oil and gas search and prospecting pool

PTEG 201

Lecture 4

Teaching PlanPRE-UPSTREAM•Operating Agreements

UPSTREAMExploration and Production

DOWNSTREAM• Refinery Operations and Marketing

• Exploration Operations• Drilling Operations• Completion Operations• Production Operations• Introductory Well Control• Gas Management

MID-STREAM• Transportation from producing field to refinery

•Search and Prospecting Pool: Geological and Geophysical Methods•Drilling and Petroleum: Drilling Rig, Power System, Drilling Fluid and Circulation•System, Bits, Drill Pipe, Directional Drilling•Well Logging, DST•Casing and Cementation•Perforation and Well Activation•Production Tubing and Well Head Assembly•Self Flow and Artificial Methods of Production of Oil/Gas•Separation and Storage•Transportation, Field Processing and Refining•Marketing and Distribution

Course Outline

3 Lectures per week

Prof. Sokari Braide College of Engineering Studies

Search and Prospecting Pool:Geological and Geophysical Methods

………………..continued


• Principles• Acquisition– Land and Marine, 2D – 3D

• Processing– Improve signal to noise ratio, resolution

• Interpretation– e.g. Amplitudes (DHIs), Inversion, 4D– Reservoir model building– Reservoir model maintenance

Seismic Data Processing


Field Data

Signal to noise ratio

Vertical resolution

Imaging

Stacking

Deconvolution

Migration

What to Improve

Processing Technique




Once the data is collected, it must be processed through complex computer programs.

Stacking is a procedure to improve the signal to noise ratio. Several seismic records are “added” together. Noise is considered random (incoherent), and adding the records together will cause the noise to cancel out.

Deconvolution removes some of the “filtering” behavior of the rock layers and the recording equipment (think of ringing of a tower bell)

Migration moves the seismic response to the appropriate position in space rather than just vertically under the geographical midpoint.


DeconvolutionDigital recording of seismic data in the field and computer processing of the seismic data has greatly improved the accuracy and usefulness of seismic exploration.

A correction (statics) is made on the seismic data for elevation changes and the thickness and velocity of the near-surface, loose sediments called the weathering layer or low-velocity zone.

As the seismic energy travel through the subsurface rocks, the relatively sharp impulse of seismic energy tends to become spread out, and some portions of the energy is lost.

Deconvolution is a computer process that compresses and restores the recorded subsurface reflections so that they are similar to the original seismic energy impulse.

This makes the reflections sharper and reduces some of the noise.


Undeconvolved Deconvolved

Vertical Resolution - Deconvolution


Unmigrated seismic events:(a) A bow tie as a result of a deep, steep syncline(b) Crossing events due to a fault.

• A seismic section is accurate only flat, horizontal rock layers.• Dipping rock layers have a different path for the seismic energy from source to detector than horizontal rock layers in the same position.• Because of this, dipping rock layers do not appear on the seismic record in their actual position.• They are shifted to a down dip position and appear flatter than they are. • This effect causes anticlines to look larger , and synclines look smaller than they actually are.• It causes the rock layers in a deep, steeply dipping syncline to cross forming a bow tie.• Rock layers sharply terminated against a fault, appear to cross with rock layers on the other side of the fault.• A computer process called migration moves the dipping rock layers into more accurate position on the seismic record.

Migration


Imaging - Migration

The reflection recorded at the surface from a dipping interface does not originate from a point vertically below the detector, but sidewaysMigration aims to reposition the data to its proper place from where it was reflected.

So the reflection is moved from the point where it was recorded to the point vertically above the reflection point.The recorded time is adjusted to the vertical traveltime.


True shape

Non-migrated time response

Arches or Fishtails need Migration

Arches are the stacked (zero-offset) expression of an anticlineFishtails are the stacked expression of a syncline


Time section Migrated section

Migration


Migration is the procedure applied in reflection seismology for the purpose of shifting the reflections from positions vertically below the midpoint between the shot point and the receiver to the correct spatial positions.

This shifting is necessary if the layers are dipping.

The need for migration is clear in the previous slide. Note the overlapping arching reflections at the bottom of the left panel.

Layers of rock certainly cannot overlap each other.

This reflection pattern is caused by the dipping beds.

Migration


Many basins such as the Gulf of Mexico and the North Sea have extensive salt layers. Passage of seismic energy through the salt blurs the seismic image of any potential petroleum structures below the salt.

A computer processing technique called prestack migration helps in giving a Clearer seismic image of the deeper structures but involves significantly more computer time.

Each seismic line is run to intersect another seismic line (tie in) so that the reflections can be correlated from one record to another.

If the reflections from two intersecting seismic records do not correlate, it is calleda mis-tie.

A typical seismic record shows the structure of the subsurface rocks and identifiessedimentary rocks by their characteristic layering. It does not, however, identify the individual sedimentary rock layers.

Prestack Migration


A seismic record is more valuable when the individual sedimentary rock layers have been identified, and potential reservoir rocks and seals can be traced.

To do this, the seismic line is often run through a well (tied in) that has been already drilled.

The well logs from that well then provide the basis for identifying subsurface rock layers on the seismic record. If no well is available, a stratigraphic test well or strat test is drilled on the seismic line. The primary purpose of the well is to collect subsurface samples and run wireline well logs.

This identifies the ages and composition of reflections on the seismic profile.

Because seismic data is so expensive, and new fields can be discovered by reprocessing old data, the seismic data is kept secret and called proprietary.It can however be traded for data considered to be of similar value.

Well-to-Seismic Tie

Kielwindeweer

OldNew

Annerveen 1

OldNew

Zuidlaarderveen-1

OldNew

Well-1 Well-2 Well-3

Well Correlation

SPREAD 3SPREAD 2

SPREAD 1

1 Fold 2 Fold 1 Fold

Stacking – CMP Spread

During the seismic survey, the survey is repeated with the shot being detonated at different points along the seismic line. Each of these “folds” sees the same points in the reservoir from slightly different angles. When these traces are added together, or “stacked”, the signal to noise ratio improves.

-6.0 -4.8 -3.6 -2.4 -1.2 0.0 1.2 2.4 3.6 4.8 6.0 km

Midpoint OffsetOffset

3

2

1

0

Reflectiontime (s)

CMP – Common Mid Point gather

A quick analysis shows the linear direct wave (orange line), the reflected signals from several layers (the green lines), and the refracted signals (the red lines).

-6.0 -4.8 -3.6 -2.4 -1.2 0.0 1.2 2.4 3.6 4.8 6.0 km

Midpoint OffsetOffset

3

2

1

0

Reflectiontime (s)

CMP with Statics

Topography is superimposed on “horizontal reflector”; results in fuzzy response

1300 1350 1400 1450 15000

1

2

3

SPStack after pre-processing: elevation statics only

Time (s)

Stack – Without Statics

Statics is another term for small scale variations in traveltime.

In this case, the seismic surveys have been corrected for only elevation related variations in traveltime before being summed or stacked.

Variations in traveltime due to high frequent variations in the near surface are NOT incorporated.


1300 1350 1400 1450 15000

1

2

3

SP

Stack after NAM statics

Time (s)

Statics150-150

Stack – Without Statics

On land the small variations in traveltime are not only due to changes in the elevations but, more importantly, also due to variations in the velocity of the layer close to the surface, called the weathered layer.

After proper correction of these variations in traveltime (called aligning) the reflectors are much stronger and the sub-surface structure is much clearer.

Here the right-hand side of the section shows fairly large shifts + / - 150 msProf. Sokari Braide College of Engineering Studies

Time-to-Depth ConversionSeismic data is recorded in seconds (time domain), and a well log is recorded in feet or metres (depth domain).

The vertical scales on both are therefore different and cannot be directly compared.

If the seismic velocity through each rock layer is known, a time-to-depth conversion can be made on the seismic data.

TWO WAYS TO DO THIS

1. Checkshot SurveyA geophone is lowered down the well. The seismic source is then detonated on the surface. The geophone is then raised up the well a distance of 200, 500, or 1,000 ft (161, 152, or 305 m), and another measurement is made. This is repeated until the geophone is at the surface.

2. Vertical Seismic Profiling (VSP)This operation is the same as a checkshot survey except the geophone interval is shorter (59 to 100 ft or 15 to 30 m).


1

.8

1.6

3 5 7 11 13 159

twt(s)

Shot point2 4 6 8 12 14 1610 .0

.4

1.2

Time section

V = 2000 m/s

V = 4500 m/s

Salt

Reservoir

Depth section

Depth(m)

Shot point1 3 5 7 11 13 1592 4 6 8 12 14 1610

1000

0

2000

Time-to-Depth Conversion –”Layer Cake”



Once the data has been processed, the seismic model needs to be converted - time measurement on the y-axis to a depth measurement.

The time (two-way-travel time) is converted to depth by applying the appropriate (or estimated) acoustic velocities of the overlying layers.

This is the solution to an exercise which asks one to find the proper place to drill a well based on the data in the upper panel.

With the depth conversion, the trap in the reservoir becomes clear.


Depth Migration

Time Migration

Time versus Depth migration



Simple approach:

Stack – (Time) migration – Time-to-Depth conversion only works for simple geology with simple travel path of seismic energy

Complex geology results in complex ray paths that requires proper approach which incorporates the true travel path.

The proper approach integrates the migration and depth conversion in one: Depth migration


time

Time Migration


depth

Pre-stack depth migration – Model 2





Seismic Methods


Amplitude versus OffsetAmplitude versus offset (AVO) is an analysis of seismic data to locate gas reservoirs and help identify the composition of the rock layers.

Offset is the distance between the seismic source and the receiver.

The amplitude of a reflection usually decreases with increasing offset distance.

Gas reservoirs and different sedimentary rocks such as sandstones, limestones, and shales have different reflection amplitudes versus offsets.

Some increase, and others decrease with offset.


3-D Seismic ExplorationThe 3-D seismic survey is divided into horizontal squares called bins.

All reflections whose midpoints fall within a particular bin are combined for common-mid-point (CMP) stacking.

The CMP fold is the number of mid-points in each bin.

Bins are commonly 55 by 55 ft., 110 by 110 ft.,20 by 20 m., or 30 by 30 m.

After computer processing, a 3-D view of the subsurface is produced. Rocks layers are migrated more accurately, and details are shown better than on a 2-D seismic image.

A cube display is very common.

Cube display of 3-D seismic data

3D seismic image with a time slice.

Horizontal slice from a 3-D seismic survey showing a subsurface, meandering submarine channel on a submarine fan, offshore Nigeria.

3-D Seismic ExplorationThe cube can be made transparent so that only the highest amplitude reflectors are shown.

A time or horizontal slice of the sub-surface is a flat seismic picture made at a specific depth in time (milliseconds).

Various reflectors that intersect the slice can be displayed.

A single seismic reflection can be displayed as a horizon slice, and a fault surface can be shown as a fault slice.

Seismic sections

Following the extensive acquisition and processing of the seismic data the final result is a seismic section (2D) or seismic cube (3D)


3-D Visualization

Special rooms called visualization centres are used to display 3-D seismic images in three dimensions.

In some rooms, there are screens on three walls and the floor. The viewer uses stereoscopic glasses.

The viewer is immersed in the 3-D seismic image and can walk through the subsurface.


Mid North Sea

Lower North Sea

Chalk

Delfland

Aalburg BunterChalk

Zechstein SaltRotliegend Reservoir

HollandVlieland

Muschelkalk/Keuper

Posidonia

FieldField

Geologic interpretation


Channel / Crevasse Splay

Facies from seismic Facies from Seismic

Example chanel minimum amplitude extraction

What's this? it’s a crevasse splay

Why is resolution so good? – shallow <1000ft

Seismic interpretation techniques – amplitude extractionExample is 3D inline through wells 6, 9here red is soft, black hard max, top and base objective for extraction is indicated


Interpreted facies

Log shape data supports seismicexample – interpreted sketch of the min amplitude extraction with log traces from wellsshaded red log data supports seismic interpretation – channel and crevasse GR motives


Low amplitude off-structure

sand

Acoustic impedance

z

Hydrocarbon Water

Flat event

High amplitude on-structureShale

Direct Hydrocarbon Indicators


Sometimes the seismic section will display bright-spots (high amplitude anomalies) or direct hydrocarbon indicators (flat events under arching or dipping events).

Bright-spots have been shown to be associated (though not always) with hydrocarbon accumulations, and the flat events are consistent with the oil-water or gas-water contacts in the traps.

Direct Hydrocarbon Indicators

Anticline showing a flat spot on the oil-water contact.





Seismic Methods


4-D Seismic Exploration4-D or time lapse seismic exploration uses several 3-D seismic surveys over the same producing reservoir at various time intervals such as two years to trace the flow of fluids through the reservoir.

As a reservoir is drained, the temperature, pressure, and composition of the fluids change.

Gas bubbles out of the oil, and water replaces gas and oil as it is being produced.

Time slices of the reservoir are compared, and changes in the seismic response such as amplitude can document the drainage.

Undrained pockets of oil can be located and wells drilled to drain them.


Amplitude difference at the top reservoir horizon

4-D Seismic Exploration


Aim: identify undrained fault blocks

Main uncertainty: sealing capacity of faults

4-D Seismic for Reservoir Model


The Producing OWC can be mapped all over the field!

4-D Seismic for Producing OWC Map





Seismic Methods


• Reservoir maintenance objectives– Maximise HC production / minimise produced water

(optimum well placement)

– Integrated Reservoir Models (honour Seismic and Production data)

4-D Seismic for Reservoir Maintenance


Optimise infill wells to maximise HC / minimise produced water

Identify / target bypassed oil

Identify connectivity, fault sealing

Identify thief zones, water/gas coning and cusping

Integrated Reservoir Models honour 4D Seismic and Production.

Effects of close-the-loop

Increase ultimate hydrocarbon recovery

Accelerate hydrocarbon recovery

Reduce infill well costs

Reduce separator requirements (delay water/gas breakthrough)

4-D Seismic for Reservoir Maintenance


Originally proposed well path

Optimise well location in new model to delay water cut as long as possible.

Original

Well pathafter time

lapse

4D impact on field development


4-C Seismic Exploration4-C or multicomponent seismic exploration records both compressional and shear waves that are given off by a seismic source.

Seismic waves (a) compressional waves, (b) shear wavesA compressional wave (P-wave) is how sound travels through the air. Particles through which the compressional wave is travelling move closer together and then further apart.

A shear wave (S-wave) is like a wave on the surface of the ocean. The particles move up and down. Shear waves are slower than compressional waves and cannot pass through a liquid or gas.


4-C Seismic ExplorationThe conventional seismic method records only the compressional waves with a one-component geophone.

The 4-C seismic method records both the compressional (one component) and also uses three geophones that are perpendicular to each other to record the shear wave (three components).

4-C seismic exploration is used to locate and determine the orientation of subsurface fractures and determine the composition of the sedimentary rock layers and their fluids through which the seismic energy passes.

Compressional waves are distorted by gas in sedimentary rocks, but shear waves are not affected.

Seismic shear wave recording produces a more accurate picture of any sedimentary rocks that contain gas.


Once the data has all been "shot", the information is fed into computers for processing. The result is a "line" of seismic, shown below.Layers of rock deep in the earth can be clearly seen. On this "2D" line, stratigraphic traps containing possible oil or gas have been shaded green. The line is called "2D", or two-dimensional seismic, because it shows a single cross-section through the earth along a relatively straight line.


What to remember about 2-D Seismic Data

You might think that 3D seismic would solve all the problems of exploration. It's not quite that simple.

• First of all, 3D seismic is extremely expensive. • Second, 3D seismic is good at showing structural traps but not so good at showing stratigraphic traps. • Finally, 3D seismic reduces the uncertainty in the search for hydrocarbons. • Only by drilling a well can this be established.

What to remember about 3-D Seismic Data


Many different types of colourful and exciting displays can be created with 3D seismic datasets.

Petroleum Geologists and Geophysicists study the images carefully before they decide to drill a very expensive well!

3-D Exploration CostsThe high cost of 3-D seismic survey is because of the expensive data acquisition and computer processing.

A 3-D seismic can have 500 GB of information.

However, more 3-D seismic exploration, both on land and in the ocean, is being run today than 2-D seismic exploration.

It saves money by decreasing the percentage of exploration dry holes.

It also saves money during developmental drilling by accurately imaging and defining the subsurface reservoir.

The optimum number of developmental wells can then be drilled into the best locations to drain the reservoir efficiently.


Once We've Found It, What Do We Do With It?

Exploration is only a part of the E and P business. There are many technologies available to extract petroleum from the ground and then process it into one of the hundreds of everyday items made from petroleum-based products.

The only way to know for sure if a trap contains commercial amounts of gas and oil is to drill a well.

A well drilled to find a new gas or oil field is called a wildcat well.

Next? Drilling to Produce It


Revision Aids

1. a) Briefly describe the principle of acquiring seismic data.

b) Explain the differences in 2-D and 3-D seismic data

2. a) Why is there a need to process seismic data?

b) Explain any three techniques

3. In interpreting seismic data, some key techniques are useful in identifying hydrocarbons.

a) Briefly describe what AVO is and its usefulness

b) What are DHIs and how useful is this interpretation technique?

oil and gas search and prospecting pool

Documents

seismic records

seismic response

seismic section

seismic energy travel

field processing

computer processing

prospecting pool

drilling rig