qualitative or a quantitative approach ?

What corrective actions can I take ?

What are the costs and benefits ?

Because reservoirs are even more complicated than the initial image we have, we can say that 4Dseismic bring always new information to manage the production (in case of phenomena detectable byseismic). In any case, it is the only source of dynamic information between wells. The benefitsobtained are determined by this information.

, ,

In general, answering the last 4 questions is very difficult. Because 4D seismic is a geophysical toolused by production managers, it is necessary to convince many different people with this technique.4D seismic is a marvelous tool to integrate geophysicists, geologists, reservoir engineers andproduction engineers.

For the decision mechanism, two different situations appear:

Some problems or major events occur during production, such as no fit between real production andforecast, or some infill wells to drill or choose to enhance the gas recovery, etc... In this situation, theeconomic balance is easier to compute. Costs and risks can be clearly evaluated and a good decisiontaken. The published cases are all in this class, and they show a benefit which is 3 to 10 times thecost of 4D seismic.

Before production, spending money for data can appear to be buying an insurance. This is the samedecision mechanism, that is spending money to have better chance to avoid the correct future risks.The impact of 4D is bigger than in the previous case. Nevertheless computing the financial balance of4D seismic is a challenge. The production manager has to take into account the effect of havingaccurate dynamic information on the reservoir during the development step. For a phaseddevelopment, this is easier (we join the previous case), but for a regular development, the managercan consider the impact on the drilling schedule and possibly change it to adapt to the seismic results.This study must be performed by scenarios’ analysis and comparison. A part of the ongoing researchis working on that point, to find confident reasons for decision.

Two parameters are commonly the most important : gas saturation and pressure. The major changesin impedance are related (90Yo) to these parameters.

4.6- Technical recommendations

This chapter presents technical recommendations for performing a 3D or 4D study, and avoidingsome traps. We consider this chapter from a theoretical point of view. In real situations, some pointswill not be a problem and for others, you would choose between 2 non perfect solutions. For thisreason, storage and production are considered all together.

4.6.1- 3D Seismic

. For structural tmmoses

The main goal of the 3D survey is to determine the structure. For this purpose, statics and thevelocity model (for appropriate time to depth conversion) need precise surface calibration usingshallow well (uphole) surveys and deep wells. The depth of uphole should be calculated in order toreach, if possible, the first layer that can be picked out the seismic. The well positions should bechosen in order to solve long wave statics, with the travel time measured into the well according tothe topography and soils nature. The density should be chosen depending on the structuralcomplexity in order to obtain a precise depth map of the first layer (about 1 per km2). It is important toadjust survey parameters and seismic processing to obtain an image as good as possible of this firstlayer.

To build the velocity model, use first deep well information (VSP, travel time or depth). Secondly, usestacking velocities to get the velocity drift that should be confirmed by variations of Iithology. Thismodel can be used for time migration in spite of only stacking velocities. A time to depth conversion,using the layer cake method working down from the calibrated shallow layer, is recommended. Theuse of geostatistical methods is recommended to estimate velocity fields and depth maps fromseismic and well measurements. This method is efficient to filter residual noise and computeaccuracy maps.

Offshore is different. It concerns only the production class. The key parameter is the velocity field

which has a direct effect on the shape of the structure. In this environment, static corrections are verysmall. The previous recommendations are still useful. In this case, don’t hesitate to use differentapproaches to obtain good velocity fields. Several are now available on the market:

- Stack velocity analysis- Focusing analysis in pre-stack or post-stack migration- Residual move out study which give access to an anisotropy measurement

Anisotropy can be crucial for the shape of the structure. New migration algorithms can take it intoaccount.

. For reservoir characterization

For reservoir analysis, 3D seismic is essential to know reservoir characteristics away from wellpositions. If you have no available seismic data, a feasibility study is recommended to decide newacquisition. This study must be based on borehole measurements: sonic, density logs, Iithologicalinterpretation based on core analyse and petrophysics measurements. Acoustic or elastic seismicmodeling should be carried out to compare with real seismic traces from 2D seismic or VSP for anyreservoir quality. The method determine the seismic ability of reservoir parameters according hisspectral domain (wavelength) to identify reservoir characteristics. At this stage, the most sensitiveseismic attributes can be selected.

If you have 3D seismic, already processed in preserved amplitude, the feasibility study should beperformed by extracting some 2D lines, to check very quickly and inexpensively base if the seismicshows any changes.

If the feasibility study is positive, the full 3D study can start. 3D processing amplitudes must bepreserved with surface consistent amplitude corrections and deconvolution processing. The result isgood if there is no correlation between amplitude along stable interfaces and surface conditions(topography, nature of soils). Processing steps for which you don’t trust the amplitude conservation(DMO ?, F-K filter ?,...) should be avoid.

At this stage, qualitative stratigraphic interpretation is the most profitable way of using commercialsoftware. This should use amplitude variation along the reservoir layer and specific seismic attributes(surface or volume) selected by feasibility study. Methods based on neural networks give goodresults. A good interpretative synthesis results from both supervised and unsupervised methods.Supervised methods give the extension of reservoir characteristics observed at wells. Unsupervisedmethods give the distribution of seismic facies, they must be correlated on reservoir facies or seismicartefacts.

Acoustic impedance inversion of reflectivity is a good way of making a quantitative interpretation. Forthis specialised and costly process, the feasibility study should aim to verify if the variation ofreservoir parameter can be quantified and observed using impedance. For example, sometimes it isimpossible to estimate porosity using impedance in a gas bearing shaly sand reservoir, because theimpedance of a good gas bearing reservoir is the same as a shaly reservoir. In the case of carbonatereservoir, it is generally easy to quantify porosity with impedance.

The use of geostatistical methods is recommended first, to estimate reservoir parameters fromseismic attributes, and secondly to integrate stratigraphic seismic results into the reservoir model.This is efficient for filtering residual noise and compute accuracy maps.

Don’t forget that AVO techniques can be used. Theoretically, this approach gives access to shearwave impedance contrasts. The shear wave impedance contrast is a new independent elasticparameter of the rocks. AVO interpretation is non unique solutions and you need absolutely wellinformation to trust the result. We recommend recording full wave sonic in two or three wells, in thereservoir and the overburden (200 m or 300 m above the top of reservoir) to be able to study rockbehaviour and performing a well-seismic tie for both near and far offsets. The study of the overburdenis necessary because changes in seismic amplitude can be due to a change either in the reservoir or

in the overburden (lateral variations of pressure, mineralogy ...).

4.6.2- 4D Seismic

Because 413 seismic comprises several seismic surveys over a period of time, most of the previousrecommendations for reservoir characterisation apply. Here we only focus on specific 4D relatedpoints.

Before any 4D study, a feasibility study is essential to check that gas variation into the reservoir canbe measured or detected by seismic. For this purpose, sonic, density logs and velocitiesmeasurements in cores at different stage of saturation must be used to estimate differential seismicresponses of the reservoir. Amplitudes variation in the reservoir and time shift measurements belowthe reservoir must be greater than the noise level estimated for a recent seismic survey. At this stage,we can estimate the minimum thickness of gas detectable with seismic monitoring. This minimum gasthickness detectable is very useful to predict the accuracy of gas bubble boundaries in the case of flatstructures.

Because gas and liquid have large contrasts of density and velocity the feasibility study is generallypositive from the petrophysical point of view in case of fluid substitution. The most favorablesituations are porous to very porous sands (>15Yo) and permeable carbonates. Pressure effects arenot so obvious and are strongly dependent on the properties of the work matrix.Expected time shift should be greater than % of a time sample rate, and resulting in the decreasing invelocity by gas to water substitution in the reservoir. Expected impedance change should be greaterthan 7%. The signal to noise ratio of the seismic should be better than 10 dB.

Repeatability of seismic can be controlled well except if the base survey is very old (before 1987).Some acquisition conditions will be different such as weather, tides, weathered zone variation, etc..,Concerning repeatability of measurements, the level can be estimated in the best conditions at 25 dBto 30 dB (5 to 3’%. of differences in amplitude). In the case of very old survey compared to a new one,repeatability can drop down to 2dB... !!! This is equivalent to 80’%. of change in the amplitude. In anycase, we have to avoid explosive sources which are not repeatable due to source coupling variations.Offshore, positioning can be crucial. Onshore, use of markers to position shot points and receiversallows repositioning within 1 meter.

After preserved amplitude processing, a cross-equalisation step is necessary only as a final step forlast and small discrepancies. This cross-equalisation works better in the pre-stack domain than post-stack.

A qualitative or quick look interpretation can be made using amplitudes and time shift anomalies. Acoherence between both these seismic attributes is generally suticient for checking gas bubbleboundaries and maximum gas volume area. Integration in reservoir evaluation will be perform mainlyby visual correlation between a map of seismic results and maps of previously computed reservoirscenarios.

For a quantitative interpretation of gas thickness and gas saturation, acoustic impedances areneeded. For this purpose, repeated velocity measurements in a well are recommended to calibratethe real velocity response with saturation and pressure variations in the reservoir.

Integration in the reservoir evaluation is not easy. Some researches are studying this particular point.History matching (HM) is the conventional way of determining the best description of reservoir duringits life. To integrate quantitative seismic results into the HM process, miscellaneous problems have tobe solved.

Seismic monitoring using borehole seismic methods is suitable for checking gas evolution near a wellin the reservoir or in the aquifers above. The maximum distance of investigation is lower than 1/3 ofthe depth of the reservoir. This method was used to increase the controlled area around a control wellin the case of an heterogeneous reservoir.

.

Acquisition, processing and interpretation must be done in the same way as 2D seismic monitoring.Recommended receivers are permanent borehole geophones installed along the tubing. 3components geophones are preferred to enable removal of S waves during P wave processing.

4.6.3- Microseismic

Microseismic surveying is recommended for fractured reservoirs or compacted seals. This methodmust be used for reducing risk of breaking or initiating fractures in the reservoir or seal layers, as thepressure varies in the reservoir. This method also helps to prevent gas linkage in the cement of thecasing. For this purpose, a network of producer wells must be equipped with 3 levels of permanentsensors in the tubing close to the reservoir. The spacing must be sufficient for detecting waves atdifferent times ( 50m). Geophones must be 3 components and their orientation must be measuredusing a calibration shoot. It is recommended to put a permanent sensor below the reservoir. Apressure measurement of the reservoir in each well bearing permanent sensors is alsorecommended. A permanent recording system with an automatic signal detection mode is essential.The signal processing must be automatic to avoid a tedious interpretation task. The main steps are:

1)2)3)

The usepurpose.

The signal to noise separationThe classification of events according to their seismic attribute and originAnd if possible, the localisation of the source

of P and S travel time and wave polarisation with triangulation is recommended for thisA poromechanical modeling of the reservoir is recommended to confirm the interpretation.

4.7- Results from questionnaires - State of the art

This part is a synthesis of all the 37 responses to the questionnaires. Many companies answered tothe 3D part and some answered to 4D part. Only 2 companies performed 4D studies in the past (4studies for each). Several considered 4D seismic, but did not take a decision to perform it.

>

‘“’”s’avia~

““it’d-::= EISERVICING

❑.STORAGE

Netherlands I9PRODUCTION

Korea [

Japan

Germany

France I

Denmark I

Czech Republic I

Croatia I

Canada I

Austria I

Argentina

o 1 2 3 4 5

Figure 4.3: Number of responses by country.

4.7.1- 3D Seismic applications

● General context

Storaae:3D technique is widely used. Thirty-two 3D surveys were carried out for storage. 8 out of 12companies use this technique. 75% of 3D surveys concern depleted fields, 25’% concern aquifers.Most of 3D surveys are more recent than 1995.

Production :In these responses, we observed more onshore activities than offshore activities. 409!. recorded often3D surveys, but 40% recorded it rarely or never. The main purpose for 3D is for appraisal anddevelopment of fields, less for exploration and even less for production. These ratios are similar foronshore and offshore activities. The average size of the surveys is between 100 and 500 kmz. Feware greater than 1000 km2, assumed for exploration.

Mostly sand and sandstone reservoirs are studied, but the geological environment is not a main factorof choice to record a 3D, except for fractured reservoir and low resistivity sands, for which more than15 to 25% companies frequently recorded 3D surveys.

● Ex~ectations and objectives

Storaue :70% of 3D surveys are dedicated for exploration and appraisal, 25% for storage operations and

, .

optimisation, and for solving complex reservoir problems. A few are dedicated for safety (sealingefficiency of cap rock and faults).

3D is used for multi-objectives purposes (first structural and secondly reservoir). For storageoperations context, 50% of 3D are dedicated for Iithology and fluid distribution purposes. Inexploration context, the main objective for 90’XOof 3D surveys is structural.

Production :For all operators, a precise structural definition is the main result expected from surveys (90%).Stratigraphic differentiation and Iithological determination are less expected (80 to 60% respectively).Net to gross (N/G) and fluid determination are the less important goals (30%). This could be related tothe satisfactory index. Structural and stratigraphy are generally confident (up to 91%) but fluiddistribution and N/G results are considered to be good for only 30% of responses.

. Results

Storacie:Users are rather satisfied with the 3D results. They are verified by wells and 50% by other means(production, stratigraphic correlations ...). Main results are: improved structural mapping, faults andseals extensions, additional compartments, reservoir extension and properties, potential volume,contribution for planning new wells and improvement of the reliability of the reservoir model.

Production:Results are confirmed by wells with a big ratio of success (81%) and by other data (74%). On a moregeneral point of view, 74% of companies are generally satisfied with 3D results. The most importantbenefits are an improved precision of structural definition, fault pattern and vertical resolution(detailed structural features). Another benefit is a better geological model and a more precisereservoir description (distribution of reservoir bodies). This fact increases the confidence in theresulting data, and decreases tisk before drilling.

4.7.2- 3D Seismic techniques

● Acquisition

Storaae :3D area varies from 2 kmz (Reef in Canada) to 220 km2. Generally, the area is between 20 to 40 km2and covers only a part of the field area. Large 3D are dedicated for depleted field. 75’%0of companieshave used seismic modeling in preparation of 3D acquisition.

Types of used sources are vibrators (75’%0)and explosives (25Yo). In Canada, for shallower objectives,they use unspecified source and 75% explosive. Geophones are mainly vertical 1C (90%). The binsize varies from 12.5 x 12.5 to 25x 25 m with a coverage of 15 to 40 folds (average 24). The averageacquisition rate varies from 60 to 200 shot points per day. Out of towns, the environmental constraintsare mainly farming, then forests and finally lakes. The impact varies usually from 1.5 to 22 103$/ km2(average 1O).

Production :70% of companies prepare the survey with seismic modeling, but 30% not. Vibrators are used by amajority of companies (75Yo) and explosives in the other cases. Offshore, air gun are used by allcompanies.

Onshore, acquisition is 4 times more expensive than offshore acquisition (20 103$/ km2 versus 5 103$/km2) plus some environmental extra cost (1O 103$/ km2). Costs of 3D project are composed of 82% ofacquisition costs and 8% for processing and interpretation (4Y0 for each). The budget of a project isbetween 1 and 7106$.

Precision of bin positioning is better than 20 m onshore and 10 m offshore. Static corrections are

●

reliable (85 to 95Yo). The on-field processing is usually good (only 11YO are unsatisfied). Thepositioning accuracy is always better than 5 m for onshore acquisition and the efficiency of QC duringsurvey is satisfactory.Well seismic is acquired in complement with 3D survey. Vertical seismic profile (VSP) very oftenand occasionally Walk-away. Few companies recorded 3D VSP.

Downhole Seismicity

F!?Erl m

W+Q,,,I

IEEEzKl

Figure 4.4: Downhole seismicity techniques : Walk-away and VSP techniques.

● Processi nq

Storaae :In-house processing software is not very common (15%). The use of external commercial softwares isthe rule. The main ‘goal is to improve seismic quality by noise reduction and static correction. DMOand time migration are used before structural interpretation. Amplitude preservation (30Yo) forimpedance inversion (40Yo) (reservoir objective). Specific programs like antimultiples, Q factor, pre-stack migration, AVO processing are rarely used. Operators are generally satisfied by processing.Depth migration is only used for the specific objectives like reef reservoir or complex faulted area.

Production :The processing is performed in-house for 45% of surveys. Some use commercial softwares (86%),but 50% have developed in-house modules for specific processes. Operators are generally satisfiedwith processing. Ampfitudeare unsatisfied).

preservation and noise reduction should be enhanced ~ the future (18?/0

. Structu rat inter~retation

Storaae :For well calibration, logs are commonly used, VSP rarely. For time to depth conversion, operators useSonic logs with or no drift correction and stack velocities. Most use 3D velocity model. The accuracyof depth evaluation is always good enough. The most common deliverable are 2D maps andaccuracy maps. 70’% use a 3D modeling. Geostatistical methods are less used.

●

Production :Time to depth conversion of seismic information requires the use of velocity field. The velocity fieldscan be obtained through Sonic logs or drift corrected Sonic logs and VSP interval velocities. The dataavailable only at well position can be interpolated by adjustment on stacking velocities. Occasionallythe inversion of travel time is used to obtain the velocity fields.

2D mapping is the most common presentation for structural events (95Yo). New techniques areemerging like the 3D modeling (45Yo). For the accuracy evaluation, 40% present a map of accuracy.Geostatistics seems not to be very useful for this purpose. 30% use it occasionally. The satisfactoryindex for structural mapping is well to very well for 100’7. of responses.

In processing sequence, the three main techniques used by all companies are: static correction fromrefraction seismic (63’?4.),DMO (83Yo) and post-stack time migration. Other processing modules areless used. They are dedicated to specific problems like depth post-stack migration and pre-stackmigration (15’%. used them often). Well calibration is often performed with time to depth conversionand seismic modeling. To perform a well calibration, following tools are used : logs (90’Yo), VSP(77%). Only 30% of the companies calibrate often with VSP inversion results, dip measurements oranisotropy (2070).

● Lithostratiara~hic seismic

Storaae:For Iithostratigraphic interpretation, the use of petrophysical facies, sedimentology, Sonic and densityis systematic. Full wave Sonic is not often used. The first objective is the qualitative interpretation.For quantitative interpretation, amplitudes and acoustic impedance are used for porosity and gasdetection. AVO and instantaneous frequency for gas determination are rarely used. Generally, gaswater contact (GWC) is the first indication required, followed by porosity. Results are more reliable forporosity and Iithology than fluid evaluation (unsatisfied).

Production :The use of preserved amplitude processing is quite common to gain Iithological information. 57% ofcompanies use amplitude very often and only 11 Y. never. This processing is performed in order touse AVO indicators, intercept amplitude and gradient or substacks. Post-stack acoustic impedanceinversion is currently used (55Yo) but not pre-stack or stochastic inversion (only 207.).

Information well measurements are usually included in the seismic interpretation. Systematically,seismic boreholes and logs (957.) are first included, and less often geological data as core descriptionand sedimentology. The most recent tools are used occasionally (full wave Sonic and VSP inversion).

The interpretation is qualitative and mainly based on a non supervised attributes classification. 60%of the companies use it very often. The other techniques (constrained inversion or geostatisticalapproach) are occasionally used. The quantitative determination of petrophysical parameters ismainly based on 2 seismic attributes: amplitude and acoustic impedance (80?4.). These two featuresare used for the following parameters : porosity, N/G, fluid content, mineralogy and over pressurezones. Instantaneous frequency is used (10!4.) for fluid detection. All other attributes represent only10%. 70% of interpreters are satisfied for the porosity evaluation. 50% are satisfied with otherparameters. Nobody computes accuracy maps for these parameters.

4.7.3- 4D Seismic applications

4D means the fourth dimension which is time-lapse (repeated seismic surveys) for any kind ofseismic experiment (VSP, Walk-away, 3D VSP, 2D seismic, 3D seismic).

This technique is quite new. So it is particularly interesting to get a snapshot of the industrial situationand to know how 4D seismic is considered by gas operators.

*

Storaue :607. of operators have considered to perform 4D, but only 2 have decided to perform it. The mainreasons for not doing it are the too high cost and the negative conclusions of feasibility studies. 4Dmethods used are borehole seismic (VSP and Walk-away), and repeated 2D lines. In Canada, formonitored reef storages, repetitive VSP method is used. Only one company use permanent sensors.

3 aquifer storages and 1 depleted ~eld have been monitored at a depth between 400 to 900 m.Reservoirs are sandstones and reef carbonates.

Feasibility studies are performed with elastic modeling using all information available. Most of 4Dstudies are experimental, consequently financial balance between cost and benefit is not performedsystematically because of the funding through research project. Duration of projects varies from 2 to 7years. One study is pursuing over 7 years. Most of experiments are recent (after 1990). Themonitored area is limited (1O kilometers of 2D).

Production :56% of companies considered 4D seismic and only 2 performed studies. The two main reasons arethe high price, and sometimes the fact that no 3D seismic data are available as base survey. Thehigh cost related to the development phase is a key factor for reluctance.

The two companies performed 4 times a 4D seismic study, each in a sandstone reservoir: 4 timesrepeated 2D lines, 1 time 2D lines repeated on 3D survey, and 3 times repeated 3D surveys. Nopermanent sensors have been used. In any case the feasibility is performed in investigating rockphysics and full wave Sonic measurements. 1D seismic simulation allows to evaluate the seismicresponse related to fluid substitution. Economic balance has been evaluated for 50% of the studies.

● Ex~ectations and objectives

Storaae:Objectives are usually the detection of gas distribution and extension, followed by GWC definitionand fluid barrier detection during storage development phase. An accuracy of 1 to 2 m of gasthickness and 50 m laterally are needed.

Production :Two main objectives can be defined : gas liquid contact (GLC) detection andwells (wells on unswept areas). The first repeated 2D study has been recordedlast repeated 3D is ongoing now.

. Results

Storacie :

positioning of futurelate eighties and the

The 4D interpretation for the cases described have been verified by wells and other works likereservoir simulation. The main results tell that the gas bubble changes near a well. This is recordedwith permanent downhole geophones (PDG) using transit time tomography and Walk-away method.Qualitative and semi-quantitative evaluation of gas bubble extension and evolution along 2D seismicprofiles is carried out with a sensitivity of 2 m in gas thickness.

Production :No control by wells is now available because the results of 4D was not to drill, or because it was notyet drilled. The validation of results is performed by matching with other information like reservoirevaluation or history matching. Both companies are satisfied with their results.

*

4.7.4- 4D Seismic techniques

. Acquisition

Storaae:The few 4D experimentations for a storage target show a limited monitoring area. The environmentalconditions have a major impact on 4D design ; for example the use of pre-existing tracks or roads forimproving repeatability. Usually, watching activities are dependent on reservoir status (ex: maximumor minimum gas volume).

For 2D profiles, classic acquisition method can be used (non permanent source and geophones), butfor borehole monitoring, cemented geophones in the well or PDG on tubing are needed.

Production :The studied areas were located in offshore. The environment can have some impacts on the surveybut never prevent to record it. The operation management of the field has occasionally an impact on4D seismic design but never prevent to run it.

● Re~eatabilitv

Storaae :For onshore seismic, usually time and signal differences generated by source and geophonescoupling and weathered zone effect are measured. Positioning accuracy has an important effect too.The use of explosive sources is not recommended because of bad effect on repeatability. The use ofpermanent geophones improve repeatability of seismic measurements.

Production:Repeatability is a key point of 4D seismic study. It is very constrained by changes of surfacecondition, shooting direction changes and offset range. Every companies are confident in thereproduction quality of the source, the signal and the positioning.

All operators who have performed 4D seismic study enhance repeatability by reprocessing. Based oncross-equalisation technique, the reprocessing concerns mainly the post-stack data. Static correctionsand signal processing (deconvolution for instance) are the key points fdr repeatability. This is thereason while reprocessing the surveys for a monitoring purpose is systematically performed. Use ofsame softwares and same parameters for processing is essential. Amplitude presewation and post-stack migration are always performed, sometimes pre-stack migration and impedance inversion canbe added to the process sequence.

. Intemretation

Storaae :Companies interpret first amplitude changes, time shift under the reservoir and acoustic impedancevariations in order to evaluate GWC and saturation changes. The quantitative interpretation is notfulfilled. GWC is the most satisfying parameter.

For gas saturation estimation, companies use impedance and time shift measurements. For GWC,they rather use time interval differences and time shift using specific interpretation environment within-house software.

Additional information are integrated in 4D interpretation like logs data, wells data (petrophysics andpetro-acoustic). Always GWC and pressure, often an output of reservoir simulation (pressure,saturation) are used for a quantitative calibration purpose. For quantitative interpretation, forwardmodeling is usually used on differences. Up to now, 4D seismic results are not integrated in reservoirmodeling.

.

Production :As for 3D Iithoseismic interpretation, amplitude and impedance are the two most used seismicattributes. Time shifts or Poisson ratio changes are rarely used. These attributes are commonly usedto perform a quantitative interpretation concerning the magnitude of water table movement. Theresults are satisfying. No approach for saturation porosity are performed. One case for depressureevaluation with good results has been performed. To validate seismic interpretation, data arecorrelated with petrophysics or well information derived from logs and well seismic. Position of gasliquid contact (GLC) in the well and gas saturation from reservoir modeling are also used to validateseismic.

● 4D moiect cost

Storaae:The cost level of a project using 2D or borehole seismic is, for equipment, less than 0.5 10s$.Acquisition costs are always less than 1 1Os$ and processing including interpretation is less than 0.610s$ for the processing sequence development. For a standardised processing, the cost is less than0.1 10s$. For each the full level of cost out of equipment is comprise between 0.2 and 1.5 10s$ for amaximum of 20 km 2D profiles.

Production :The cost range from 1 to 10 10s$ includes seismic acquisition, processing and interpretation. In anycase acquisition is 10 times more expensive than process and interpretation.

4.7.5- Microseismic applications

The Microseismic Techniques includes recording, processing and interpretation of micro-earthquakesproduced by mechanical stresses in the reservoir.

Only ho companies answered to this part of the questionnaire : one for production and one forstorage activity. This fact underlines the still very experimental side of these techniques forproduction application. In the following subchapters, we will prefer a short description of the two casesinstead of a statistical treatment of the data which is not significant.

● General context

Storaae:The storage survey was made in the framework of a research project to investigate the behavior of areservoir installed in an aquifer sandstone layer. The experiment has been run since 1992 and is stillgoing on. Rock measurements and full wave Sonic recording have been used.

Production :The microseismic survey started for safety reasons in relation with human and equipment protection.The survey ran over thirty years during the depletion of a carbonate and fractured dolomite gas field.Both surface equipment and downhole sensors have been used.

● Ex~ectations and objectives

Storaae :After a short experimental acquisition showing that the microseismic survey was recording typicalevents during reservoir production, this new survey had two objectives : first, the proof thatpermanent sensors were reliable over a long period, and second, the typical events were not linked toa specific reservoir but to the production phases.

The expectation is to understand the physical reasons of the events, and to be able to simulate andpredict them (in frequency and in magnitude) for safety reasons.

Production :Following the safety reasons, the main expectation was to locate highly fractured area and to forecastthe best productive zones.

. Results

Storaue:The main results obtained are the following

1) Validation of microseismic survey2) The microseismic events are related to production3) The events are mainly located around injection wells

The future research efforts are still required to understand the mechanical behavior of the reservoirenvironment.

Production :The survey covered 1000 kmz for a field size of 120 km2. The results are well correlated to the wellsproduction and the fractures model (seismic data, correlation of logs). They enable to understand themechanical behavior of the reservoir and allow to evaluate the seismic risk of the area (subsidence,seismic activity).

4.7.6- Microseismic techniques

● Acquisition

Storaae:3 production wells have been equipped with permanent sensors clutched on production tubing. The 3components sensors are located above the reservoir with a spacing of 50 meters. The recorders arein-house or commercial. A QC is performed during acquisition.

Production :A surface network has been installed within 12 nodes. Shallow wells have been equipped ofvelocimeters with 1 or 3 components. Accelerometers with 3 components have been used too during2 years. Several kinds of numeric recorders have been installed, either in-house or commercial type(Sislac). One deep dedicated well has been also equipped with one cemented sensor, 3000 m abovethe reservoir. A QC has been performed during acquisition.

. Processi nq

Storaae:The preliminary processing is automatic. Then the data are analysed through triangulation,and S waves, and through polarisation processing using an elastic velocity model.

Production :

P waves

The preliminary processing was automatic to avoid human movements on site. The data wereprocessed in order to locate the events by : triangulation, PS and polarisation analysis, and relativelocalisation. The processing was based on a 3D velocity model.

. Intemretation

Storaae :The interpretation is based on a comprehensive set of data : petrophysical descriptions andmeasurements, full set of wireline logs, well seismic survey and daily pressure measurements. Theresults are the location of the events, their frequency and their magnitude. Their occurrences are in

,

relation to the production.

Production :The additional data were : core description, standard Sonic log, regular pressure measurements, andseismic well survey; and occasionally: petro-acoustic measurements, productivity index andgeomechanics.

The analysis of microseismicity proved a close link between the level of activity and the increase ofwell production. The understanding of the faults network has been improved with events localisation.An interpretation of the variability in azimuth of the main stress in the reservoirs has been developed.

4.8- Expectation in seismic monitoring

This chapter deals with the future of 4D methods and try to evaluate the balance of costs to riskregarding economical aspects.

4.8.1- Producer’s needs

The ranking in producer’s needs are as followed : spill point control and gas extension, GWCproblems, unknowns and problems resulted in faulting in carbonate reservoir.Needs of production information are in order of priority: pressure measurements, gas extension,seals control, then GWC and gas saturation.

4.8.2- Economic

Responses are very scattered in the cost domain, but a trend can be observed. Benefits related togas volume are estimated between 10 to 30106$. Costs related to low production wells are between 1to 10106$ (few production wells). Costs related to seal failures are estimated at the level of the totalstorage costs (30 to 100 106$ or more). Benefits expected from monitoring by producers are : tooptimize the number and to improve the positioning of infill wells and to minimize economic risk. Thepossible benefit may be important (1O to 100 106$) regarding the costs of 4D operations (less than 1106$ per year).

4.8.3- Techniques

Companies think that 4D techniques are not yet mature (5 responses out of 7) with the 2 followingreasons : not enough practical experiences and unsatisfactory sensitivity and reliability. 9 companiesexpect to use 4D techniques in the future and they think that 4D seismic can give access to quantifiedproduction parameters. The general prediction for the future is between 50 to 200 fields under seismicmonitoring during the 5 next years. 6 companies consider doing study works in the future. For storagepurpose, 7 projects would have a proactive objective (at the beginning of the storage development)and 2 for solving production problems (reactive).

4.9- Bibliography synthesis

This part is a contribution of each participant to develop a specific topic above 3D or 4D in technicalor strategic point of view. The source is a combination of bibliography and personal experience.

A) 3D Seismic Survey Applied for Underground Gas Storage Development Planning byJmgen E. Knudsen, Dansk Olie og Naturgas AIS.

A 3D-Seismic land survey was acquired over 39 km2 for the development of an underground natural

●

gas storage in Denmark. The storage is planned in a nitrogen filled Triassic sandstone reservoir(bunter sandstone) at a depth of about 1600 m in fluvial sands interfingering with sabka deposits. Theplanning follows reservoir appraisal by a few wells drilled.

The primary objectives of the survey were:

Identification of nitrogen filled reservoir bodiesDelineation of the extension of the innate nitrogen fillingCharacterization of the reservoir qualityDepth mapping of reservoir horizons and identification of other structural elements

The data quality was excellent. Post-stack stratigraphic inversion calibrated to the (sparse) well dataallowed successful identification and characterization of various reservoir bodies and the location ofGWC.

The extent of the gas zone was further verified by AVO analysis.

It was possible to map the porosity from the inversion data.

Also the continuity and the extent of the caprock was verified.

The interpretation confirmed the suitability for natural gas storage of the potential storage zones and itserves as the basis for planning of prolific well locations for the storage development.

B) Repeatability of 4D Seismic by /+w/ Fink (OMV)

It is still an uncommon situation that, through the production cycle, a field is covered with identicallyacquired repetitive 3D surveys. More often an initial “exploration” 3D seismic is available and onlyyears later, while both acquisition and processing technology have significantly advanced, another 3Dsurvey is made available to be compared with the initial baseline survey. It is therefore quite evidentthat processing techniques are faced with the major challenge of how to harmonize two drasticallydifferent seismic surveys. But, as will be mentioned below, even in an ideal world, i.e. the identicaluse of acquisition geometty, equipment and processing algorithms, environmental changes over timestill represent a significant difficulty to overcome, before one can even think of relating differencesbetween surveys to changes in the reservoir related to production history. Challenges to overcome in4D operations are :

Acquisition parameter related :Changes in fold, azimuth distribution, offset ranges and bin spacing can all lead to significantamplitude and time differences not related to production.In general, the problem is more significant for land surveys because of a generally lower S/N ratioand less sampling, allowing for less flexibility in the subsequent processing and differencingoperations. Both in marine (with restricted azimuthal distribution of marine surveys and streamer tailbuoy mispositioning) and land operations, positioning can be a significant source of errors. It istherefore essential that the processing software provides enough capability to quickly detect andhandle positioning errors.

Environment related :In any different operations, noise is a major issue. With changing weather conditions, ambient noisecan change accordingly within several orders of magnitude and even become stronger than anyseismic signal, including first breaks.Rainy or very windy conditions are an issue for land operations; bad conditions (wave motions) andinterference from other vessels are an issue for marine operations (cable noise).

An ever-present problem are changes in near surface velocities which can introduce severalmilliseconds of timing error. In land operations the main cause is a seasonally changing water table.In marine operations, changing temperature and salinity can change traveltimes up to 10 ms.For 4D-interpretation, it is therefore extremely useful to generate 3D-maps of ambient noise levels

present during acquisition.

, ,

Recording equipment related :Using different recording equipments over time causes the most obvious unwanted change.Mathematically this comes equal to a change in impulse (and phase) response of the recordingequipment. The same is true for source related parameters like time variant coupling conditions, airgun signatures, gun/explosive depths.

Although a lot of mathematical correlation techniques exist to handle equipment related changes, thebest solution is probably to zero phase the data and match it to a reliable well log.

Processing related :Despite the fact some seismic data processors still follow the strategy to “force” a match betweendifferent surveys, a careful and consistent selection of processing parameters seems to yield the bestresults for the time being. After all, the 4D processing philosophy is significantly different toconventional 3D processing, as one is caused by production changes, and not after optimizing thegeological subsurface image.A good example is multiples suppression. Multides are reaarded as noise in 3D, but not in 4D. And itis this differencing process where the main attractions of 4D seismic are to be found.Firstly, the vertical resolution of 4D with regard to changes is, as a rule of thumb, at least 5 timeshigher than that of conventional seismic. Secondly, the corresponding percentage change inreflectivity is usually much higher than the underlying change in acoustic impedance, caused i.e. byproduction or gas injection/depletion operations.Therefore 4D is extremely sensitive to changes in the reservoir which cause acoustic impedancechanges.

The major challenge to overcome is to make sure, that reflectivity changes are not related toprocessing artefacts. At the beginning of the 4D processing sequence, a common time-zero has to beestablished for the two surveys. Modeling studies indicate that a time shift of 4 ms can cause rrns-amplitude changes as high as 70’Yo.

Other processing steps within the pre-processing sequence, which require special attention, are themute function (in the reservoir zone) and the pre-stack-deconvolution (PSD) operator. Again,modeling indicates, that changes as small as 4 ms in the operator gap, can create significant artefactsnot related to real changes in the reservoir.

C) 4D UGS Case History by Fredetic F/uguef (GDF)

. The clas storaae of Cere-La-Ronde (Gaz de France~

The Cere-La-Ronde gas storage reservoir in the Loire valley is used as a test site to study andimprove reservoir monitoring by Gaz de France (GDF). The Cere-La-Ronde gas storage is a water-beanng sandstone reservoir in a faulted anticline structure located at a depth of 900 meters. Itconsists of sandstone channels of an excellent reservoir quality with an average thickness of 20meters. A R2 reservoir exists below RI, and is made up of complex communicating channels. Twotops were identified and wells CEI 2 and CEI 12 were drilled for the first gas injection. The sitepresents favorable characteristics. It is relatively shallow and gas injection results in major variationsin impedance and time shift. But the data quality is poor. Modeling this reservoir resulted in a 1.5 mstime shift variation following gas injection.

● Eaui~ment and woaram

Two methods of monitoring were developed in order to control the moves of the gas bubble aroundthe wells during the development of the new Cer6-La-Ronde gas storage.

The time-lapse seismic :The 4D technique means the fourth dimension which is time-lapse (repeated seismic survey) for any

kinds of seismic experiment (VSP, Walk-away, 3D VSP, 2D seismic, 3D seismic). The time-lapseseismic consists of installing permanent sensors between the tubing and the casing and of scoringwaves from a seismic source to the surface. After a specific seismic treatment in order to correct thedifferences due to the pulse and crossover of the weathered area by the pulse, the result is theseismic image. On it, the amplitude difference between the hvo geophysical surveys and the measureof the delay of the reflections, which are located under the reservoir, enable to monitor the evolutionof the gas bubble between the two periods. The four surface seismic lines were chosen to give goodcoverage on the central top, and to check communication with the eastern top, and also the spill pointarea further west.

Six 4D seismic surveys were carried out in collaboration with IFP around the well CEI 2. The firstmeasurement survey has occurred before the first gas injected as unbiased reference and fiverepeated seismic surveys were acquired from February 1994 to April 1997 during storagedevelopment. The last 4D seismic survey carried out in April 1997 is made up for the drawdownsurvey. All together, the seismic lines cross ten wells. A second well CE1 12, just 10 m from CEI 2,contains 15 permanent seismic sensors which allow repeat VSP’S to be obtained. The VSP toolsdeveloped by Gaz de France and IFP have been used for expanding investigation distances aroundwells. The interpretation of these measurements demonstrated an increase of the anomaly in theamplitude difference and in the time shift compared with December 1995. Also, during 1997, adecrease of anomalies was observed after the drawdown to 200 meters of the well along the twocrossed profiles.

The major difficulty of time-lapse seismic in this area is related to a strong shallow heterogeneity,responsible for refraction resonance, high energy surface waves and fairly unpredictable static’s. Alarge part of this heterogeneity is a result of a moisture content which changes with weatherconditions, giving variations in seismic response and making repeatability a real challenge. Theseuncertainties lead to increase the noise amplitude of the seismic signal.

The surface 2D seismic :The repeated surface 2D seismic enables to observe the limits of the gas bubble and particularly thecritical structural saddles. For the aim, GDF installed in 1997 a permanent device constituted ofseismic traces buried to control the structural saddle of the Cere-La-Ronde storage. In order tovalidate the technique of monitoring by the 2D seismic, GDF in collaboration with CGG scored threeseismic profiles with a long extension which was shoot conventional (the positioning of the seismicsources and traces for each seismic survey) in June 1993, December 1994 and December 95.

. Results

The results, after a seismic specific treatment in order to compensate the differences due to theacquisition respect to the seismic base survey, are the measurements of the amplitude differencesand the time differences, which affect the reflection under the reservoir. The extension evolution ofthe anomalies in time and in amplitude along the profiles correspond to the gas bubble developmentalong the same profiles. The limit of the sensing corresponding to the middle level of the noise isappreciated approximately to the gas thickness of 2 m.

This experiment was successful in providing an accurate mapping tool for the gas bubble extensionafter initial injection. But it nevertheless failed in its ambition to precisely monitor the saturationvariations associated with alternate extraction-injection cycles. The unwanted differences are duefirstly to the inaccuracy positioning and to the very rapid weather zone variations, and secondly to thesurface variations. An effort has been made to harmonize the transmitted wavelet at eachtransmitting point as well as between surveys. Therefore, an attractive solution was to burygeophones on a permanent basis. Receiver positioning repeatability is optimized. Source positioningcan be enhanced by using permanent markers at short intervals. So that, coupling conditions ofreceivers are stabilized.

. Economics

Thanks to seismic monitoring, we are able to explain and solve production problems in a secondarytop. The benefit expected in gas recovery is estimated over ten times the cost of the project.

D) 4D for Gas Field by Paul Fink (OM~ and Maciej Gorski (Geofizyca - Torun)

The series of papers (six-part), by the Lament 4D technologies Group, described the comingrevolution in production management : 4D Reservoir Monitoring. The main benefits for manager’sperspectives are: verification of past drainage, prediction of location of bypassed pay to be drained inthe future so that revenues can be maximized and risk minimized over the entire life of the field.

Succeeding papers described:How to conduct a 4D project (3 phases : feasibility study, pilot study and field application).How 4D software processes multiple 3D seismic survey to normalize power spectra, 4D work-flow(differencing techniques) 4D inversion work-flow, how provide reliable information about fluiddistribution in the reservoir, integration the results with reservoir simulation, 4D reservoir simulationand management decisions.

● Future directions

Technology development : technology can reduce drastically the cycle’s time.Data acquisition : greater use of 3 components geophones (3C) recording, development of very-deepwater ocean-bottom cables.Data processing : integration of data to conform to a single earth model.Data analysis: gradual move from qualitative to quantitative analysis, integratedtechnologies/softwares.

E) 3D for Exploration and Field Development by Taska Vasi~evic (NIS Naftagas, OD “istrazivanjei technolog~a’~

The rising cost and demand for seismic vessels and drilling rigs has created an urgent need tocompress cycle times at every stage of exploration and exploitation. Inspiring new play concepts inarea of great geological complexity demands that geoscientists employ the most advancedgeophysical technology.

Geoscientists are often faced with the formidable task of balancing the accuracy of the seismic imageand resulting earth model with the actual time and cost required creating them. 3D seismic hasundoubtedly revolutionised geophysics and greatly reduced the incidence of dry hole drilling over thelast few years. The significance of the 3D method is best illustrated by the rapid expansion of itsapplication. The power of this method to generate high-resolution images, unparalleled by 2Dseismic, is an invaluable asset. 3D seismic are used not only in the early exploration, but also in thesubsequent reservoir extension, development and production stages of the overall petroleumoperations.

. Acquisition

A fundamental factor in the cost of seismic acquisition is the efficiency of source and receiver effort.The increase in seismic channel count resulted in increased spatial sampling and more efficient 3Doperation.

Ray-tracing techniques and modeling :Seismic feasibility study (existing seismic data analysis, ray-tracing and wave propagationmethodology) and processing tests provide the acquisition and processing parameters, and the wholeoperative flow optimization, particularly Quality/(Time+Cost) ratio.

4C-acquisition technology:Recently there has been considerable focus on conducting 3D with multicomponents seismicacquisition (Gulf of Mexico, North Sea) named 4C-acquisition technology. Deepwater complexacquisition design has purpose to make detailed seismic data analysis possible by acquiring not onlyconventional compressional-wave (P) seismic data, but shear-wave (S) seismic data as well. By thesimultaneous acquisition, combined P- and S-wave, survey becomes an important tool in delineatingstratigraphic traps, even when they are not visible on conventional P-wave data.

● Processing

The prime goal of seismic interpretation is to extract more information out of processed data and useit for interpreting subsurface structural, stratigraphic and Iithological features.One way to obtain such information is through generation of seismic attributes in term of geometry,kinematics, dynamics and statistical features of seismic data.But, all these applications are possible only when seismic attributes are extracted from the seismicdata without distortion of their characteristics while processing.The reliability of the results of all kind of amplitude analysis depends on seismic processing. Suchprocessing is often known as amplitude preserving processing, relative amplitude, true amplitude orcontrolled amplitude processing.

● Intemretation svstem

The tasks before petroleum explorations are growingly demanding geophysical methods, and they areexpected to hit the mark by supplying highly accurate geological models. The focus of geophysicalsurveys moves from conventional structural features to unconventional sub TLE traps. The highestimportance for efficient petroleum explorations can be attached to 3D seismic and the interactiveinterpretation system, which begin with survey design and processing scheme.

Seismic stratigraphy is an attractive framework for interpretation of 3D. Automatic detection oftermination’s can be run on entire seismic volume or to a limited time interval by horizoninterpretation. The density of termination’s is attribute for 3D-volume or surface attribute – input to 3Dfacies analysis.

Reservoir classification with seismic attributes :Mapping seismically sensitive reservoir characteristics derived from horizons picked on 3D seismicdata. It is preferable to select those seismic attributes, which are sensitive to desired reservoirproperties, and then analyse the data in multi-dimensional attribute space forming clusters.(ex: horizon time, reflection strength, instantaneous frequency and peak-to-trough time - a mapwhich matches the net porosity-feet map ; gas-water contact – time and amplitude from horizon in itsproximity ; Amplitude and dominant frequency - a replacement well to drain probable bypassedhydrocarbons).

The modeling of geodynamic fields based on seismic 3D volume, the deformation of compressionmodules and the shear modulus from relations between Vp, Vs, Ro and drilling data, allows :

- To evaluate the stress-deformations’ field of the geomedium- To arrange the efficient system of geophysical reservoir monitoring- To select the optimal system of production well arrangement- To modify development plans

. Exrdoration ~hase

Analysis of seismic amplitude and phase are the most reliable and cost effective method. Processedand modelled, the data could be analysed to predict Iithology that will be drilled at the target area, torecommend the drill site. There are today different Iithology prediction methods : seismic stratiura~hvt

reaional amditude analvses, faults analvses, velocitv analvses, AVO etc.With geostatistics, variogram analysis of reservoir parameters and seismic attributes try to establishspatial relationships.

● Develo~ment ~hase

Seismic data have a key role to play in the process of reservoir characterization. They reduceexploration risk, and optimize the reservoir management.Integrated 3D-reservoir modeling, allows to fully exploit 3D seismic data and rapidly assessmultiple geological scenarios in a pre-development stage.Determination of reservoir pore fill from 3D (combination of supervised classification and principalcomponent transformation) allows to know the production behavior and reduce development risk.Reservoir Rock Type (sub-seismic scale changes of rock properties observed in the wells) allows toshow which geological scenario is the most likely to compare with the real data. The best fit 3Dproperty models can then be upscaled and used for full field simulation to produce forecast.

● Production ~hase

Stratigraphic inversion of 3D seismic (interpolation of reservoir properties on a layer-by-layerbasis), to improve the stratigraphic description of the target layers.Model-based 3D inversion. It emphasizes the strong improvement of vertical resolution and tracksaccurately the correct position of top and base of the resetvoir.Post-stack amplitude inversion of 3D data. It demonstrates how the reservoir heterogeneity andfluid distribution between MO wells can be detected.New seismic technology can help to mitigate risks in mature fields.

F) Microseismic Techniques by fran~ois Verdier (GDF) and Jean-Pierre Deflandre (Instifutfran~ais du Petrole)

In oil production, as in the area of hydrocarbon storage or for any other activity based on fluidinjection in a geological rock formation (or even in a cavity), the producer change, on a more or lesslarge scale, the constraint scope, due to different fluid pressures in the rock. The same phenomenonappears in case of a significative change of temperatures in the rocks. These constraint changes canlead to a environment damage (compaction, fissuring, breaking ...) which will modify thepetrophysical properties of rock formations and, consequently, drills’ productivity. Also this damagecan be the origin of the activation of a sliding on a fault plane. These phenomena very often go with amicro-seismic activity : the apparition of micro-seisms, in relation with the pressure evolution and/orthe temperature evolution in the ground.

The monitoring of these phenomena allows, through a seismic-sensors networking set nearby theinterest’s areas, on one hand, to locate the mechanically active areas, and on the other hand, tocharacterize the area mechanical response, subjected to a certain rate of exploitation (fixed depletionrate, production or injection flows, pressure threshold reached in injection ...). According to the case,the target will be :

I . To map, in hydraulic fracturation, the source area, so that to know the fault directionI

● To know the maximal acceptable injection pressure in the rock formation in case of fluidstorage (for example, a drilling mud) in a specified geological layer

I ● To determine an optimal production flow allowing a long-term high productivity rate

● To assess results of an injection locating the emissive areas (drained areas in case of amassive cold water injection through a warm rock massif)

With the development of permanent downhole geophones, it is now possible to detect micro-seismicevents with a low magnitude on a sufficient number of ways. This technological progress, surveys onimplicated processes and poromechanical phenomena taken into account, and brought about realconstraints evolution, will ensure a better characterization of the geomechanical behavior of an oilstructure and, to full term, an optimization of the exploitation conditions for the site.

For about twenty years, the application of these methods are more and more usual. The first listedsurveys match to the mapping of hydraulic faults both to the deep geothermal power and to oil wellstimulation. Mostly, those surveys were initiated by researches’ organisms and otlen subsidized byorganisms like Gas Research Institute in the USA. In case of geothermal power, surveys on theFenton Hill (New Mexico, USA) and Soultz-sous-For~t (Alsace, France) are the most famous. In thearea of the hydraulic fracturation, results were get in different types of rocks formations from thepetrophysical and mechanical’s point of view. On a reservoir scale, measurements were carried outon more or less important terms (a couple of weeks to more than 10 years). GasIi’s field (Uzbekistan),Lacq’s field (France), Groninguen and Edberg’s fields (the Netherlands) are or were under monitoring.On Lacq’s field, an intense activity was recorded and ermitted to better understand the structure’s

Pbehavior in response to a depletion of about 35 10 Pa in the reservoir. In the Netherlands, inresponse to a strong seismic activity near Groninguen’s field, a surface watch was created on the newfields such Edberg’s fields for the purpose to foresee these phenomena. In the Ekofisk’s chalkreservoir in the North Sea, where problems of compaction are very important, same measurementswere carried out, in shorter times, putting sensors in a borehole.

G) 3D UGS Case History by Lothar Gese//e (RUHRGAS) and Maciej Gorsky (GEOFIZYCA -TORUN)

A 3D seismic survey has been recorded to investigate potential traps both aquifers and depletedfields which are considered to be used as a gas storage. The accurate high quality acquisition andstate-of-the-art processing are indispensable to successful interpretation. Precise wavelet control,applied zero phase processing and 3D migration considerably improved the data resolution (bothvertical and horizontal). The preservation of true amplitudes is expected. Interpretation should betaken over structural and Iithofacial description. All sophisticated techniques as an amplitude anomalydetection, a 3D inversion, an AVO etc. should be used. All studies demonstrate the impact of 3Dseismic data for the construction and development of UGS. The information originated from 3Dseismic is used for enhanced simulation purposes, for the estimation of storage capacity and foroptimal drilling paths with respect to horizontal wells.

The 3D seismic land survey has been performed over a depleted gas field to determine as detailed aspossible the structure, fault pattern, and GWC of the UGS to further optimize the reservoir modelingand storage operation. In order to achieve the greatest possible precision and security ofinterpretation, state-of-the-art acquisition, processing and interpretation technology are indispensable.

Advanced interpretation techniques in addition to the structural interpretation will be carried out todescribe and quantify the internal structure geometry as well as the lateral and vertical distribution ofpetrophysical parameters and reservoir characteristics such as : sand/shale sequence architecture,porosity, permeability, pore filling, saturation/GWC and fault characteristics.

Maximum exploitation of seismic data and attributes, in combination with well log and core data, plussedimentological expertise is essential to successfully perform processes as: seismic modeling,AVO, 3D seismic coherency analysis, Al-inversion, trend analysis, geostatistical processing and faultsanalysis.

REFERENCES

A) 3D Seismic

1 - Alam, A., Caragounis, P., Matsumoto, S. and Hurst, C. (1995). Reservoir classification withseismic attributes. 5?h Conference, European Association of Geoscientists & Engineers, ExtendedAbstracts, paper A037.2 - Kemp, A.C. and Gallagher, J.W. (1995). Studying Iithology and fluid ;ndicators on a seismicworkstation. 5?h Conference, European Association of Geoscientists & Engineers, ExtendedAbstracts, paper P045.3- Kummer, 1., Takacs, E. and Papa, A. (1995). Seismic characterization of gas-saturated layers forvarious depths in the Pannonian Basin. 5~h Conference, European Association of Geoscientists &Engineers, Extended Abstracts, paper PI 60.4- Grotsch, J., Mercadier, C., Diebold, P., Van Der Berg, A., Lak, K. and Glass, F. (1996). 3Dreservoir modeling based on 3D seismic, Malampaya/Camago field, Philippines. 5#h Conference,European Association of Geoscientists& Engineers, Extended Abstracts, paper L054.5- Barkved, O. (1996). Successful use of seismic attribute maps and poststack inversion in horizontalwell planing. 5#h Conference, European Association of Geoscientists & Engineers, ExtendedAbstracts, paper A047.6- Birdus, S.A. (1996). The application of 3D seismic models for studies of geodynamic effects inoil fields. 5#h Conference, European Association of Geoscientists & Engineers, Extended Abstracts,paper P501.7- Trappe, H. and Hellmich, C. (1996). Seismic characterization of Rotliegend reservoirs-from brightspots to stochastic simulation. 5~h Conference, European Association of Geoscientists & Engineers,Extended Abstracts, paper A055.8- Stephens, A. R., Monson, G.D. and Reilly, J.M. (1996). The relevance of seismic amplitudes inexploring the Niger Delta. Oflshore, October 1996.9- Jensen, L., Ackers, M., Viegue, V. and Walker, L. (1997). Clustering analysis of seismic qualityattributes – a marine acquisition case study. 59fh Conference, European Association of Geoscientists& Engineers, Extended Abstracts, paper B023.10- Nerby, A.M. and Gueri, G. (1997). The role of amplitude coherency processing for 3D imaging ofa salt diapir in the Central Graben. 5~h Conference, European Association of Geoscientists &Engineers, Extended Abstracfs, paper B028.11 - Veire, H. H., S@nneland, L., Signer, C. and Reymond, B. (1997). Fluid distribution in earlyexploration phase using a stratigraphical inversion procedure. 5#h Conference, European Associationof Geoscientists & Engineers, Extended Absfracts, paper BO14.12 - Hacker, C. and Dailey, D. (1997). Structural vector field a volume attribute for seismicinterpretation. 59th Conference, European Association of Geoscientists & Engineers, ExtendedAbstracfs, paper B025.13- Pharez, S.J. and Schellinger, D. (1997). Visualization of layered acoustic impedance and itsimpact for horizontal drilling-a Nigeria case study. 5~h Conference, European Association ofGeoscientists & Engineers, Extended Abstracts, paper BOI 6.14- Guderian, K., Lahmeyer, B. and Weber, R. (1997). ,Determination of reservoir porefill from 3Dseismic data in the oilfield Ruehme. 5~h Conference, European Association of Geoscientists &Engineers, Extended Absfracts, paper BOI 3.15 - George, D. (1997). Multicomponent seismic acquisition complements conventional towedstreamer, Offshore, October 1997.16- Beasley, G.J. and Chambers, R.E. (1998). A new look at simultaneous sources. 6~h Conference,European Association of Geoscienfisfs & Engineers, Extended Abstracts, paper 2-38.17- Firmano, A., Baldino, F., De tomasi, V. and Lionger, E. (1998). Seismic feasibility study-thecomprehensive approach. 6~h Conference, European Association of Geoscientists & Engineers,Extended Abstracfs, paper 2-33.18- Kerr, J.D. and Abatzis, 1. (1998). Seismic trace shape classification-takes the magic out ofstratigraphic interpretation?. 6~h Conference, European Association of Geoscienfisfs & Engineers,Extended Abstracfs, paper 2-41.19 - Randen, T., Reymond, B. and Szmnenland, L. (1998). Attributes for automated seismicstratigraphic analysis. 6~h Conference, European Association of Geoscienfisfs & Engineers, ExtendedAbstracfs, paper 2-42.

20- Rebeck, T., Maver, K. and Clark, N. (1998). The integration of seismic inversion and coherencecube imaging-a new seismic tool. 6~h Conference, European Association of Geoscientists &Engineers, Extended Abstracts, paper 2-44.21 - Musser, J.A. and Kappius, R. (1998). Advances in 3D seismic survey design. 6~h Conference,European Association of Geoscientists& Engineers, Extended Abstracts, paper 2-49.22- De Groot, P., Krajevski, P. and Bischoff, R. (1998). Evaluation of oil potential with 3D seismicusing neural networks. 6~h Conference, European Association of Geoscientists & Engineers,Extended Abstracts, paper 2-47.23 - Prakash, A., Singh, V., Saxena U.C. and Sen, G. (1998). Impact of surface consistentdeconvolution on wavelet stability and seismic attributes-a case study; 6@h Conference, EuropeanAssociation of Geoscientists& Engineers, Extended Abstracts, paper PO02.24 - Buland, A. (1998). Relative amplitude processing-a contractor evaluation, 6~h Conference,European Association of Geoscientists& Engineers, Extended Abstracts, paper PO04.25- Leger, M. and Grizon, L. (1998). Full 3D stratigraphic inversion-toward an accurate reservoirimaging. 6~h Conference, European Association of Geoscientists & Engineers, Extended Abstracts,paper P066.26- Duboz, P., Lafet, Y. and Mougenot, D. (1998). Case study. Firsf Break, Septernbefi (p.1 2-15).27- Dunbar, T. and Lance, M. (1998). Major improvement in 3D seismic imaging expanding NorthSea opportunities. Offshore, March 1998.28- Foster, M. and Dennis, J. (1998). Seismic attribute characterization of mature fields. Conferenceon advances in seismic technology, January 1998, Stavangec Norway.29- Itko, V. (1994). The present state in the seismic exploration of oil and gas. Association ofEngineers and Technicians, December 1994, Novi Sad, Yugoslavia.30- Vuei=, S., Ivadinovi=, M., Mavrensky, B. and Parezanovi=, M. (1994). Application of 3Dseismic exploration in the field Turija-sever. Association of Engineers and Technicians, December1994, Novi Sad, Yugoslavia.31- Vueia?, S. (1996). First 3D seismic application in the region Vojvodina. 50 Years of geophysics atthe Belgrade University, Facu/ty of mining and geology, October 1996, Belgrade, Yugoslavia.32- Brown, A.R. (1998). Picking philosophy for 3-D strati-graphic interpretation. The Leading Edge9/98.33- Radovich, B.J. and Burnet, R. (1998). 3-D sequence interpretation of seismic instantaneousattributes from the Gorgon Field. The Leading Edge 9/98.34- Peyton, L., Botjer, R. and Partyka, G. (1998). Interpretation of incised valleys using new 3-Dseismic techniques: A case history using spectral decomposition and coherency. The Leading Edge9/98.35- Tonn, R. (1998). Seismic reservoir characterization of Montney Sand in the Peace River Archarea, Canada. The Leading Edge 5/98.36- Sheng-Shyong, L., Shui Chih Wu, S., Ching-Hsiang, H., Jen-Yang, L., Yu-Liang, Y., Chang-Sheng, H. and Lina-De, J. (1998). The Leading Edge 5/98.37- Burge, D.W. and Neff, D.B. (1998). Well-based seismic Iithology inversion for porosity and pay-thickness mapping. The Leading Edge 2198.38- Ross, J.M. (1997). Integrated petrophysical-geostatis-cal characterization compared to Iayercakeflow simulaiton. The Leading Edge 9/97.39- Lazaratos, S.K. and Marion, B.P. (1997). Crosswell seismic imaging of reservoir changes causedby C02. The Leading Edge 9/97.40 - Gluck, S., Juve, Y. and Lafet, Y. (1997). High-resolution impedance layering through 3-Dstratigraphic inversion of poststack seismic data. The Leading Edge 9/97.41- Raeuchie, S.K., Ambrose, W.A., Akhter, M.S. and Casas, J. (1997). Integrated reservoir study,Lower Eocene Misoa reservoirs, Lagunillas Field, Lake Maracaibo, Venezuela. The Leading Edge9/97.42- Raeuchle, S.K., Hamilton, D.S. and Uzcategui, M. (1997). Integrating 3-D seismic imaging andattribute analysis with genetic stratigraphy: Implications for infield reserve growth and field extension,Budare Field, Venezuela. The Leading Edge 9/97.43 - Chen, Q. and Sidney, S. (1997). Seismic attribute technology for reservoir forecasting andmonitoring. The Leading Edge 5197.44- Lewis, C. (1997). Seismic attributes for reservoir monitoring : A feasibility study using forwardmodeling. The Leading Edge 5197.45- Castagna, J.P. and Swan, H.W. (1997). Principles of AVO crossplotting. The Leading Edge 4/97.

46- Hesthammer, J, (1998). Evaluation of the timedip, correlation and coherence maps for structuralinterpretation of seismic data. First Break 5/98.47- Trappe, H., Hellmich, C., Jones, G. and Knipe, R.J. (1996). Seismic attribute maps ; applicationto structural interpretation and fault seal analysis in the North Sea Basin Veroffentlichungen u.Vortrage. First Break 12196.48 -. Trappe, H. and Hellmich, C. (1998). Seismic characterization of Rotliegend reservoirs: Frombright spots to stochastic simulation. First Break 3/98.49 - Trappe, H., Krajewski, P. and Aust, S. (1996). Seismische Reservoircharkterisierung desStraBfurtkarbonats im Westernsland. ERDOL ERDGAS KOHLE 3/96.50 - Trappe, H., Kraft, T. and Schweitzer, C. (1995). Neuronale Netzwerke zurPerrneabilitatsbestimmung in Rotliegendsandsteinen. ERDOL ERDGAS KOHLE 4/95.51 - Trappe, H. and Hellmich, C. (1997). Image processing and geostatistics - upper carboniferousreservoir description. CO04 EAGE EXTENDED ABSTRACTS 5197.52- Trappe, H. and Hellmich, C. (1997). A real prediction of porosity thickness from 3D seismic databy means of neural networks. CO07 EAGE EXTENDED ABSTRACTS 5/97.53 - Trappe, H. (1997). 3D seismic for gas storage investigation. C039 EAGE EXTENDEDABSTRACTS 5197.54- Trappe, H. and Hellmich, C. (1996). Seismic characterization of Rotliegend reservoirs - from brightspots to stochastic simulation. A055 EAGE EXTENDED ABSTRACTS 6196.55 - Trappe, H. Einsatz der 3D Seismik in der Planung von Gasspeichern. DGMKTAGUNGSBERICHT 9701.56 - Trappe, H., Kraft, T. and Schweitzer, C. Anwendung neuronaler Netzwerke zurPerrneabilitatsbestimmung in Rotliegendsandsteinen. DGMK TAGUNGSBERICHT 9402,57- Trappe, H. and Hellmich, C. (1996). Seismische Reservoir Charakterisierung : Vom Bright Spotzurstochstischen Simulation. MINSTROP-SEMINAR 5/96.58 - Trappe, H. (1994). Potential neuronaler Netze in der Kohelenwasserstoffexploration undproduction. MINSTROP-SEMINAR 5194.59- Egden, J. and Matthews, K. (1990). A 3D Seismic Case Study of the Dawn 167 Storage Pool.29th Ann. Ontario Pefr. Inst. Conf. (London, Ontario 14-16. Nov. 1990). Proc. Paper no. 1. (English)60 - Gluck, S., Juve, E. and Lafet, Y. (1997). High-Resolution impedance layering through 3D-stratigraphic inversion of poststack seismic data. The Leading Edge, Vo/. 76,9 (Sept. 1997,), p. 1309-1315. (English)61- Gorski, M., Kunicka-Gorska, W. and Trela, M. (1998). Wienchowice - najwiekszy podziemnymagazyn gazu (PMG) w PoLsce. Cz. /. Studium geometrii i wlasciwosci serii zbiornikowej oraz budowynakladu na podstawie sejsmiki 3D. (Wietzechowice - the greatest underground gas storage (UGS) inPo/and. Part f: Geometry and reservoir description based on 3D seismic survey).Ptzeglad Geologiczny, Vol. 46,3 (1998), p. 255-263. (Polish, w. English Summary)62- Huguet, F., Papouin, M. and Verdier, F. (1997). La sismique en 3D et Ies stockages souterrainsde gaz en nappes aquiferes (3D Seismic Surveys for Underground Gas Storage in Aquifers). Gazd’Aujourd’hui (Paris), 1997, no. 3, p. 121-126. (French w. English summary)63- Lentz, J.E. and Anstey, N.A. (1998). ANR Storage Company Experience with and Utilization ofSwath and 3D Seismic in the Development of Devonian Reef Gas Storage, New York State. AAPGEastern Section meeting 7-10 Ocf 1998, Abstract: AAPG Bull. Vol. 82 no. 9, p. 1770. (English)64- Morris, J.R. (1997). Cyclone Reef 3D Study. AAPG Eastern Section, Lexington KY., USA, 27-30Sept. 1997. Abstract: AAPG Bull. Vol. 81 no. 9, p. 1559. (English)65- Pinson, C. and Huguet, F. (1994). Apport de la sismique 3D pour la caracterisation d’un reseaude stockage de gaz (The Contribution of 3D Seismography to the Characterization of an UndergroundGas Storage). Petrole et Technique Vol. 391 (Nov. 1994), p. 6-11. (French)66- Schaefer, S.F. and Dixon, R.A. (1994). A 3D Seismic Investigation of the Ray Gas Storage Reefin Macomb County, Michigan. AGA Operating Section Operations Conference (San Francisco 5/8-11194) Proc. p. 762-773 (1994).Abstract: AAPG Bull. Vol. 79 (9), Sept. 1995, p. 1479. (English)67- Trappe, H. (1998). Einsatz der 3D-Seismik in der Planung von Gasspeichern : Aufsuchung undGewinnung. (3D Seismic for Gas Storage Investigations). Erdo/, Erdgas und Koh/e, Vo/. lf4.6 (Juni1998), p. 317-319. (German w. English summaty)68- Trappe, H. (1997). 3D-Seismic for Gas Storage Investigations. EAGE 5@h Conf And Tech.Exhibition, Geneva, 26-30 May 1997. (English)69- Walsh, P.R. (1997). The Use of 3D Seusmic to Enhance Deliverability in Underground Storage

Reefs. SPE Eastern Regional Meeting in Lexington KY., USA, 22-24 Oct. 1997, Proc. p. 99-11.SPEK39224. (English)

B) 4D Seismic

70 - Anderson, R. N., Boulanger, A., He, W., Sun, Y. F., Xu, L. and Hart, B. (1996). 4D seismicmonitoring of reservoir production in the Eugene Island 330 Field, Gulf of Mexico. AAPG Studies inGeology, No. 42& SEG Developments Series, No. 5, p. 9-19.71 - Anderson, R.N., Boulanger, A., He, W., Sun, Y.F. and Xu, L. (1995). 4D seismic imaging ofdrainage and pressure compartmentalization in the Gulf of Mexico. Od and Gas Journa/, Part 1: vol.216, No. 3; Part 2: vol. 216, No. 4.72- Anderson, R.N. and He, W. (1995). 4D seismic interpretation techniques for the detection ofdrainage and migration in oil and gas reservoirs. U.S. Letters patent app/ied for.73- Anderson, R. N., Boulanger, A., He, W., Teng, Y-C., Xu, L., Meadow, B. and Neal, R. (1997). Thefourth dimension in reservoir management, Part 1: What is 4D and how does it improve recoveryefficiency. Oi/ and Gas Journa/, vol. 218, No. 3, p. 43-48.74- Boulanger, A., Anderson, R. N., He, W., Teng, Y-C., Xu, L., Meadow, B. and Neal, R. (1997). Thefourth dimension in reservoir management, Part 2: Why 4D ? The Information Technology revolutionhas changed the way we do business. Oi/ and Gas Jouma/, vol. 218, No. 4.75- Sparkman, G., Xu, L., Anderson, R.N., Boulanger, A. and Teng, Y-C. (1998). Time lapse or timeelapse. The Leading Edge 10198.76 - Meunier, J. and Huguet, F. (1998). Cere-la-Ronde : A laboratory for time lapse seismicmonitoring in the Paris Basin. The Leading Edge fO/98.77- Roche, S.L., Talley, D.J. and Davis, T.L. (1998). Dynamic reservoir characterization of VacuumField. The Leading Edge 10/98.78- Deutsch, C., Gundes@, R., Thiele, M., Biondi, B. And Lumley, D. (1998). Reservoir monitoring : Amultidisciplinary feasibility study. The Leading Edge 10/98.79- Boulanger, R., Mello, U., Anderson, R.N. and Guerin, G. (1998). 4-D seismic reservoir simulationin a South Timbalier 295 turbidite reservoir Wie He. The Leading Edge 70/98.80- Almond, J., Johnston, D.H. and McKenny, R.S. (1998). Time-lapse seismic analysis of FulmarField J. VerbErdol Erdgas Kohle. The Leading Edge 10/98.81- Guerin, G., Anderson, R.N. and Mello, U. (1998). Time-dependent reservoir characterization of theLF sand in the South Eugene Island 330 Field, Gulfof Mexico Wie He. The Leading Edge 10/98.82 - Houston, L.M. and Kinsland, G.L. (1998). Minimal-effort time-lapse seismic monitoring :Exploiting the relationship between acquisition and imaging tim~lapse data. The Leading Edge 10/98.83- Batzle, M., Christiansen, R. (1998). Reservoir recovery processes and geophysics. The LeadingEdge 10/98.84- Weathers, L.R. and Poggiagliolmi, E. (1998). Wavelet control allows differencing of SD from 2-D. The Leading Edge 10/98.85- Scott, L., Ebrom, D., Krail, D. and Ridyard, D. (1998). 4-C/ 4-D at Teal South. The Leading Edge10/98.86- Reymond, B., Signer C., Pedersen, L., Ryan, S., Saters, C., S@nneland, L. and Hafslund Veire,H. {1997). Seismic reservoir monitoring on Gullfaks. The Leading Edge 9/97.87- Capello de P, M.A., Batzle, M. (1997). Rock physics in seismic monitoring. The Leading Edge9/97.88- Bee, M. F., Jenkins, S.D. and Waite, M.W. (1997). Time-lapse monitoring of the Duri steamflood :A pilot and case study. The Leading Edge 9197.89- Waite, M.W. and Sigit, R. (1997). Seismic monitoring of the Duri steamflood : Application toreservoir management. The Leading Edge 9/97.90- Zhijing, W., Lumley, D.E. and Behrens, R.A. (1997). Assessing the risk of a 4-D seismic project.The Leading Edge 7197.91 - Zhijing, W. (1997). Feasibility of time-lapse seismic reservoir monitoring : The physical basis.The Leading Edge 9/97.92 - Boss, C.P. and Suat, M. (1997). Time-lapse seismic monitoring : Some shortcomings innonuniform processing. A/tan Minstrop-Seminar 6/97.93- Treppe, H. and Schade, J. Reservoir Monitoring von Gasspeichern unter Einsatz seismischerMethoden. DGMK TAGUNGSBERICHT 9501.

C) Microseismic Techniques

94 - Albright, J.N. and Pearson, C.F. (1980). Location of hydraulic fractures using microseismictechniques. Armua/ Fa// Technics/ Conference and Exhibition, Dallas, TX, paper SPE 9509, 55th,September 21-241980.95- Deflandre, J.-P., Laurent, J., Michon, D. and Blondin, E. (1995). Microseismic surveying andrepeated VSP’S for monitoring an underground gas storage reservoir using permanent geophones.First Break, Vol 13, No. 4, April 1995.96- Grasso, J.-R., Fourrnaintraux, D. and Maury, V. (1992). Le rde des fluides clans Ies mecanismesd’instabilites de la croile supeneure : I’exemple des exploitations d’hydrocarbures. Bu//. Sot. Gee/. deFrance, t.163, No.1, p. 27-36.97 - Grasso, J.-R., Volant, P. and Fourmaintraux, D. (1994). Scaling of seismic response tohydrocarbon production : a tool to estimate both seismic hazard and reservoir behaviour over time.Eurock’94, Delft, The Netherlands, paper SPE-ISRM, 29-31 August 1994.98- Jupe, A., Cowles, J. and Jones, R. (1998). Microseismic Monitoring : listen and see the reservoir.World 01, pp.1 71-174, December 1998.99- Grasso, J.-R. and Wlttlinger, G. (1990). Ten years of seismic monitoring over a gas field. f3u//efinof the Seismo/ogica/ Society of America, Vol. 80, No. 2, pp. 450-473, April 1990.