Finding fractures in deep and tightrocks has become a high priorityamong explorationists in the MiddleEast. Recent discoveries have shown thatfractures can play an important role in theproductivity of low permeabilityformations. This is because they form aninterface with the rock matrix which ismany times greater than that provided bythe borehole.

In this article authors from differentorganizations discuss the origins offractures, their importance within MiddleEast oil reservoirs, and outline the variousfracture interpretation techniquescurrently being used.

Authors:

Schlumberger: Mahmood Akbar, Roy Nurmi, Eric Standen and Sandeep Sharma.

Oil and Natural Gas Commission of India (ONGC): Piyush Panwar and J.G. Chaturvedi.

ONGC-Schlumberger Wireline Research Center: Bob Dennis.

Middle East Well Evaluation Review26

Fig. 3.1: UNRELIABLEOUTCROPS: As rocklayers return to thesurface, stress releaseallows new fracturesto develop. Thesefractures do not occurin the same formationat reservoir depth.

Fractures are 3D features - a factthat is often neglected in the earlystages of reservoir development.While fractures seen in the wellbore willbe analyzed to determine aperture andprobable production rates, little effort ismade to develop a detailed model offracture distribution. This kind of studyonly takes place when the reservoir isformed almost entirely of fracture poros-ity (as in a fractured basement) or whensome aspect of reservoir behaviourstrongly contradicts the existing reser-voir model - for example, in cases wherethere is sudden and unexpected waterproduction.

In general, fractures are importantbecause of their influence on tight reser-voirs, not because of their actual oil stor-age capability. Although fracture volumemay be negligible in comparison withthe total reservoir volume, they providean interface with the matrix which ismuch larger than the borehole. Conse-quently, very small primary permeabil-ity values are sufficient for production ina fractured reservoir.

Factors controlling the occurrence ofnatural, open, permeable fractureswithin Middle East reservoirs are thenature and degree of folding and/orfaulting, in-situ stresses and changes inrock properties such as porosity, bed-ding and lithology, especially shaliness.Such geological factors are oftenmapped in reservoir studies. By accu-rately defining the relationship of thesefactors to the fracturing in a number ofwells within a field, it should be possibleto extrapolate the fracture data through-out the reservoir.

The major problem with large-scalefracture studies based on either boreholeimagery or oriented cores is that the frac-

tures in the borehole wall may not berepresentative of the large-scale fracturenetwork which controls production. It isnecessary, therefore, to relate geometri-cal information from borehole data to thereservoirs geological characteristics(structure, stratigraphy, sedimentology,diagenesis and geostatistics). Simplecubic models used in reservoir studiesare sometimes appropriate but often morecomplex geometric models are required.

Borehole-scale characterization andgeological modelling must be fully inte-grated with dynamic testing and produc-tion data, not simply used for compari-son as a type of quality control.

Using outcrop data to characterizethe fracture pattern of a reservoir is frus-trated by the stress release whichoccurs as rocks come to the surface.Uplift and erosion of overburden oftenresult in tensional breaking of brittlebeds due to deformation along ductilebedding planes (figure 3.1). As a result,fracture density in reservoirs is com-monly lower than values recordedwhere the same formation outcrops.Consequently, outcrop data is unsuit-able for modelling reservoir fractures.

Fig. 3.3: The fracture orientationscommonly found in the MiddleEasts anticlinal reservoirs arehighly variable. In most cases,information from a single wellcannot be used to characterizethe fractures for an entire field.

UPLI

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Fig. 3.2: HIDDENAGENDA: Surfacefractures can causemajor problems atreservoir depths.Unconformities inlimestones oftenlead to thedevelopment ofkarst and leachingof fractures. Afterburial, thesesurface weatheringeffects can lead tostrange reservoirbehaviour. Theleached fractures inthis photo are fromJebel Hafit, UAE,with Roberta Nurmifor scale.

Number 14, 1993. 27

Unfortunately, it is becoming clearthat many unconformity-related features(figure 3.2), such as karst and exfoliationfractures (both due to subaerial weath-ering), are present in Middle East reser-voirs. While such fracture systems usu-ally occur over a very narrow depthrange, they may have greater connectiv-ity than tectonic fractures, which aremore numerous, and so exert a stronginfluence on reservoir behaviour.

How fractures are formed

Fracture orientations can vary consider-ably between reservoirs but the orienta-tions are neither random nor chaotic.Early fracture studies, which were basedon poor or insufficient data, providedvery misleading information about frac-ture distribution in reservoirs.

Fractures are usually formed duringfolding or doming of a reservoir, with themost intense fracturing being concen-trated in low-porosity rocks. In areaswhere the reservoirs have little matrixpermeability fractures are critical to pro-ductivity. This is especially true of base-ment reservoirs where fracture porositymakes up most of the reservoir.

Borehole imagery and 3D seismicsurveys have improved fault mappingand horizontal wells are providing anew insight into the fracturing associ-ated with reservoir faulting.

Fracturing typically occurs in one oftwo ways - either parallel or perpendicu-lar to normal or reverse faults. Obliqueorientations are associated with wrenchor shear movements.

Crack concentration

Fracture density, or the intensity of frac-turing, is defined as the number of frac-tures per unit length inside an interval of adefined height. This value has to be cor-rected for the orientation bias created bychanges in angle of the fracture planesand the borehole axis.

Anomalous increases in reservoirtectonic fracturing are sometimes seenin wells. This is usually associated withfaulting. Other variations in fracture den-sity have been attributed to changes inlithology, porosity or shaliness. Lowporosity and shale-free intervals gener-ally contain more fractures, althoughthese may or may not be mineralized.

Clearing up the chaos

In the past, wide variations in fractureorientations within anticlinal reservoirs,coupled with poor quality orientationdata and fracture characterization, led towidespread pessimism about the useful-ness of fracture data in reservoir mod-els. However, improvements in fracturedetection and analysis techniques haveshown that there is order in the appar-ent chaos. Fracture orientations can berelated to specific geological parametersand structural events.

The common fracture orientationsfound in Middle Eastern anticlinal reser-voirs are shown in figure 3.3. Changes inorientations can be caused by later faultmovement associated with variations intectonic stress through time. In the car-bonate reservoirs of Turkey and Iran,the orientation of karstic fractures asso-ciated with erosional unconformities, ismuch more variable. Low-angle, stress-relief, exfoliation fractures, which occursub-parallel to the unconformity surfacein these reservoirs, are particularlyimportant features.

Variable orientations of open frac-tures in a reservoir are often associatedwith the rotation of principal stress inproximity to some of the observedfaults. When the principal stress is per-pendicular to the fault strike compres-sional forces are probably at work onthe fault.

Pass the salt

Fracture orientations in salt dome reser-voirs depend on the shape of the struc-ture and the nature of the regional anddoming stresses. As a result, reservoirsof the same age in different fields canhave different fracture orientations if thefractures formed at separate times underdifferent regional stress conditions (fig-ure 3.4).

For the same reason, fracture orienta-tions vary through geological time. Inthe same field it is possible to find frac-tures formed in the Permian with verydifferent orientations to those formed inthe Jurassic or in Cretaceous reservoirs.In addition, fracture orientations can dif-fer between reservoirs of identical age inadjacent fields if doming and fracturingdid not occur at the same time in thetwo reservoirs.

Early fracturing around The Gulf wasaffected by extension stresses during theTriassic which were later followed bycompressional forces from the east due tothrusting associated with obduction(pushing of ocean crust onto continentalcrust) in Oman. In contrast, orientations oflater fractures were determined by com-pressional forces of the Zagros Orogeny.These are generally oriented NNE.

Definition of reservoir fracturing inMesozoic salt structures is important inThe Gulf. These fractures rarely dominateproductivity as their apertures and densi-ties are usually much lower than thoseassociated with tectonic fractures. (Pres-sure and flow tests indicate that intergran-ular pore systems in both shoal and reefalfacies generally contribute more flow).However, it is clear that the fractures,which are often characterized by thinapertures, are connecting heteroge-neously distributed porosity (Nurmi et al.,1990, WER Structural Geology Supplement).

Cretaceous Cretaceous

A B

Fig. 3.4: AGE GAP:Reservoirs of thesame age in differentfields can display awide variety offracture orientations.Patterns of fracturingvary throughgeological time;fracture orientationbeing controlled bythe timing offracturing, not theformation age.

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Middle East Well Evaluation Review28

Recognising faultsThe presence of a fault is often indicatedby rapid increases in fracture intensity orspacing. This means that once the relativeintensity of fracturing has been deter-mined for each reservoir zone anyincrease in permeability, especially inmore porous intervals, can usually beattributed to faulting. In such situations, afocused search using borehole imagerycan reveal the actual fault plane or zoneaffecting the fractures (figure 3.5).

Fault detection and analysis have onlyrecently been recognized as a criticalcomponent in the characterization of frac-tured reservoirs. In early boreholeimages few faults were recognizedbecause none of the available tools werespecifically designed to find them. Frac-tures related to faults are much less abun-dant than fractures related to the foldingor doming of a field, but the vertical conti-nuity of faults often results in dramaticwater movements through fault-associ-ated fracture systems.

A major fault can be a single, high-angleplane cutting a reservoir horizon and maynot be intersected by wells. However,there are associated smaller faults andfractures which formed at the same timeand it is these zones which are morelikely to be crossed by wells.

Indications of faulting can arise dur-ing drilling. These may take the form ofmissing rock sections in areas of normalfaulting, repeated sections in areas ofthrust or reverse faulting or loss circula-tion material in areas affected by wrenchfaulting or any other type of open frac-tures associated with faults.

Recognising faults depends on ourability to detect the bedding plane above

and below the feature. However, even ifthe bedding is not recognised, the geome-try of the fault plane can be measuredwhere it intersects the well. This informa-tion can be used to project the fault planeaway from the well and through any for-mation above or below. Unfortunately,this is not as simple as it may seem (fig-ure 3.6).

Natural fractures

Natural fractures are usually assumed tohave been created by tectonic stresses.They are more common in carbonaterocks than in sandstones and typicallyoccur in specific directions which are dic-tated by the regional tectonic stresses.Induced fractures associated with natural

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Fig. 3.5: FAMILY OF FAULTS: The four major fault types (top row) can be distinguished using borehole imagery (bottom row).Knowing the precise geometry of the fault plane where it intersects the well allows us to project the fault away from the well and sopredict its effect in other locations.

Fig. 3.6: THINGSARE NOT ALWAYSAS SIMPLE ASTHEY APPEAR:These diagramsillustrate situationswhere normalfaulting hasdeveloped on thehanging wall of amajor reverse fault(a) and where smallreverse faults havebeen created on alarge, normal faultstructure (b). Theinterpretation ofeither structurewould dependprimarily on thelocation of the welland its depth ofpenetration.

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Number 14, 1993. 29

fractures might also be expected to havea preferred orientation, although thiswould not necessarily be parallel andcould even be perpendicular to the nat-ural fractures.

Drilling-induced fractures

Borehole electrical images, especiallythose which have been computerenhanced, provide useful informationabout the borehole wall and the reser-voir rock.

Borehole enlargement is a commonfeature and often occurs in the samedirection in a number of wells. Theenlargement is related to stress releasefailure and is usually greatest parallel tothe least principal horizontal stressdirection. A complex mixture of inducedfracturing with both shear and extensioncracks is often present (figure 3.7).

Repeat logging of special processwells has shown that borehole failureand breakouts occur within days ofdrilling. Large fractures induced perpen-dicular to the direction of boreholeenlargement are usually long straightcracks in an axial position lying on oppo-site sides of the borehole. These inducedcracks are extensional fractures whichform and are propagated in front of thebit during drilling. Cores taken from suchzones often reveal these fractures.

Axial drilling-induced fractures havea modified appearance when the axis ofthe borehole is not parallel to any of the

principal stress directions. This kind ofcrack has a jagged appearance, resem-bling a lightening bolt, in contrast to thelong and straight cracks which are per-fectly parallel to the axis of the borehole(figure 3.8). Such cracks are most com-mon in horizontal or highly deviatedwells where the orientation of the bore-hole with respect to the stress field wasnot accounted for in positioning the well.

Recognizing and analyzing inducedfractures is valuable in determining theorientation of the principal horizontalstress which may vary within a reservoir.The orientation of mini-frac jobs hasalso been ascertained and confirmed bylogging before and after fracing.

Enhanced fractures

In addition to natural and induced frac-tures, there are pre-existing fractureswhich are extended or opened in theborehole by drilling. These have beencalled enhanced fractures (Standen,1991). It has also been observed andreported that even totally mineralizedfractures can be re-opened during thecourse of drilling.

The enhanced fractures are usuallyoriented sub-parallel to the principalhorizontal stress as are drilling-inducedfractures. Generally, drilling-enhancedfractures do not seem to affect the pro-ductivity of an interval as they usuallyhave very small apertures in the undis-turbed state.

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Fig. 3.7 (Left): UP THE WALL:Drilling-inducedfractures are commonbut not easilydistinguished fromnatural fracturesoccurring in thewellbore.

Fig. 3.8: (Right):PRINCIPALPATTERNS: The pattern fordrilling-inducedfractures depends onthe orientation ofprincipal in-situstresses relative to theborehole. Thesestresses may beparallel (a, b) oroblique (c, d) to theborehole axis.

In-situ stresses normal andalong the borehole axis.

In-situ stress not normal andnon-axial to the borehole.

Principal stressesnormal and parallelto borehole axis.

Principal stressesacting normal butnot parallel to axis.

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Middle East Well Evaluation Review30

Understanding India'sfracturesBorholla-Changpang

In the mid-1960s, a basement high wasmapped near Borholla, Assam in India.The first well, drilled on the highest partof the antiformal feature was dry. How-ever, a second well, drilled severalyears later, located oil in a Palaeocenesandstone overlying the granitic base-ment.

Further wildcat drilling in the plainsaround Borholla was unsuccessful but alast ditch attempt was made to drill atest hole in the nearby Naga Hills. ThisNaga Hills guess was rewarded whenthe first well, drilled close to Chang-pang, in Nagaland, struck oil in fracturedgranite rock (Middle East Well Evalua-tion Review Number 7 1989, India - 100Years of Oil). This discovery provedthat commercial accumulations of oilcould be found in fractured basementrocks and had a radical effect on explo-ration in India.

Conventional seismic investigationscarried out around Borholla in the 1960swere followed by six-fold CDP (Com-mon Depth Point) surveys in the early1970s and 12- and 24-fold CDP surveysabout five years later. These studieshelped to clarify regional tectonics, butthe low quality of seismic images didnot throw light on structures within thefractured basement.

The inadequacy of seismic datameant that the only way to decide onthe location for a delineation well was toanalyze data from previous wells. Asmore wells were drilled a structuralmodel emerged.

This approach allowed the basementstructure to be described as a singledome near Borholla. However, asclearer information was gathered, themodel was refined to show two separateculminations in the basement, one atBorholla and another at Changpang. Thecontour maps for the fields becameincreasingly detailed and eventuallycontained faults with throws of less than25 m. At this stage the structure began tobe redefined as a mosaic of several faultblocks and sub-blocks (figure 3.10).

Geology - the first analysis

The explorationists began detailed cor-relation of micro-features on electrologs,in order to improve their understandingof the faults. By analyzing well data forsand thicknesses in the Sylhet andKopili formations, it was possible toinfer the location of faults by their effecton the sediment. This approach pro-vided an indication of the underlyingbasement structure, but not a detailedimage.

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Fig. 3.9: TWIN PEAKS: The twinoilfields of Borholla-Changpang inAssam and Nagaland states ofnortheastern India werediscovered in the late 1960s.Located on NE-SW trendingbasement highs, the fields produceoil from fractured granite. 3Dseismic surveys were carried outto define the details of reservoirfaulting.

Number 14, 1993. 31

3D seismic survey -the final analysis

3D seismic data was gathered between1987 and 1988. Difficult terrain, poorlogistics and subsurface complexitycombined to produce lower quality datathan had been anticipated. However,the high density of 3D data enabledinterpreters to produce a much betterimage than would have been possibleusing 2D techniques.

Unfortunately, the reservoir map pro-duced from the 3D survey was viewedwith reservations because it containedno reflection event corresponding to thebasements top surface. This deficiencywas explained as absorption and dissi-pation of seismic energy by the frac-tured basement.

More importantly, the fault patternfrom the 3D survey did not match thepattern which had been inferred fromwell data and the 2D survey.

Mechanical contouring performedusing GEOPIC failed to provide a logicalstructure over one producing field. The3D seismic showed the structure to bemuch steeper than had been previouslybelieved, which meant that there wouldbe a marked reduction in closed area.

As a result of these apparent incon-sistencies, the 3D survey was treatedwith scepticism and the earlier map wasused for delineation purposes.

However, the 3D seismic data indi-cated a fault wherever the sandisopachs showed reduced thicknessesor missing stratigraphic units. Subse-quent wells confirmed the patternrevealed by the 3D survey (figure 3.11)and prompted its use for fault and struc-ture delineation. It was concluded that,in the absence of a strong reflectionfrom the top of the basement, a goodreflection of the overlying Sylhet Lime-stone would be sufficient to deduce thebasement fault pattern as well as depth.

This model proved to be highly suc-cessful. Instead of two distinct culmina-tions seen at the basement level, thestructure now appears to consist of alarge number of discrete antiformalcumulations separated by basementlows. These lows are the sites of majornormal faults typically trending NNE-SSW which generally avoid the antifor-mal highs.

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Fig. 3.10: BASE MODEL: This structure contour map represents the top of the basement in theBorholla-Changpang area. This model is based on data gathered from 2D seismic surveys andnumerous delineation wells. At this stage the structure appears to be a mosaic of a few majorfault blocks, each comprising many sub-blocks.

Fig. 3.11: BASEMENT JIGSAW: A 3D survey revealed that the basement structure did not containtwo large oil-bearing structures, but many small antiformal highs separated by basement lows.These lows correspond to major normal faults which trend NNE-SSW.

0 400 800 m

Drilled wellFault

Middle East Well Evaluation Review32

More than one way to image a reservoir

From a detailed examination of all thenew data which emerged from the 3Dsurvey it became apparent that the max-imum fracture intensity would beencountered on the flanks of the antifor-mal culminations. Production tests ofwells located on the flanks confirmedthis model and showed that these arethe most prolific oil producers in thefield. These wells have very low initialgas-oil ratios and a negligible decline inpressure during production. Some wellshad the capacity to sustain self-flow withmore than 60% water cut - proof thatthey operate under active bottom-waterdrive conditions.

In contrast, wells drilled in the syn-formal lows often registered good dailyflow rates but produced only moderateamounts of oil. Recent wells in the lowshave either ceased production as aresult of water loading or producedwater containing only traces of oil.

These observations support the ideathat oil accumulation in the basement iscontrolled not only by fractures but alsoby structural constraints. Below a cer-tain level virtually all fractures arewater-bearing and the aquifer has gener-ally flooded the troughs in the vicinity ofgood producers. This is the reason whyperipheral wells have short productionlives and wells in the lows surroundingthe main field produce little or no oil.

Reservoir models evolve as the tech-niques used to investigate them change.A major obstacle, in this case, was theuncertainty about replacing or modify-ing an older model with the contradic-tory information derived from a 3D sur-vey. Perhaps the most important benefitof the 3D survey has been in develop-ment drilling. In situations where previ-ously there had been uncertainty, themodel provides clear targets in thequest to extend the field.

In the Borholla-Changpang area, a con-tinuous compressive regime has resultedin extensive basement fracturing. In anattempt to define the tectonic framework

Fig. 3.12: This FMS image highlights thefractures in the Changpang reservoir. Thenatural open fractures (1) are seen toextend across the diameter of the wellbore.Drilling enhanced fractures (2) have thesame orientation but are open only in thedirection of the principal horizontal stress.The drilling-induced fractures (3) are near-vertical cracks which are found onopposite sides of the wellbore.

of the region, 3D seismic surveys wereplanned and carried out in 1987 and 1988.

A recent reservoir study highlightedthe possibility of using horizontal wells toexploit this fractured basement reservoir(figure 3.11). But before any decisioncould be made on developing the field anumber of parameters had to be deter-mined - fracture orientation, dip, spacingand fracture density.

The replacement ratio Fr can be cal-culated by taking into account the differ-ences in drainage areas of horizontaland vertical wells. This ratio representsthe number of vertical wells whichwould be required to produce at thesame rate as a single horizontal well. Inthis case, the Fr value increased withlength of horizontal section and the com-puted value was never more than 4.8.

Production performance is a functionof the number of intercepted fractures.Since the major fracture trends are ori-ented NW-SE and E-W (figure 3.13), thehorizontal section of each well had to bedrilled in NNE-SSW orientations to maxi-mize production. Environmental factorssuch as land acquisition and logisticsalso contributed to the argument thathorizontal, rather than vertical wellswere more suitable for developing thisfield.

Oil from Bombay High

Drilling in the giant Bombay High Field,offshore India, has revealed large vol-umes of oil and gas in basement rockscomposed of basaltic and graniticgneiss. The field, discovered in 1974, issituated approximately 150 km off India'swestern coast. Most of the oil and gas iscontained in Miocene limestone reser-voirs and a smaller proportion in thebasal clastic sandstone reservoirs whichoverlie the fractured metamorphicrocks.

The main limestone reservoir isencountered at a depth of approxi-mately 1300 m but the basement hydro-carbons occur at 1900 m, on the crest ofthe structure.

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Number 14, 1993. 33

Away from the well-bore, characterization mayalso be divided into staticand dynamic approaches. Thestatic method consists of project-ing fracture systems laterally, away fromthe wellbore, guided by input from high-resolution seismic data or offset VSPs.Dynamic characterization beyond thewellbore requires well test data to deter-mine fracture length, boundary condi-tions, vertical communication andextended flow capacity for reserve cal-culations.

Core studies, borehole images andStoneley acoustic permeability logs allshow the presence of a high-angle (75)open fracture set and a low-angle (20)healed fractures in the study wells. Thehigh-angle fractures strike NNW, parallelto the adjacent faults.

The low-angle fractures, which aregenerally filled with either calcite orquartz, have apertures between 5 mmand 10 mm. These fractures may haveresulted from a rebounding effect andexpansion in the basement with subse-quent fluid movement.

Fig. 3.13: DIRECTIONS FOR BORHOLLA-CHANGPANG:These stereonets indicate the fault and fracture directionsin the Borholla-Changpang fields of Assam, India. Thepredominant fault orientation (large stereonet) is E-W,while the fractures recorded in three separate wells(small nets) show variable orientations but areconcentrated in E-W and NW-SE directions. These

results were obtained using aStratigraphic High Resolution

Dipmeter (SHDT)* Tool.

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Formation MicroScanner (FMS*) imag-ing was used to determine orientation,dip and density of the fractures. In addi-tion, acoustic waveform data was used todelineate open fractures by means of theStoneley-derived Permeability Index. Thestudy indicates that the open fractures are,in general, high angle and striking in a N-Sdirection. Low-angle fractures recorded inthe field are generally closed and filledwith calcite and quartz. Fracture densityseems to be higher in granitic basementthan in rocks with a basaltic composition.

Four wells (BH-36, BH-19, SY-5 and II-7)were drilled to depths of 200 m in thebasement. Only SY-5 failed to produce oilor gas and required hydrofracturing. Afterinitial stimulation the other wells pro-duced oil and/or gas at a rate exceeding1000bpd.

Acoustic and petrophysical logs, coreand well test data were integrated tolocate open fractures and determinetheir relative flow capability.

Fracture characterizationtechniques

Fracture characterization at the wellboreis a two-step process, requiring staticand dynamic approaches. The staticcharacterization involves determiningin-situ fracture location, density and ori-entation using a range of wellbore imag-ing tools, petrophysical anomalies, coreand drilling information. The dynamicapproach makes use of acoustic data(SDT- and LSS-derived Stoneley) for rela-tive fracture conductivity at the well-bore. This can be verified with produc-tion logs from flow and injection tests.

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Fault directions in Borholla-Changpang fields.

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Fig. 3.14: LOOKING A LITTLE DEEPER:These recent analyses of basement fracturingat Borholla-Changpang reveal a clearvariation of fracture orientation with depth.The diagrams illustrate dominant strikeorientations, at various depths, derived fromFMS image analysis.

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Middle East Well Evaluation Review34

Egypts basement bonusThe fractured granite reservoirs of theGulf of Suez, Egypt, are flanked byporous and permeable reservoir sandsand carbonates. The permeability of thegranite horst blocks and the fact thatthey are in hydraulic communicationwith these flanking sands and carbon-ates makes them ideal drainage systems.The Precambrian-age granites wereprobably first fractured in Paleozoictimes as the Suez and Aqaba shearzones were developing (figure 3.16).Porosity system studies of these granite

Fig. 3.15: Shear fractures, which affectrocks at the southern end of the Gulf ofSuez, have produced a number of large

granite reservoirs. In some fields,granites produce more hydrocarbon

than adjacent sandstone or carbonatereservoir rocks.

reservoirs indicate the development ofsecondary porosity along fractures and apervasive leaching of feldspars. Thebasement complex was later covered bythe Nubia sand sequence and a thicksuccession of Cretaceous sediments,prior to the start of rifting in the Gulf ofSuez (figure 3.17).

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Number 14, 1993. 35

Fractal fractures

Statistical analysis of fracture data in theGulf of Suez indicates a fractal or fre-quency relationship between the com-puted fracture apertures. Figure 3.18shows mean aperture of fractures in asection of basement granite reservoir.The increase in aperture towards thetop of the granite and the variation ofaperture at all vertical scales are particu-larly important features.

Fracture spacingdata also shows varia-tions which are usefulin predicting expectedranges of valuesthroughout the reser-voir. However, this kindof data does not give

absolute values atany location. Datasets from the frac-tured granite reser-voirs of the Gulf ofSuez show a partic-ularly interesting

variation of fractureaperture with depth.

The fracture datarevealed up to 4000 frac-

tures in each 1000 ft. Frac-tures were selected by hand and a

computer program was then used to cal-culate mean aperture and meanhydraulic aperture for each one. Fracturedensity and porosity were then calcu-lated using an averaging technique with afixed sample rate.

Several wells were analyzed in thismanner, with the results showing a simi-lar range of porosity values but differentfracture densities and average aperturevalues for each well. This variation inaverage aperture was further corrobo-rated by production results whichshowed that density of fracturing andwidth of fracture aperture were themain criteria for initial production(Taleb et al EGPC 1990).

Fig. 3.16 : SPLITPERSONALITY: Fractures in theZeit Bay area of the Gulf of Suezshow two dominant orientations - oneparallel to the Suez spreading axis and theother parallel to the Gulf of Aqaba axis.

Graben

Rift graben60/70

Fig. 3.18: This plot shows the mean aperture of fractures in part of a basementgranite reservoir in Egypt. It should be noted that aperture size increasestowards the top of the granite and that aperture is variable at all vertical scales.Cyclic variations can be seen at each scale of examination.

Fracture orientation

A consistent relationship could not befound between orientation and aperturefor the granite reservoirs. In certain cases,where a strong, principal, far-field stress ispresent in the rocks, the fractures seemedto open parallel to the principal stress.This was usually accompanied by drilling-induced fractures in the section. In onegranite reservoir, flowmeter datarecorded in the barefoot completed sec-tion of the well, correlated with a pseudo-flow profile from the cumulative fractureporosity results. This, however, was onlyafter the drilling-induced fracture porositydata was added, indicating a contributionto the flow by the drilling-induced frac-tures (Taleb et al EGPC 1990).

Production changes (coning of waterand gas) with respect to fracture orienta-tion have also been noted in the granitereservoirs. In general, there is a highervertical permeability when a single set ofhigh-angle fractures is encountered. Oilproduction is better with a mixed orienta-tion of fractures such as at the margins ofthe granite blocks (El Wazeer et al EGPC1990).

Fig. 3.17: PULLING FRACTURES APART:Fractures often occur in situations wherethere is a shear component of extension(a).This is the case in Yemens MaribGraben and Egypts Gulf of Suez whereshear movements produce non-perpendicular fractures. The simple, no-shear model (b) is unusual. In this modelfracture orientations are parallel andperpendicular to the rift.

x400 m

Mean aperture (1:1000) Mean aperture (1:200) Mean aperture (1:40) Mean aperture (1:5)

x400 m

x450 m

x500 m x440 m

x434m

x432m

x430 m

x430 m

x420 m

x600 m

x800 m

Gulf ofSuez

EasternDesert

SinaiGulf ofAqaba

Suez trend

Spreadimg axis

Aqab

a tre

nd

Suez trend

a b

Middle East Well Evaluation Review36

Only the StoneleyAnalysis of acoustic Stoneley wavesoffers a way of assessing the permeabil-ity of fractures and porous beds. Thedispersive Stoneley waves move alongthe interface between the borehole andthe formation. While travelling alongthe borehole wall, the wave propagateswithout much energy loss. However,the wave decays when it encounters apermeability change or a break in thewall.

In a cylindrical plane, such as aborehole, the detailed borehole geome-try becomes a very significant factor inwave propagation. At very short wave-

lengths, the boreholes influence on thewave is similar to that of a flat surface.However, at an operating frequency ofapproximately 10 kHz, the Stoneleywavelength is approximately 6 inches,roughly comparable to the diameter of aborehole. In these circumstances thewave decays only slightly as it crossesthe borehole and is referred to as theTube Wave. The low-frequency Stoneleymode of the Dipole Sonic Imager* (DSI)tool operates under these conditions.

At low frequencies, the tube wavemay be considered as a simple pressurepulse propagating in a cylindrical bore-hole. When it intersects a permeablefracture crossing the borehole, pressure

is released into the fracture. This pres-sure drop causes an attenuation of thedirect arrival and produces a secondary(or reflected) Stoneley wave. Thereflected Stoneley wave may beregarded as being generated by a sec-ondary source located where the frac-ture crosses the borehole. The strengthof this secondary source (reflection) isdependent on the amount of energy lostby fluid displacement inside the fractureand, therefore, the permeability of thefracture (figure 3.19).

The Stoneley waves generated by theDSI tool have frequencies which havebeen selected for maximum sensitivity tofluid motion. In fast formations the Stone-ley wave propagates faster than the fluidslowness. In slow formations the wave ismore strongly coupled to the formationand propagates at slownesses greaterthan that of the shear slowness.

Stoneley attenuation andpermeability

At low frequencies, propagation of theStoneley wave actually causes fluid flow.This makes it an ideal technique for esti-mating permeability (figure 3.20). TheStoneley wave can be thought of as adynamic micro-drill stem test. Acousticpressure in the borehole forces fluid intothe formation. The volume of fluid flow,and the size of the Stoneley wave attenua-tion which it causes, is directly controlledby permeability.

Combining the low-frequency mono-pole Stoneley with dipole shear from theDSI has greatly enhanced acoustic wave-form logging. Low-frequency Stoneleywave data can now be analyzed in termsof the formations dynamic permeabilityresponse. An additional benefit is thatthe high-quality shear data obtainedwith the dipole measurement offers away to account for non-permeabilityeffects recorded in the Stoneley wavevelocity. This is possible even in veryslow formations and allows accuracy farbeyond that achieved by standard sonictools.

Fractures derived from the DSI analy-sis can then be compared and combinedwith images from the FMS tool to improveour understanding of the reservoir.

Fig. 3.19: Stoneley wave data gives clear indications of fractures. This example is taken from afractured basement in the Ashrafi Field, Gulf of Suez, Egypt.

X600 ft

X700 ft

X650ft

Number 14, 1993. 37

Stoneley slowness andpermeability

Stoneley slowness can also be used toderive permeability measurements.Although less sensitive to changes inpermeability than attenuation data, slow-ness can be measured more preciselyand is less sensitive to mudcake andlithology. Data gathered using this tech-nique compares well with informationtaken from cores and RFT/MDT tool data.

When using Stoneley wave reflec-tions to detect fractures it is important tofirst identify thin shales in the sequence,as these can create reflections and atten-uations similar to those from fractures.

Integrated fracture interpretation

One way of improving reservoir modelsinvolves combining FMI/FMS imageswith Stoneley permeability profiles andcomparing the estimates from both tech-niques:

(1) Where the fracture aperture esti-mates from both techniques are in closeagreement the fractures are likely to beisolated planar features of large extent(ie greater than Stoneley wave penetra-tion away from the borehole).

(2) When fracture aperture measuredon electrical images is larger than frac-ture aperture from the Stoneley waves itgenerally implies that the typical frac-ture is shorter than the wavelength for

Tim

e

Shear

Stoneley

Compressional

R

Ston

eley

Shea

r

Com

pres

siona

l

T

Fig. 3.20: LATEARRIVAL: Stoneleywaves arrive later thanthe compressional andshear waves and havelower frequency and,usually, higheramplitude. Stoneleywaves can be thoughtof as a 'dynamic micro-drill stem test' since thevolume of fluid flowand the size ofStoneley waveattenuation is directlycontrolled bypermeability.

the Stoneley wave (approximately 10 ftat 500 Hz). They are also likely toinclude features such as vugs and shal-low drilling-induced fractures.

(3) If the fracture aperture recordedon electrical borehole images is lessthan fracture aperture from reflectedStoneley waves then the borehole frac-tures are probably connected to a net-work of fractures which extend farenough from the borehole to be beyondthe scope of the FMS.

Marked lithology changes, whichcause significant variations in formationshear modulus (eg the interbedding ofcalcite with soft shale) can give measur-able reflected Stoneley wave responsesbut will have little effect on FMS toolresponse. The common occurrence ofthin, washed shales below the resolu-tion limits of most logging calipers, oftencauses false increases in recordedStoneley permeability. However, theirbedding geometry, seen with boreholeimaging techniques, indicates that theyare not permeable fractures.

From this it should be clear that nosingle technique should be employed forfracture evaluation. Borehole imaging,high-resolution calipers, lithology indica-tors and Stoneley wave data should beused in conjunction in order to discrimi-nate against environmental effects and toarrive at a reliable interpretation of frac-ture properties.

Sounding out Bombay HighAn Array Sonic (SDT*) tool was evalu-ated on well BH-19 in the Bombay HighField. The SDT has a single, low-fre-quency transmitter with an array ofeight receivers spread 6 inches apart.This configuration allows a better deter-mination of the Stoneley event than datagathered using the Long Spaced Sonic(LSS*) tool.

This technique was used at two verti-cal resolutions. The first, based on thestacked Stoneley energies over thetransmitter-receiver spacings offers reso-lution of approximately 10 ft and can beused to evaluate gross fractured inter-vals. Resolution of approximately 2 ftcan be achieved by examining differen-tial Stoneley energies between selectedadjacent receivers.

Since most of the wells logged in thebasement have only LSS data, the SDTStoneley technique was adapted for LSSwaveform processing. Further workresulted in the development of a Stone-ley Permeability Index from differentenergies. Results on BH-36 comparedwell with the actual flow entries in theproduction logs.

The Stoneley signal from the LSSwaveforms has less energy than the SDTwaveforms and contains broad-spectrumpseudo-Rayleigh guided waves. At first,these effects appeared to completelymask the permeability variations. How-ever, the introduction of a select band-pass filter allowed the Stoneley waves tobe isolated and interpreted.

Fracture

Middle East Well Evaluation Review38

Appraising aperturesComputer modelling has shown that thewidth of a fracture is proportional to thefracture conductivity on an electricalborehole image (Luthi and Souhaite,1990). Fracture character depends pri-marily on the drilling fluid whichinvades the crack.

The resistivity of the drilling mudwithin the borehole should be mea-sured directly and accurately. Miscalcu-lations can occur due to changes in theconductivity of the fracture-filling fluidor because of conductive material alongthe fracture plane. Overall, the data cal-culated in the modelling tests suggeststhat reasonable fracture aperture val-ues are obtained from water-filled frac-tures in a wide range of lithologies.

Fracture apertures often increase insize along paleo-unconformities withinMiddle East reservoirs in Turkey, Iranand the United Arab Emirates. In Iranthe karstic fractures in Asmari carbon-ate reservoir sequences were detectedusing both imagery and core. Coreexamination showed that most of thesecarbonate fractures were filled withporous red sandstone.

Fig. 3.21: OPEN ORSHUT?: Theapparent reductionin aperture at thetop of this section(left) is due to thereduced volume ofwater in the oilzone. The plot onthe right shows thesame apertures aftercorrection.

Fig. 3.22: High fracturedensities are oftenassociated with faultsbut can be caused by anumber of geologicalfeatures. Basementreservoirs are frequentlycut by smaller igneousbodies such as this dykewhich is characterizedby a higher density offracturing than thesurrounding granite.

x000 ft x000 ft

x200 ft x200 ft

x600 ft x600 ft

x400 ft x400 ft

Number 14, 1993. 39

The variation of the apertures of base-ment fractures in Egypt's Gulf of Suezshows an unusual cyclic nature. Eachsequence of downward increasing aper-ture widths is followed by another cyclewith a similar range of aperture varia-tion. Even more surprising is the fact thatthese cycles have a fractal nature. Eachcycle is composed of smaller cycles ofdownward increasing apertures whichare in turn composed of still smallercyclic variations of aperture.

The presence of hydrocarbons withinreservoir fractures can lower apparentfracture apertures in some wells (figure3.21). This shift is due to a reduction inthe volume of water within the fracturewhich is not taken into account by com-puter software during the initial aperturecalculation. However, the location of theshift in apparent aperture seems tooccur at the oil-water contact within thefracture network even in cases wherethere is little to no oil within the sur-rounding rock matrix. In these cases,reservoir fracture porosity can be cor-

Fig. 3.23: ALTEREDAPERTURE: Thisoutput shows areduction inapparent aperturein the oil zones of acarbonate reservoirin Turkey (lefttrack). Test resultsconfirmed the oil-water contact andcorrection of theaperture data wasperformed byincreasing the Rmain the FracView*program. Thecorrected results areshown in the righttrack.

rected empirically. Careful analysis ofaperture variation indicates whether oilor water will be tested or produced.

The correction to the data is found bydividing the input mud resistivity (Rm)by the average calculated water satura-tion (Sw) from standard log interpreta-tion. This gives an estimate of the correctinput apparent mud resistivity (Rma)which can be used in the workstationfracture analysis program (figure 3.23).

Production information

Massive mud losses are generallycaused by open fractures being encoun-tered or created during drilling. Somereservoirs in the Middle East have evenbeen discovered as a result of severemud losses. In the Ain Zalah Field inIraq, the mud losses typically correlatedwith the productive potential of devel-

opment wells. Since mud losses canresult from either induced or naturalfractures, it is recommended that bore-hole imagery be used to analyze thenature of fracturing and the geometry ofthe fracture system. Careful monitoringof mud losses may reveal intervalswhich ought to be logged that might nototherwise be scheduled for logging.However, during normal monitoringconditions only the largest fractures willbe detected, and mud losses are notnecessarily related to fracturing.

Fig. 3.24: Aperture uncertainty in highlyfractured core.

x500 ft

x600 ft x600 ft

x700 ft

x800 ftx800 ft

x700 ft

x500 ft

FracView oil-water contact

Middle East Well Evaluation Review40

High hopes in thehorizontalThe growing use of highly deviated wellshas thrown new light on the distributionof fracture planes in reservoirs. Thereare obvious limitations in using a verticalwellbore as a method for sampling thedensity of vertical or sub-vertical frac-tures and fracture spacing. A wells ori-entation to the major fracture sets can becontrolled to avoid or to intersect themaximum number of fractures, depend-ing on the wells intended role in anygiven field. (Middle East Well EvaluationReview Number 8, 1990 Putting a bitaside.)

The productivity of horizontal wellscan be optimized by drilling the well inthe most effective orientation - parallelto the major fractures. Horizontal drillingappears to be most beneficial whenthere is little communication betweenfracture systems, for example whenthere is a single fracture orientationand/or when the fractures are not highlyinterconnected.

Borehole imagery in horizontal wells(figures 3.25 and 3.26) has provideddirect evidence of variations in fracture

orientations because of the greater num-ber of fractures encountered in highlydeviated wells. This has shown thatwhile fracture spacing may be fairly con-sistent in some fields it is highly variablein others, with the greatest single causeof variation being faulting.

Fracture spacing

Fracture spacing has been one of themost elusive parameters for reservoirmodellers. This is mainly because of thesmall number of fractures intersected byvertical wells. As petroleum companiescome to appreciate the improvement infracture detection and accept that frac-ture systems can be modelled, it is cer-tain that highly deviated developmentwells will be used to increase the statisti-cal sampling of fracture spacing.

The information will also improveour understanding of fracture-relatedproduction effects (figure 3.27). In 1987Nolan-Hoeksema and Howard (AAPGBulletin) suggested a method for com-puting optimal drilling direction in reser-voirs where there are a number of frac-ture sets or populations.

Numerous outcrop studies haverevealed a statistical relationshipbetween fracture spacing and thin beds.This has led many people to expect asimilar relationship in the subsurface butexamination of imagery and core sug-gests that such a relationship is not ascommon as occurs in outcrops. This maybe because uplift and overburden ero-sion cause stress relief and tensionalbreaking of brittle beds which deformalong ductile bedding planes.

Horizontal FMS image

0(360) Top

180 Bottom

0 Top

Horizontal FMS image

0/360

180

0

3-D block

Top

Bottom

Top

Bottom

Top

Fig. 3.25:HORIZONTALHEADACHE 1:The three verticalfractures shownin this block allhave verydifferentappearances onthe FMS image(immediatelybelow) of thishorizontal well.The actualorientation of thefractures withrespect to thewellbore can bedetermined fromthe shape of thetraces.

Fig. 3.26:HORIZONTALHEADACHE 2: In this case, threedipping fracturescross theborehole. Onceagain theorientations canbe determinedfrom their traceon an FMS image(below).Explorationistsaccustomed toexaminingfracture data invertical wellsoften find thefracture traces inhorizontal wellsdifficult tovisualize.

Number 14, 1993. 41

Improved interconnectivity -hydraulic fracturing

The interconnectivity of a fracture sys-tem can be further increased hydrauli-cally by creating deeply penetratingfracs. Research by Schlumberger Dowellhas led to the development of tech-niques designed to obtain minimumleak-off and deeper penetrating acid fracs(>200 ft).

The intervals most suitable for fracingand the best methods for fracture propaga-tion within a reservoir have been discov-ered by modelling the fracture process.Tests of a new fluid-loss-controlled acidfracturing system indicate that it is possi-ble to connect more natural fractures tothe borehole.

Information concerning the vertical dis-tribution of natural fractures is importantwhen planning a hydraulic fracture job.Even the recognition of induced fracturesand their orientation from imagery is valu-able for frac planning. In some cases, it isnot possible to generate fracs from perfora-tions that are not parallel or subparallel tothe principal horizontal stress. Moreover,fractures induced from perforations whichare not parallel to the principal horizontalstress have a tendency to lose frac effec-tiveness as a result of later closure.

Fig. 3.27: WATER INTHE WELL: Verticalfractures are veryimportant inhorizontal wells. Thefirst example (a)shows a sub-horizontal wellpassing through akarst (deeplyweathered)unconformity surfacewith high aperturezones which arepotential water-producing intervals.Section (b) illustratesa case where highfracture densitiesassociated with faultsled to severe waterproduction problems.(a)

Recent developments infracture detectionNew tools have been developed to com-plement the FMI. Recently, two new imag-ing devices have been introduced. TheAzimuthal Resistivity Imager* (ARI) toolappeared in 1992, and the UltrasonicBorehole Imager* (UBI) tool entered ser-vice in 1993.

The main area of overlap betweenthese tools occurs in fracture and thin-bed analysis. The techniques theyemploy to detect fractures or bed bound-aries are different and the images pro-duced by each tool may or may not besimilar. Clearly, the interpreter mustknow when and why the images will besimilar and the reasons for any discrep-ancies when they occur (figure 3.28).

ARI: A second opinion

The ARI tool is a standard dual laterologmodified by the addition of 12 azimuthalelectrodes. This electrode array providesa 360, quantitative and calibrated resis-tivity image of the formation.

In common with other resistivity tools,the ARI tool response is strongly affectedby fractures filled with conductive fluids.The ARI tool provides the best indicationof fracture porosity in partially mineral-ized fractures and is capable of distin-guishing shallow, drilling-induced frac-tures from tectonic fractures.

The ARI tool also provides a linkbetween geological data and traditionalformation evaluation of fractures andthin beds.

Numerical models have been con-structed to show that fracture aperturecan be determined from tool response.Individual fractures, which are more than8 inches apart, can be distinguishedusing the ARI tool. In cases where frac-ture separation is less than 8 inches, thecomputed aperture value will be the sumof the individual fracture apertures.

Providing estimates of fracture andstructural dip data will be valuable inwells or zones where FMI/FMS logs willnot be run.

UBI: A third opinion

The UBI tool was developed from theUltra Sonic Imager* (USI) tool and is suit-able for open hole use. The UBI tool hasa revolving transducer which emits ultra-sonic pulses and receives returningechoes from the borehole wall. Two-waytransit time and echo amplitude can beobtained and, with a high sampling rate,borehole images can be rendered usingeither time or amplitude.

If the velocity of ultrasound in theborehole fluid is measured, the transit

Oil-water contact

Potential water entry zones

400 m 500 m 600 m

Shale

Tight

Non-reservoir

Porous

(b)

FMI image ARI image UBI amplitude

Middle East Well Evaluation Review42

time can be converted to boreholeradius. Either transit time or radius mea-surements give a high-resolution, quan-titative and continuous scan of boreholeshape. If the borehole wall is disrupted,by fractures or features which are largecompared to the spot size for the tool(about 0.25 inches for the 250 kHz trans-ducer), they can be seen in the transittime values. The main limitation is thatthe UBI tool cannot provide quantitativeaperture information. The amplitudemeasurement facility available with UBIoffers a more detailed image than transittime, but is difficult to analyze quantita-tively.

Under normal logging conditionsthere are many factors which influenceamplitude. These include the angle ofincidence, scattering by irregularities onthe borehole wall and the acousticimpedance contrast and loss betweenmud and formation.

However, fractures and vugs shouldbe visible in the amplitude image pro-vided that losses in mud and those fromhigh angles of incidence do not elimi-nate a particular echo. UBI data is ana-lyzed using the FracView utilities devel-oped for the FMI/FMS image analysistechniques. Dip and strike are easilydefined in FracView. However, it shouldbe recognized that accurate measure-ments of borehole radii are necessaryfor good fracture orientation analysisand identification of bed boundaries.

Field tests using the UBI tool haveshown a dramatic improvement in bore-hole acoustic imaging. The introductionof advanced tools has helped to removemost of the low-quality images andbroad black bands that have frustratedgeological interpreters over the years.

Pictures in oils

The UBI tools acoustic technologyprovides high-quality borehole imagingin oil-base muds. While this representsa significant step forward, there are afew limitations. For example, the verysmall fractures which can be resolvedusing electrical imagery cannot bedetected by the UBI tool or any otheracoustic imagery tools. This presents aproblem because small fractures areimportant for statistical analyses offracture geometry. Acoustic imagescontain less bedding information thantheir electrical counterparts. Thisbedding data is critical for differentiatingfault types. Therefore, in wells with oil-base mud the Oil-Base Dipmeter*(OBDT) tool is sometimes run alongwith the UBI tool to ensure thatfractures and their associated structurescan be defined.

Fig. 3.28: A SECOND (AND THIRD) OPINION: In addition to the FMI and ARI tools, the fracture-seeking geologist or engineer can now call upon the UBI tool for locating fractures. The UBI toolhas been developed for wells with oil-base muds where the FMI tool is not suitable. The ARItool is being used to help identify deep open fractures and the fluid they fractures contain.

x40 m

x45 m

Number 14, 1993. 43

Slip sliding away

Cross-sectional plots of wellbore shapes(figure 3.29) show the detailed geometryof various types of borehole damage;including stress release, borehole break-outs and key seat wear by drill pipe. Acareful examination of borehole shapesby Schlumberger Etudes et Productionin Paris has revealed slippage or sheardisplacement along fault and/or fractureplanes in response to drilling (figure3.30). This slippage, which is oftenimaged on UBI tool surveys, may resultfrom reduced friction along the faultplanes, with drilling fluids acting as alubricant.

In a few cases, the slippage may beso large that production tubing or casingwill be damaged. Vincent Maury of Elfsuggests that some wells in southeasternFrance may have been completely lostas a result of this movement. This faultslippage data may prove to be an impor-tant source of tectonic information.

References

R Dennis, V Saxema. and A Rajvanshi (1991):Fracture Characterization in the Basement of theBombay High. ONGC-Schlumberger WirelineResearch Centre Report.

F El Wazeer, F Ismail and E Standen. (1990):Fracture Geometry and Hydrocarbon Productivity inthe Basement Rocks of the Zeit Bay Field - Gulf ofSuez, Egypt, presented at the 10th EGPC Explorationand Production Conference, Cairo, Egypt.

O Faivre (1993): Fracture Evaluation fromQuantitative Azimuthal Resistivities, presented at theSPE Fall Annual Meeting (SPE Paper 26434).

SM Luthi and P Souhaite (1990): Fracture Aperturesfrom Electrical Borehole Scans. Geophysics (55) pp821-833.

V Maury and A Etchecopar (1992): Shear Induced byOil and Gas Wells Drilling and Production AlongFaults and Discontinuities, presented at the WorldGeological Congress, Japan.

Fig. 3.29: This UBI logshows a combination ofbreakouts and naturalfractures in a Cretaceousreservoir in the Middle East.

Fig. 3.30: ON THESLIDE: During drilling ,movements can occuralong fault or fractureplanes which maydamage or evendestroy productiontubing or casing.Evidence for thismovement can be seenon many UBI images.

2

1Fault plane

Borehole side view

Borehole plan view

2 1

Breakout

Natural fractures

BB Neogi, BS Josyulu and KVB Singh (1991):Hydrocarbon Detection through Seismic AttributeParameters of Fractured Basement, Borholla-Changpang Fields, Assam. ONGC Bulletin.

E Standen (1991): Tips for Analysing Fractures onWellbore Images. World Oil (212) pp 99-118.

E Standen, R Nurmi, F El Wazeer and M Ozkanli,(1993): Quantitative Applications of Wellbore Imagesto Reservoir Analysis presented at the AnnualSPWLA Meeting.

HA Taleb, I Helal and E Standen (1990): GraniticBasement Fracture Study in the Geisum Field - Gulf ofSuez, presented at the 10th EGPC Exploration andProduction Conference, Cairo, Egypt.

Depth(feet)

Amplitude

Conductivity from 2.94 to 22.67 Conductivity from 4.46 to 4.86

Borehole radius

x200

x210