geology middle east

8/4/2019 Geology Middle East

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F O C U S O NF R A C T U R E SRock fractures and outstanding dailyproduction rates go hand-in-hand in theMiddle East. The 1 9 2 7 discovery well of theKirkuk O il Field, with its production rate o f90,000 barrels per day, dram aticallyemphasized the significance of fracturedrock. Since then, oil explorers have tried tofind and tap similar highly permeablereservoirs.Their task has not been easy. Fracturedetection and analysis has posed a thornyproblem for the past 60 years and it is onlynow that we can routinely investigatefractures in bo reholes. This article, byMartin Waterhouse, Mouhab Charara andRoy Nurmi, outlines som e of these newtechniques.Additional contributions by Carl Poster, an d Carl Montgomery


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KIRKUK - SIXTY YEARS AGO THIS MONTH" As soon as the b i t crack ed throug h thecemen t at the casing sh oe, o i l star ted to f lowinto the w e l l . I n a few mome nts i t was b low ingover the cro wn of the der r ick to a height ofabou t 140 feet - a typica l old t ime gu sher, thefirst and last to be seen in Iraq "" The dr i l ler ran to the boi lers andext ingu ished the f lames and when day l igh tcame the scene of the o i l d iscovery wasmarked by a b lack pal l o f gas and o i l vapoursand the o i l was f lowing l ike a r iver down theWadi Naf t . At two o 'c lock in the af ternoon theheavy dr i l l ing st r ing was b lown up in to theder r ick wi th a mighty roar .

Three days passed before the contro l va lvecou ld be c losed . Soon the g round a l l a roundthe wel l was saturated with o i l to a depth ofseveral inches and the sl ight breeze whichrose dai ly was b lowing some of i t longdistances. Men ar r iv ing f rom southern areasrepor ted that it began to fog their w inds creen sas far as 10 miles from the w e l l . With a l l th isin f lammab le l iqu id l y ing in poo ls eve rywhere ,one care lessly st ruck l ight might have causeda most d isastrous f i re .The famous Burning F iery Furnace ofShadrack, Meshak and Abednego, hal f a mi leaway, was hur r id lv smoth ered wit h ear th and a

constant wa tch ha d to be kept to ensure that itwas not relit nor any other f ire started. Hadthere been one heavy ra in storm al l the hast i lyth rown up dams in the wad i wou ld have bu rs tand thousand of tons of o i l which they wereho ld ing up wou ld have run downst ream in tothe River T igr is wi th consequences that wouldhave been catastrop hic. Yes, we were verylucky indeed with that w e l l . "Eyewitness acco unt of the dramatic discoveryof oil by the Kirkuk I well (then called BabaGugur No. lion 14th October 19 27. The uellproduces from fractured limestone.

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T his well-documented oil discovery in the Kirkuk Field highlightsthe importance of fractures toproduction rates. But a largenumber of the Middle East's prolific oilfields owe their fame to fractures whichhave formed the permeable highwaysthrough the reservoir along which oil canrapidly migrate. If the fractures were notpresent, many of these well-known oilfields would not be viable.As early as 1908, wo rker s in the Masjid-i-Suliman Field suspected that fractureswere responsible for a major proportionof the reservoir's permeability. This wasthe first major oil field to be exploited inthe Middle East bu t since then, numerousimportant fields have been located andmany of these c ontain a variety of fracturetypes.The type an d distribution of fracturesdepends on the geological setting, therock's material properties an d the kindsof stresses to which it has been subjected. It is difficult to provide a com

prehensive summary of fracture charac

teristics in the region, but they can becategorised according to the subsurfaceand plate tectonic movements whichresulted in the formation of fractures.Fractures in the Middle East can begrouped into three main types:

1) Fractures in orogenic fold beltsMany fractured anticlinal reservoirs liealong the Zagros-Bitlis fold belt whichextends from southeast Turkey to thenortheastern part of the Arabian Peninsula. Similar fold and fault belts extendnorthwards through Pakistan, withimportant fracture production in the region around Islam abad. The importance ofrock fractures in these regions cannot beoverstated. If fractures were not present,the production rates of many reservoirswould be crippled.

Most cracks, joints and fractures seenin rock outcrops exhibit a regular patternfrom which their distribution and orientation can be determined. Over 30 years

ago, L. U. De Sitter studied aerial photographs of l imestone anticlines andsynclines and found that the fractures onthe crest of the anticlines had a differentorientation to those off the crust. Thiskind of observation eventually led to afracture classification which will bedescribed here only briefly. (For a complete summary of the fracture classification, readers are recommended to readNelson's book on fractures which is citedin the references below ).Geologic Analysis of Naturally Fractured Reservoirs, byRonald A. Nelson, 1985 Published by Gulf PublishingCompany, Houston. Texas. USA. 320ppFundam entals of Fractured Reservoir Engineering, by T. DVan Golf Rachl, 1982 Published by Elsevier ScientificPublishing Company. New York, 710pp.Naturally Fractured Reservoirs, by Roberto Aguilera. 1980Published by PennWell Books, Tulsa, Oklahoma, 700pp.Advanced Interpretation of Wireline Logs , by Oberto Serra,1986 Published bySchlumberger, 295pp.

Fig 2.1: Map showing the location of themajor types of fractured zones within theMiddle East.

Number:!. 1987 17


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The block diagrams (Figure 2.2a, b,and c) show the orientations and distributions of various fracture types. Themost common types of fracture in theMiddle East are Type I and Type II, both ofwhich ca n be found in the same reservoir.However, it appears that only Type Ifractures develop on gently folding anticlines. Type I fractures oc cur as continu o u s , single breaks, or as zones of parallel

sub-fractures across an entire subsurface. These fractures can take the form ofX-shapes, which create a sort of latticepattern along the flanks of the anticline

(see Figure 2.2a). They are sometimescontinuous over large distances alongthe strike of the fold and also verticallythrough several hundred feet of rock.Some Type I fractures can be clearly seenin aerial photographs (such as that onpage 14) whilst others may be almostinvisible to the naked eye.Evidence from the field suggests thatType II fractures only app ear on th e antic

line when the folding has progressed farenough to deform the rocks elastically.

Type II fractures have an orientatiparallel to the anticline's axis, and alfrequently exhibit X-shaped patternGood ex am ple s of Type II fractures cbe observed along the Zagros-Bitlis fobelt but they are rarely as impressive size as Type I fractures. In gen eral, Typfractures are only a few feet long bmuch longer examples have been founHowever, they can extend to great deptand therefore have considerable infence on reservoir permeability.Type III fractures are less common the Middle East. These develop as a resuof shear stresses in the rock and their tyical orienta tion is sh own in Figure 2.2Onc e again, the fractures exhibit an shape pattern. In Type Ilia fractures, tX shap e is vertical, crossing the beddiplan es wh ere as the X sha pe of Type 1fractures is parallel to the bedding plan

Type I

Fig 2.2a: Type I and II fractures - themost common types in the MiddleEast.

Fig 2.2b: Type 111 fracturesType lllb Type

Fig 2.2(a,b,c): Examples of the typesof fractures fou nd in anticlines in theTurkey Oman Pakistan fold belt andin shear fault zones (After Stearns).


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2) Fault'related fracturesFractures can be produced in shear faultzones of which the best examples in theMiddle East can be found in the granitebasement rock of Egypt's Gulf of Suez ZeitBay and Shoab Ali fields. Major shearfault zones also occur between Turkeyand Pakistan. These kinds of fracturesdevelop the sam e orientation a s the faultand so it is a relatively simple task to predict the direction of fluid communicationwithin the reservoir.

Minor displacem ent is com monly seenin shear fractures associated with normalfaulting. The resulting rock permeabilityand porosity depend on the planarity ofthe crack a nd the rock rigidity. Rigid rock,which often has little porosity prior torupture, may have relatively higher permeability after fracture than more porous,plastic rock.Wrench faults usually result in thedevelopment of tension gash es which areoriented at a high angle to the fault plane

strike.3) Salt domes and vertical upliftsIn the interior Arabian Platform, numerous anticlinal reservoirs have beencreated by vertical uplift. In addition , saltdomes along the northern edge of theplatform have caused uplift and fracturing. Although the se features frequentlycontain fractures, they may have only aminor effect on the reservoir's overall permeability. However, in the deep Khuffand pre-Khuff formations, fracture permeability is extremely important. Fractures are also significant in the anticlinalfields of Egypt's Western Desert, in particular the El Alamein Field.

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A MAZE OF FRACTURES: A drorthogonal north-south, east-wesystem above the nothern portionsupergiant G ha war Field in Saubeing examined by Martin WateThese exposed fractures have beenlarged by surface water actio

Fig 2.2c: Fault related fractures

Number H, 1!W 19


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Fracture detectionWe now have a classification for fracturetypes, but is it possible to detect variousfracture types and use this information toour advantage in newly discovered fractured reservoirs?

To distinguish fracture types, we needto be able to analyse the geometry of thefractures intersected by the well. Untilrecently, the only way this could be donewas by core analy sis, which is costly overlarge intervals. For instance, it would beexpensive to core the prolific reservoirsin the Middle East's Asmari Limestone

Fig 2.3: This Formation MicroScannerimage shows Type 1 fractures in a sectionof reservoir in the eastern ArabianPlatform. The fractures are filled u uthconductive fluids an d are therefore seenas dark strips on the electrical images.

Formation, which average 1150ft thick iIran, or the deeper C retaceou s Bangestanreservoirs which average over 2,300fthick. In addition, the removal of fractured cores, intact, poses major problems.However, the re are several tools - thdipmeter, Borehole Televiewer (BHTV*and the Formation MicroScanner (FMS*which enable us to exam ine the geometr

of fractures in the borehole wall.Th e dipmeter is run in conductivmuds and detects open fractures whichshow up on the log as conductivanom alies. The tool has 4 pads , 90apartwhich allow the fracture plane to bdetected at four positions around thborehole. The position of the tool, in timeand space, is continuously recorded tallow accurate determination of the truedip of bedding and fractures. The dipmeter is particularly useful for fracture evalu

SUPER SONIC FRACTURE DETECTIONOne of the best acoustical methods ofdetecting fractures is to look foranomalies in Stoneley wave data. TheStoneley wa ve is on e of a family of wavetypes produced by borehole sonictools. Each wave type can be distinguished by examining particle motionin the formation generated by the passing wave.

Compressional waves, which travelthrough either solids or fluids, induce aparticle motion parallel to the wavedirection. Shear waves move particlesat right angles to the wave direction,and therefore can only travel in solidswhich can support this shearingmotion.The Stoneley wave produces amore complex elliptical motion, withboth shear and compressional components. In addition, the Stoneley waveonly occurs at the interface betweentwo mediums of different elastic properties - fluid and rock in the ca se of theborehole. Therefore, the amplitude ofthe elliptical particle motion dropsquickly with increasing distance awayfrom borehole and hence the Stoneleywave does not ' s ee ' very deeply into thesurrounding formation.

With the Array Sonic tool (AS*), theStoneley wave is generated by pressuretransducers at the base of the tool, andtravels along the borehole wall and isdetected by an array of eight receivers.The presence of the Stoneley wave iseasily identified in the recordedwaveforms as the wave motion isslower than either compressional orshear waves, has the lowest frequency,and suffers least attenuation thusappearing as a large amplitude event.However, fractures in the borehole wall

45

FMS IMAGE

may alter some of the Stoneley wavecharacteristics.A water-filled ope n fracture in thepath of the Stoneley wave w ill result in aloss of wave energy and speed as theStoneley wave 'pushes and pulls' thefluid in the fracture. Similar effects willoccur when the wave encounters interconn ected pores in the rock. In general,fractures produce sharper and morespiked disruptions of the sonicwaveforms than interconnecting poresand these can be clearly seen on

waveform displays of the kind shownin Figure 2.4.


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ation when there is either a high anglebetween the borehole axis and fracturesand/or a high dipmeter tool rotation during logging.Th e Borehole Televiewer (BHTV) is anultrasonic logging tool that records sonicenergy reflected from the borehole wall.The amplitudes of the ultrasonic pulsesare used to produce acoustic images ofthe borehole wall. The BHTV can be runin low density conductive and non-conductive muds and gives 360 boreholecoverage.Th e Formation MicroScanner hasbrought a significant leap forward in fracture detection as it allows us to clearlydetermine the orientation of both largeand hairline fractures. Open permeablefractures, containing conductive drillingfluids, appear as dark lines on the FMSimage while sealed fractures, which areresistive, appear as light coloured lines.

Fracture analysisIn fracture analysis, it is sometimes dif-ficult to distinguish between natural fractures (those that were present in the rockbefore drilling) and induced fractures(produced as a direct result of the drilling) from core and log data. However,electrical imagery is proving to be aninvaluable aid in distinguishing betweenfracture types.

Induced fractures can be caused inthree main ways.a) by hydraulic fracturingb) by therm al fracturingc) due to unloading stressesHydraulic fracturing, which occurs atdepths greater than 2000ft, produces vertical cracks which are oriented along thelines of least principal stress across the

borehole wall . They are commonlytermed double winged fractures.Hydraulic fractures are created whenthe wellbore pressure exceeds the combined reservoir pore pressure and rocktensile strength. Several studies haveshown that the anisotropy of the rock cancontrol the induced fracture orientationat pressures below 200psi. Anisotropicscan be caused by natural fractures and so

it is important to take these into accountbefore embarking on any hydraulic fracture stimulation.Thermal fracturing often occurs duringdrilling and coring operations when largethermal gradients are generated betweenthe hot rock and coole r drilling fluid. Thisresults in tensile fractures appearing inthe rock around the well in a directionperpendicular to the temperature gradient. Serious thermal fracturing can leadto the collapse of the borehole wall .

STONELEY WAVEFORMS46

7500 ft

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Cr'M I

& \

x. ;H^r |\ lf\jf-Tr \i * ii i

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II

+ SLOWNESS (/xs/ft)+ POWER (dB)

Fig2.4: SPOTTHESPIKE: This figure shows a Formation MicroScanner image (left)and a data se t from the Array Sonic tool which ha s been processed to examineStoneley wave events. The central plot shows the Stoneley wave slowness (^slft)plotted in green with the scale increasing from right to left. On the same plot, theStoneley wave power, from a single receiver, is plotted in red with the scale alsoincreasing from right to left. The yellow area indicates where anom alous valuesof power and slowness were recorded.The display on the right shows waveforms measured by a single receiver whichhave undergone filtering to Stoneley wave frequencies. The positive events arecolour-coded according to their amplitude.The Stoneley slowness and power plot show that a zone around 7500ft hashigher Stoneley slowness and lower power than adjacent zones. The powermeasured by the single receiver shows a distinct spike in the middle of this zone,an d a corresponding disruption of the Stoneley wavforms can be seen on the right.This anomaly is a good indication of an open fracture.The existence of an open fracture is confirmed by the section of the FormationMicroScanner image. The dark image of the fracture is clearly visible.

Nu mb er H. l !W 7 21


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FRACTURES: THE KEY TO TURKEY'S OIL

Fig 2.5: Induced fractures often bisect the borehole vertically (as the diagram on theleft indicates) an d therefore appear as vertk al lines on the FMS images. Secondaryfractures sometimes branch out of the main fracture (centre). The plane of theinduced vertical fracture can also have an irregular shape (right).

(EYTOTURKEY'SOIL

Unloading fractures occ ur as a result ofthe reduction in overburden pressure andare found near the surface of unloading.They propagate in a direction perpendicular to the unloading direction andcan b e easily identified in the wellbore orin cores.Numerous unloading fractures occurin cores as a result of the core samplebeing removed from subsurface stresses.However, the same kinds of fractures donot form in the borehole wall and caremust be taken when comparing FMSimages with cores. Figure 2.5 summarises how induced fractures ma>appear on images.

Much of Turkey's present oil production of 50,000 b/d comes from itssoutheast region (north and east ofDiyarbakir), which lies at the northernend of the fractured Zagros-Bitlis foldbelt.

The internal complexity of thenorthern part of the Zagros-Bitlis foldbelt is shown in the cross-sectionbelow. During folding, the shalesabove and below the Cretaceous Mar-din Limestone deform plasticallywhile the more brittle limestone isfractured. This relatively thin Cretaceous limesto ne forms a distinctly imbricated unit with each of the structuralhighs being a prospective reservoir.The main producing area is alongthe southern portion of the section. Inthe central part of the cross-section,the porous Mardin interval has lost itshydrocarbons due to groundwaterflushing. Furthermore, the reservoirquality deteriorates rapidly towardsthe north because intense pressurehas led to recrystallization of the rock.Recent exploration has uncoveredreservoir potential in the Paleozoicinterval.Although found at shallow depths,the faulted anticlines which contain

the oil are difficult to locate. Evenwhen exploration w ells hit their structural targets and find oil, economicproduction may only be possible inreservoirs which contain a well-developed fracture system. Fracturesare equally important in the deeperPaleozoic sandstone targets, such asDevonian Katin Formation and d eeperOrdovician prospects. Oil also migrates along fractures and faults fromthe organic-rich Silurian shales upinto much younger overlying reservoirrocks.

The recent introduction of electricalimagery to southeast Turkey hasrevealed that fracture systems infaulted and folded anticlines can bemodelled using Stearns' fracture classificatio n. Type 1 fractures are oftenfound on the flanks ot asymmetricanticlinal reservoirs. Fault-relatedfracturing occurs in parallel with thethrust and tear faults wh ich cut acro ssthese anticlines. A lesser number ofType II fractures are found along theanticlinal crests but these can be veryimportant to a field's production sinc ethey frequently penetrate deeply intothe Cretaceous carbonate reservoirrock.

Both dipmeter data and electricalimagery allow us to clarify structuralgeometry which may be poorlydefined in seism ic surveys due to variations in rock properties which affectthe velocity of the reflected seismicwaves. Even in appraisal and earlydevelopment wells, dipmeter datahelps to define structure while electrical imagery reveals fracture densityand size - all of which are extremelyimportant to well completion.

By using dipmeter data to definefaults, we can determine the distribution of fault-related fractures andimprove our modelling of large-scalefracture systems. In fact, the tec-tonophysical modelling of structuraldeformation and fracture types promises to be the key to mo delling fracturedistribution throughout anticlinalreservoirs. In Turkey and other area s,successful attempts have been madeat integrating older dipmeter recordswith electrical images and this hasconsiderably improved our understanding of the reservoirs.

BTERTIARYMELANGE NAPPEOPHIOLITE NAPPE_ UPPER CRETACEOUS

_ CRETACEOUS MAR DIN LIMESTONEPALEOZOIC

Fig 2.7: TURKEY'S COMPLEX FOLDS: This is a section across southeast Turkey'smain o il producing area showing the folding and faulting of the fractured MardinLimestone.N BITLIS EXTENSION OF THE ZAGROS FOLD BELT S


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Geom etrical anal/sis of fracturesComprehensive analysis of a fracturedreservoir involves the evaluation of thefracture frequency, distribution, length,width, morphology and orientation. Thefrequency, distribution and length of fractures can be easily evaluated by applyingstatistical approaches to informationdrawn from both acoustic and electricalimages. The fracture morphology(whether the fractures are open, partiallyopen or sealed) can also be seen onborehole imag es althou gh the FMS ismuch more sensitive to such features.

Fracture width is more difficult toquantitatively evaluate and, at present, itis only possible to make general distinctions between relative sizes of fractures.However, hair-line fractures can bedetected by the FMS when there is a largeresistivity contrast between the fractureand surrou nding rock. Calibration of FMSimages with cores from a particular formation may result in a more quantitativeevaluation of fracture width.Fracture geometry evaluation tech

niques using core, dipm eter, BHTV orFMS data are all similar. Figure 2.6 s how show a dipping planar fracture, crossinga circular borehole, produces an ellipsoid section. The trace of this fracture ona plane surface (unfolding the ellipsoid)produces a sine wave as shown in thefigure. The height of this sine wave isdependent on the steepness of the fracture intersected by the borehole.For example, as the fracture dipincreas es, the ellipsoid elongates and thesine wave increases in height. A simplemathematical equation is used to determine the fracture orientation and this isprese nted in Figure 2.6. Whe n theborehole intersects vertical fractures, vertical lines, 180 apart, are prod uce dinstead of the characteristic sine waveshape. However, in this case, fractureorientation can also be obtained usingthe same equation .Conversely, as the fracture dipdecreases , the s ine wave amplitude alsodecr ease s until it becom es flat in the case

of a borehole intersecting a horizontalfracture.

Fracture attitude is calculated usingthree possib le approaches:1) Manual me asurem ent2) Sine wave overlays3) Geologica l Work Station

Manual measurementFracture attitude measurements can onlybe obtained on cores with a special orientation survey (core orientation can alsobe defined using dipmeter and/or deviation data).

The core is first encased in a tube orsheet of clear plastic. The fractures andbedding planes, visible in the core, aretraced onto the surface of the plastic. Theplastic tube or sheet is then oriented and,finally, the fracture and bedding planeattitudes are determined using the equations shown below. Flexible plasticsheets are more practical for storage andfuture reference.

Fig 2.6: This diagram shows wh\ dippingfractures, intersected by a borehole, havea sinusoidal appearance when viewed as2dimensional borehole images.AVFRAGE DEPTHOF FRACTURE

(A + B)/2

DIP ANGLE a = tan

STRIKE =V Borehole d iameter^ /orehole diameter

Dip azimuth at X 90

N E SDIP AZ IMUTH 0

w

3-D VIEW UNFOLDING THE ELLIPSOIDN i i N i h e r S . l i l . S T : : !


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The geometrical analysis of fracturesusing dipmeter data follows a similar procedure. An open, dipping fracture,appears as a conductive anomaly on themicro-resistivity curves. If theseanomalies are connected, they form asine wave which can be used to determine fracture orientation. Care must betaken in analysing deviated well data asthe angle of deviation (borehole drift)needs to be removed in order to obtaintrue dip . The FMS and BHTV are particularly useful as they provide continuousimages wh ich m ake it easy to distinguishbetween different fracture sets.Sine wave overlay techniqueThis is another manual technique fordetermination of fracture dip and strikebut instead of using equations in theanalysis, the process is simplified byapplying sine wave overlays. These aretransparent sheets of plastic upon whichare drawn calibrated sine waves. Theplastic overlay sheet is placed around thecore or on top of the images (see photograph) and the sine wave which best fitsthe fracture pattern is chosen. The dipmagnitude and fracture orientation arethen easily obtained from the calibratedsine wave. As the calibrated sine wavesare constructed assuming planar fractures and perfectly cylindrical boreholes,slight adjustment is needed if the fractures are not perfectly planar. In addition,the computed dip needs to be correctedfor borehole drift.

Geological Work StationThis is the most recent technique appliedto fracture analysis and probably givesthe most accurate results. The Geological Work Station comprises a computerand tailor-made interactive software withwhich it is possible to map and interpretthe borehole trace of any feature. TheGeological Work Station can be used toanalyse features such as fracturesfrom open-hole logs, dipmeter data andFMS images. The Work Station softwareautomatically computes a true fracturedip after first accounting for tool andborehole geometry and deviation.A further a dvan tage of the Geo logicalWork Station is its so-called Expert System Programming Technique which actsas a kind of artificial inte lligence, guidingthe user on how best to interpret geological structures. Geological Work Stationanalysis has increased the speed atwhich fractures can be interpreted and,for the first time, has made routineanalysis possible

The program logic of the GeologicalWork Station has similarities with thesine wave overlay technique in that itassumes the events, including fractures,are planar. The Work Station user firstloads the borehole data and scrolls

through it until a fracture is displayed onthe terminal screen. The computersoftware can then be used to enh anc e thedata and an approp riate vertical exaggeration can be made. The user marks thefracture trace on the images by using ascreen cursor (mouse) .Figure 2.8 show s a dip com putation onFMS images of a layered carbonate sequ

ence in the Middle East. Two fracturescan be seen between 2517.5ft and2518.0ft and the thin grey layers correspond to argillaceous limestone. The dipof the upper fracture could not be calculated because it was only observed byone p ad. The fracture sho uld be d etectedon two pads for a precise geometricalanalysis, which is the reason why morethan o ne run is usually carried out in fractured formations.Using the Geological Work Station, thedip of the fractures and argillaceouslayers were calculated. The user simply

marks the thin layers and fractures byusing the cursor. The Geological WorkStation then generates a best fit plane to

the marked events and computes theirtrue dips. The computed dips are displayed at the point where the fracture andbedding plane intersect the boreholeaxis. In the second app roac h, the Geological Work Station ge nerates several possible planes which fit the events and theuser chooses the best fi t curve.


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(Top left): SINE WAVE OVERLAY:Transparen t sheets of plastic, uponwhich are drawn calibrated sinewaves, are placed over the FormationMicroScan ner images. The calibratedsine wave which best fits the fracturepattern in the image is selected. Fromthis sine wav e, it is then possible tocalculate the fracture dip andorientation.

Fig 2.8: (Top): This shows a typicalGeological Work Station displayduring fracture analysis. The computersuperimposes a series of possible sinewaves on the left-hand FormationMicroScanner image. The operator canthen choose the sine wave w hich bestcorrepond s to the fracture pattern onthe right-hand FMS image.

The Geological Work Station (right),offers the most accurate method ofanalysing fracture geometry.

Number 3, 1987 2 . r


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Depth GR/Borehole Volume SDT Data Formation Analysisby Volume

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8 6 4 0

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WAnhvdrl tI t m i t o n eD o l o m l l oPoro(ty

BIT SIZE T

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FMS Fractures

N o . Of Frec1ure/FtO TO TFRACTURE WIDTHr~w'


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Wh at us e is fracture data?Borehole images now allow the densityand width of fractures to be determinedwithin the boreh ole and this kind of information is currently being summarized infracture logs of the type described inFigure 2.9. But what use are such fracturesummaries?

Fracture logs are of great significancein well completion where it is essential toknow the fracture system, fracture spacing and orientation.The electrical images also provideinformation on the reservoir and othergeological properties that control thecharacter and distribution of fractures,such as shale beds.Fractures and the futureAs confidence in fracture detection andanalysis grows in the petroleum industry,more emphasis is being placed on themodelling of fracture distribution withina reservoir. Such information is provingto be a valuable guide to the positioning,testing and completion of new wells.

Even now, some wells are beingdeviated at an angle which causes theborehole to intersect the greatest numberof fractures and so enhance production.

AN EYE INTO THE BOREHOLE: Photographof induced hydraulic fractures within aborehole. Th e orientation of the fracturescan be determined by referring to thecompass in the centre of the photograph.The irregular shape of the cracks,characteristic of induced fractures, can beclearly seen.

Fig 2.10: FRACTURES AND RESERVOIRBEHAVIOUR : Fracture distribution andorientation has a major bearing on theperformance of a reservoir. Knowledge ofthe fracture systems allows us to decide onthe best way of developing the reservoirand optimizing production.The map shows Type I fractures to bedominant in this anticlinal Middle E astreservoir within the prolific Turkey-Oman -Pakistan fold belt. The Type I fractures

Fracture orientation

I km

geology middle east

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