36 Oilfield Review

Rocks Matter: Ground Truth in Geomechanics

John CookCambridge, England

René A. FrederiksenKlaus HasboHess Denmark ApSCopenhagen, Denmark

Sidney GreenArnis JudzisJ. Wesley Martin Roberto Suarez-RiveraSalt Lake City, Utah, USA

Jorg HerwangerPatrick HooymanDon LeeSheila NoethColin SayersHouston, Texas, USA

Nick KoutsabeloulisRobert MarsdenBracknell, England

Morten G. Stage DONG EnergyHørsholm, Denmark

Chee Phuat TanKuala Lumpur, Malaysia

For help in preparation of this article, thanks to Ben Elbel,Dallas; Ian Walton, Rosharon, Texas; and Smaine Zeroug,Clamart, France. Thanks also to Hess Denmark ApS, DONGExploration and Production A/S, Noreco ASA, and Danoil forcontributing their North Sea case study.ECLIPSE, Petrel, TerraTek, UBI (Ultrasonic Borehole Imager)and VISAGE are marks of Schlumberger.

Stress and pressure act upon every reservoir, wellbore and completion. Drilling,

production and injection processes modify these stresses and pressures, sometimes

to the operator’s detriment. Through advances in geomechanical measurements,

modeling and monitoring, E&P companies are now able to predict and mitigate the

effects of stress and pressure as they change throughout the life of their fields—

from appraisal to abandonment.

Change the stress on a rock and it deforms,altering its volume and geometry, as well as thepaths of fluid flow within. The stress regime of aformation can be impacted by multiple factors,including rock type, depositional settings, regionaltectonics, episodes of erosion or uplift, localseismic disturbances and even tidal variations.The influence of such stressors is furthercomplicated by differences in rock fabric.

The manner in which formations react tochanging stress is becoming a focus of increasinginterest to E&P companies. In-situ reservoirstresses, having reached equilibrium overgeologic time, are altered by the process ofdrilling, production and injection. If drilling- orproduction-induced changes in stress are notanticipated, the challenges and costs of managinga prospect can far exceed an operator’s initialexpectations. To characterize stress, strain anddeformation in their reservoirs, E&P companiesemploy the discipline of geomechanics. Thiswide-ranging field applies solid and fluidmechanics, engineering, geology and physics todetermine how rocks and the fluids they containrespond to force or to changes in stress, pressureand temperature caused by drilling, completionand production.

In the past, most drilling and productiondepartments were not particularly attuned toformation stresses and geomechanics; manyreservoirs were deemed technically straight -forward and had undergone only limited

depletion. But declining resource volumes andfavorable oil prices are prompting operators todrill deeper, more intricate well trajectories, atthe same time that new technologies areextending the lives of mature fields. Operatorstherefore are becoming more mindful ofgeomechanics as they assess drilling andproduction difficulties—especially those whoendeavor to protect their investments inexpensive completions installed in high-pressure, high-temperature, tectonically activeor ultradeepwater prospects.

Failure to appreciate the importance of geo -mechanics may have severe consequences.Excessive mud loss, wellbore instability, casingcompression or shearing, reservoir compaction,surface subsidence, sand production, faultreacti vation and loss of reservoir seal may all be manifestations of stress changes within a formation.

Some operators are forced to react to changesin stress or rock fabric as they drill and producetheir wells. Others are more proactive. Throughcore testing and geomechanical model ing of rockstrength, deformation and stress behavior, theyare engineering better wells and fields. Theseefforts have recently been aided by newlyestablished centers of excellence forgeomechanics in Bracknell, England, and inHouston and Salt Lake City, Utah, USA.

This article describes advances in geo -mechanics laboratory testing techniques,stress-dependent reservoir simulations and

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 36

Autumn 2007 37

monitoring. Field studies performed at theSchlumberger Geomechanics Laboratory Centerof Excellence and the Schlumberger ReservoirGeomechanics Center of Excellence show howthis science is helping E&P companies optimizedrilling and production in increasinglychallenging reservoirs.

Stress in the SubsurfaceThe stresses acting on a formation can vary inorigin, magnitude and direction. Natural, in-situvertical stresses stem primarily from the weightof overburden. Horizontal stresses also have agravitational component that may be enhancedby tectonics, thermal effects and geologicalstructure. However, other factors such as litho -

logy, pore pressure and temperature influencestress magnitude and orientation as well as thedegree to which rock responds to stress.

Stress, a measure of force acting on a givenarea, is made up of normal and shearcomponents. Normal stress (σ) is that which isapplied perpendicular to a plane or rock surface.Shear stress (τ) is applied along the face of the

Majoreffectivestress σ1

Tensile strength Minor effective stress σ3

Uniaxial compressive strength

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 37

plane. Mathematically, there is one orientation oforthogonal axes defining the stress directions forwhich the shear stresses are zero. Thatorientation defines the principal axes of stress,wherein applied stresses are strictly normal.

In situ, these orthogonal principal axes areoften assumed to be oriented vertically andhorizontally (left); however, this condition isoften not the case. The magnitude andorientation of stresses in the earth change withthe structural dip of the formation, which canrotate the orientation of principal stresses fromthe vertical and horizontal orientations, as canthe presence of faults, salt diapirs, mountains orother complex structures.1

In the earth, where deformation is restricted,the three stress components are linked, and anychange of stress in one direction is accompaniedby changes in stress along the orthogonal axes.For example, when continued deposition bringsabout greater burial depths, the resultingincrease in overburden vertical stress cangenerate changes in horizontal stress, dependingon the degree to which the formations are able tospread out laterally. This response is generallyconstrained by the presence of adjacentformations that confine the rock deformation.Differences in formation properties also imposecontrasts in stresses between adjacent litho -logies. Furthermore, formation anisotropy canresult in greater lateral stress in one directionthan in another.

A body of rock responds to applied stressthrough various modes of strain, or deformation,causing changes in volume and shape, oftenaccompanied by changes in rock properties(left). The spectrum of deformation ranges fromreversible, or elastic deformation, to permanent,or plastic defor mation, before eventually endingin failure of the rock. Deformation caused bycompression, tension or shear can result incompaction, extension, translation or rotation,eventually ending in failure by shearing,fracturing or faulting. In addition to themagnitude of stress applied, a rock’s response tostress depends largely on rock type, cementation,porosity and burial depth. In sandstones, thesize, shape and area of contact points betweenindividual rock grains influence deformation. Inlimestones, the shape and strength of theskeletal rock framework influence deformation.2

Small increases in stress generally cause asmall strain from which the rock may recover.

38 Oilfield Review

> In-situ stresses and principal stresses. Stresses on a cube of materialburied in the earth are given the designation σV, σH and σh, where V indicatesvertical, H indicates the direction of the larger horizontal stress, and h that ofthe smaller horizontal stress. For simplicity, it is often assumed that these arethe principal stress directions, but the principal directions of stress can berotated significantly from these three axes. The principal stresses aregenerally indicated as σ1, σ2 and σ3, in decreasing order of magnitude. Whenthe principal stress directions do not coincide with the vertical and horizontaldirections, there will also be shear stresses on the cube faces in theorientation shown.

σV

σV

σHσH

σh

> Stress-strain diagram. Rocks that undergo elastic deformation store strainenergy as their volume changes. When the applied boundary stresses areremoved, the rock returns to its original state of deformation while the strainenergy returns to its original value. With application of greater stress, rocksundergo inelastic deformation as nonrecoverable, internal structural changesoccur (starting at the yield point), such as tensile microcracking, grain crushingor slippage at grain boundaries. These changes result in permanent volumetricdeformation, often referred to as plastic deformation. Higher stresseseventually cause the rock to fail (fracture point), as exemplified by crushing orfracturing of constituent grains and cement or by mineral dissolution.

Stre

ss

Strain

Yield point

Fracture point

Ductile field

Elastic field

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 38

Autumn 2007 39

Beyond a certain point, it will deform plasticallyor fail. The mode of deformation and failure isdictated by the relationship between changes inmaximum and minimum stresses (right). Thisrelationship is called a stress path.3 In petroleumgeomechanics, the stress path (K) isconventionally the ratio of change in effectiveminimum horizontal stress to the change ineffective vertical, or overburden, stress frominitial reservoir conditions during fluid pressure drawdown, more simply expressed as K = Δσ3/Δσ1. This can also be expressed in termsof changes of shear stress (Q) and changes inmean stress (P'), as shown on the P'-Q diagram.4

A sufficiently low stress-path value impliesthat the rock will fail in shear, generating a shearplane. Shear strength increases as lateralconfining stress on the rock increases. Wherelarger stress-path values are seen, the rockundergoes compaction or reduction in porosity.This is most common in soft, high-porosity rockssuch as chalk, porous sands and diatomite.5

When subjected to differential stresses, otherrocks, such as salts, will tend to flow over time toreduce shear stresses and move towardshydrostatic stress states.

To manage reservoirs, oil and gas companiesmust contend with a variety of downholestressors—not all of which are caused by over -burden or tectonics. Pore pressure, tempera turedifferences and chemical interactions can alsoaffect localized perturbations in stress orienta -tion and magnitude.

Stress and pore pressure are intrinsicallylinked.6 In formation pore spaces, stress istransmitted to liquids or gases in the form ofpressure. The magnitude of pressure applied inany one direction is the same for all directions. Ifa fluid is compressed, it reacts by exerting anequal and opposite pressure outwards. Underpressure, pore fluids often take up some of thestress imposed on a formation. Thus pore

pressure is an important component of the netstress applied to a body of rock.

Temperature is yet another contributor to theoverall stress regime. Temperature differencesbetween drilling fluids and downhole formationswill result in heat transfer between the twomedia. Given the low thermal conductivity ofmost rocks, these temperature variationsgenerate large strain gradients that may causesevere fracturing and stress realignments. Sincethermal expansion of water in the pore space ismuch higher than that in the rock matrix, theheat transferred into a formation by drilling fluid will generate a larger volume expansion ofthe pore fluid and a corresponding increase inpore pressure.7

Thermal expansion of the rock matrix underconstrained conditions will generate further

stress. A reduction in effective mud support isoften associated with an increase in porepressure. This reduction, together with thethermal matrix expansion, will lead to a lessstable wellbore condition. Conversely, coolingthe formation may result in a more stablecondition because of decreased pore pressureand tangential stress. The reduction of tangen -tial stress may also lead to a lower hydraulicfracture gradient, and, in extreme cases, thetangential stress will become negative andinitiate hydraulic fracture.

Stress and pore pressure can also be affectedby interactions between rock and drilling fluid.Shales, which account for the majority of drilledsections in most wells, are particularly sensitiveto drilling fluids. Somewhat porous and usuallysaturated with formation water, these rocks maybe susceptible to chemical reactions with certain

1. Addis MA: “The Stress-Depletion Response ofReservoirs,” paper SPE 38720, presented at the SPEAnnual Technical Conference and Exhibition, SanAntonio, Texas, October 5–8, 1997.

2. Geertsma J: “Land Subsidence Above Compacting Oiland Gas Reservoirs,” paper SPE 3730, presented at SPE-AIME European Spring Meeting, Amsterdam, May 16–18, 1972.

3. For more on stress paths: Crawford BR and Yale DP:“Constitutive Modeling of Deformation and Permeability:Relationships between Critical State andMicromechanics,” paper SPE/ISRM 78189, presented atthe SPE/ISRM Rock Mechanics Conference, Irving,Texas, October 20–23, 2002.Rhett DW and Teufel LW: “Effect of Reservoir Stress Path on Compressibility and Permeability ofSandstones,” paper SPE 24756, presented at the SPEAnnual Technical Conference and Exhibition,Washington, DC, October 4–7, 1992.

Scott TE: “The Effects of Stress Paths on AcousticVelocities and 4D Seismic Imaging,” The Leading Edge 26,no. 5 (May 2007): 602–608.Teufel LW, Rhett DW and Farrell HE: “Effect of ReservoirDepletion and Pore Pressure Drawdown on In-SituStress and Deformation in the Ekofisk Field, North Sea,”Proceedings of the 32nd US Rock MechanicsSymposium. Rotterdam, The Netherlands: A.A. Balkema(1991): 63–72.

4. A relationship exists between the stress path, shearstress and mean stress. While the stress path (K) can be expressed as K = Δσ3/Δσ1, shear stress (Q) isexpressed as (Q = σ1-σ3), and effective mean stress (P')is [P' = (σ1+σ2+σ3)/3]. In laboratory uniaxial stress tests,in which the minimum and intermediate principal stressesare considered equal (σ2 = σ3), the slope η in the P'-Qplane corresponding to the stress path K is given by thisequation, following Crawford and Yale (reference 3):

5. Doornhof D, Kristiansen TG, Nagel NB, Pattillo PD andSayers C: “Compaction and Subsidence,” OilfieldReview 18, no. 3 (Autumn 2006): 50–68.

6. Addis, reference 1.7. Choi SK and Tan CP: “Modeling of Effects of Drilling Fluid

Temperature on Wellbore Stability,” Proceedings,SPE/ISRM Rock Mechanics in Petroleum EngineeringSymposium, Trondheim, Norway (July 8–10, 1998): 471–477.Li X, Cui L and Roegiers J: “Thermoporoelastic Analysisfor Inclined Borehole Stability,” Proceedings, SPE/ISRMRock Mechanics in Petroleum Engineering Symposium,Trondheim, Norway (July 8–10, 1998): 443–452.

> Distortion and failure. Distinct modes of distortion and failure can be plottedas a function of shear stress (Q) and mean effective stress (P'). At relativelylow P' and high Q, rock failure typically occurs as localized shear along aplane oriented at an angle to the principal stress axes. At relatively high P'and low Q, rocks may undergo compaction or pore collapse. (Adapted fromScott, reference 3.)

Dilation

CompactionNear-elastic region

Mean effective stress (P'): (σ1 + σ2 + σ3) / 3

Shea

r stre

ss (Q

): σ 1 –

σ3

Impossiblestates

Critical

state

line

Compaction surfaceShear failure surface

Tens

ile

failu

re su

rface

Ductile failure surface

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 39

drilling fluids. When a formation is drilled withan incompatible fluid, the invading filtrate maycause the shale to swell, which can lead toweakening of the rock and wellbore instability.Shales may also be susceptible to time-dependent changes in effective mud supportcaused by differences between the mud pressureand pore-fluid pressure, or between drilling fluidsalinity and formation salinity.8 Furthermore,volume changes in shales arising from inter -actions between shale and drilling fluid canlocally disturb the stress orientation andmagnitude in a borehole.

Thus, while local and regional tectonicstresses play a major role in rock deformation,other downhole factors, such as pore pressure,mud weight and downhole pressure fluctuations,temperature and chemistry must also beconsidered for their distinctive contributions tothe local stress-deformation continuum. Theireffects may also be tempered by textural proper -ties unique to the local lithology, such as the sizeand distribution of framework grains and pores,mineralogy and the composition of diageneticcements. Given the variety of reactions to stress,it is crucial for an operator to know as much aspossible about the rock surrounding a wellboreand the conditions to which it will be subjected.

Changes in Stress Drilling and production activities affect localstress regimes. Problems encountered duringdrilling may portend difficulties encounteredsubse quently during the production phase.Changes in stress may result in rock failure thatcauses wellbore instability during drilling. Thesechanges may later lead to sand production oncethe well has been completed. Other activitiesduring the life of a field can cause pore pressureand temperature changes, which can modifystresses acting farther from the wellbore. Stresschanges affect not only the reservoir but alsoadjacent formations.

Drilling activity perturbs the initial equilib -rium of stresses in the near-wellbore region. Asa cylindrical volume of rock is excavatedthrough drilling, the stresses formerly exertedon that volume must instead be transferred tothe surrounding formation. This process createstangential, or hoop stresses, which must beborne by the rock surrounding the borehole.These wellbore stresses are a function of mudweight, wellbore inclination, formation dipangle and azimuth, and the magnitude andorientation of far-field stresses (σV, σH and σh).Hoop stress varies strongly as a function of

borehole radius and azimuth.9 Furthermore, itcan far exceed σH (left).

In most conventional drilling operations,drillers use hydraulic pressure from drilling fluidas a substitute for the mechanical support that islost through the cylindrical volume of rockexcavated while drilling a wellbore. They essen -tially replace a cylinder of rock with a cylinder ofdrilling fluid. However, mud pressure is uniformin all directions, and cannot balance againstoriented shear stresses in a formation. As stressis redistributed around the wall of the wellbore,shear stresses can exceed rock strength. When thishappens, the wellbore will deform or fail entirely.

Typical examples of geomechanics-relateddrilling problems include wellbore instability andfracturing of the formation. Ramifications includefinancial loss resulting from lost circulation,kicks, stuck pipe, additional casing strings,sidetracks and even abandonment. To sustainwellbore stability, operators must develop drillingand well construction plans that consider stressmagnitude and direction, mud weight, trajectoryand pore pressure before, during and after a wellis drilled.

Drillers manage pressures imposed by mudweight to avoid wellbore stability problems.Their control of wellbore hydraulics reflects a petroleum engineering approach to ageomechanical problem. During drilling, well-bores can be compromised through a variety ofmud-induced modes of failure:10

• Tensile failure occurs by increasing mud pres-sure until it causes the wellbore wall to go intotension and eventually to exceed the rock’stensile strength. This fractures the rock alonga plane perpendicular to the direction of mini-mum stress, often resulting in lost circulation.

• Compressive failure may be caused by mudweight that is too low or too high. In eithercase, the formation caves in or spalls off, pro-ducing borehole damage and breakouts (nextpage, top). Unless the wellbore is properlycleaned out, the accumulation of breakoutdebris can lead to stuck pipe as the boreholepacks off or collapses.

• Shear displacement takes place when the mudpressure is high enough to reopen existingfractures that the wellbore has intersected. Asa fracture is opened, stresses along the open-ing are temporarily relieved, allowing oppos ingfaces of the fracture to shear, creating a smallbut potentially dangerous dislocation along the wellbore.

Wellbore stability is further affected bystructural factors, such as the interplay betweenwellbore inclination, formation dip and

40 Oilfield Review

> Plan view of hoop stresses surrounding a vertical wellbore. In this model,pore pressure and wellbore pressure are equal, while maximum and minimumeffective stresses within the formation equal 2,000 psi and 3,000 psi [13.8 and20.7 MPa], respectively. However, hoop stress, which varies as a function ofradius and azimuth, is strongly compressive along the azimuth aligned withminimum horizontal stress (σh) (red shading above and below the wellbore),where it reaches almost 7,000 psi [48.3 MPa]. Wellbore failure will be morelikely to occur along this axis. (Adapted from Sayers et al, reference 9.)

σH = 3,000 psiσH = 3,000 psi

σh = 2,000 psi

σh = 2,000 psi

2,000 3,000 4,000 5,000

Hoop stress, psi

6,000

Wellbore

7,000

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 40

Autumn 2007 41

directional variations in strength between andalong formation bedding planes (below right). Itis not unusual for some degree of wellbore failureto occur in vertical wells that encounter steeplydipping shales, or inclined wells that intersectshale bedding planes at low angles. Such failuresare initiated by low shear and tensile strengthalong planes of weakness in shales.11

The issue of strength, or a rock’s capacity towithstand stress, points to an important under -lying influence on deformation and failure: thatof rock fabric.12 Rock fabric can dictate whethera given amount of stress will cause a rock todeform or to completely fail, and can influencethe extent and orientation of fractures orbreakouts in a wellbore. Thus, although boreholebreakout is typically assumed to be orientedalong the axis of least stress, the bedding,cementation, mineralogy and grain size of a rockmay actually redirect the course of a breakoutalong the rock’s weakest points.

For help in anticipating and circumventingproblems such as those described above, some

8. Gazaniol D, Forsans T, Boisson MJF and Piau JM:“Wellbore Failure Mechanisms in Shales: Prediction and Prevention,” paper SPE 28851, presented at the SPE European Petroleum Conference, London, October 25–27, 1994.Mody FK and Hale AH: “A Borehole Stability Model toCouple the Mechanics and Chemistry of Drilling FluidInteraction,” in Proceedings, SPE/IADC DrillingConference, Amsterdam (February 22–25, 1993): 473–490.Tan CP, Rahman SS, Richards BG and Mody FK:“Integrated Approach to Drilling Fluid Optimization forEfficient Shale Instability Management,” paper SPE48875, presented at the SPE International Oil and GasConference and Exhibition, Beijing, November 2–6, 1998.van Oort E, Hale AH and Mody FK: “Manipulation ofCoupled Osmotic Flows for Stabilization of ShalesExposed to Water-Based Drilling Fluids,” paper SPE30499, presented at the SPE Annual TechnicalConference and Exhibition, Dallas, October 22–25, 1995.

9. Sayers CM, Kisra S, Tagbor K, Dahi Taleghani A andAdachi J: “Calibrating the Mechanical Properties and In-Situ Stresses Using Acoustic Radial Profiles,” paperSPE 110089-PP, presented at the SPE Annual TechnicalConference and Exhibition, Anaheim, California, USA,November 11–14, 2007.

10. For more on wellbore stability problems: Addis T, Last N,Boulter D, Roca-Ramisa L and Plumb D: “The Quest forBorehole Stability in the Cusiana Field, Colombia,”Oilfield Review 5, no. 2 & 3 (April/July 1993): 33–43.

11. Aoki T, Tan CP and Bamford WE: “Stability Analysis ofInclined Wellbores in Saturated Anisotropic Shales,” in Siriwardane HJ and Zaman MM (eds): ComputerMethods and Advances in Geomechanics: Proceedingsof the Eighth International Conference on ComputerMethods and Advances in Geomechanics, Morgantown,West Virginia, USA, May 22–28, 1994. Rotterdam, TheNetherlands: A.A. Balkema (1994): 2025–2030. Yamamoto K, Shioya Y, Matsunaga TY, Kikuchi S andTantawi I: “A Mechanical Model of Shale InstabilityProblems Offshore Abu Dhabi,” paper SPE 78494,presented at the 10th Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, October 13–16, 2002.

12. Rock fabric is a term that loosely encompasses themineral content, size, shape, orientation andcementation of component grains within a rock,including their overall arrangement into microscopiclaminations or larger beds.

> Formation effects on wellbore stability. Structural and stratigraphic factorscan combine to cause well damage. Here, incompetent beds overlie a strongerformation near the crest of a structure; relative movement results in damagedcement and collapsed casing.

> Borehole breakout. Results from a UBI Ultrasonic Borehole Imager loggingtool show the extent of stress-related damage in a wellbore. In isotropic ortransversely isotropic rock, where rock properties do not change along theplane of the wellbore, such damage is generally aligned along a plane ofleast horizontal stress.

5,321

5,322

5,323

5,324

–50

5–5

0

5

Radius, in.

Dept

h, ft

5,325

5,320

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 41

operators are turning to geomechanics experts atthe Schlumberger Center of Excellence for PorePressure Prediction and Wellbore StabilityAnalysis. Located in Houston, the geomechanicsexperts in this group have a global reach, andsupport operators around the world. Thisinterdisci plinary team is actively involved inhelping clients mitigate risk in drilling,completing and producing wells in difficultgeomechanical environments, such as deepwaterexploration, subsalt drilling, unconventional gasand unconsolidated reservoirs.

Beyond the WellboreGeomechanical influences can extend past theborehole, into the reservoir and beyond—thoughtheir extent may not be recognized until areservoir is produced. The pressure sink createdby a well to induce production will result in lowerwellbore pressures than the pore pressure of thesurrounding formation, and this difference canincrease the risk of rock failure.13

With the withdrawal of reservoir fluids duringproduction, the overburden load borne by porefluids must be transferred to the rock frameworksurrounding the pore space. Resulting changes inpore pressure will prompt adjustments in totalstress and effective stress. Within the rock,increased loading will cause varying degrees ofdeformation or failure, evidenced by grain sliding

and rotation, plastic deformation, cementbreakage at grain contacts, or activation ofexisting fractures.14

On a larger scale, production-induced stresschanges on the rock framework can lead to porecollapse and compaction of the reservoir.15

(Compaction is not always a problem, however—compaction drive has helped to pressurize oil insome reservoirs, thereby increasing productionrates and improving ultimate recovery.)16 As aresult, operators have had to contend withsurface subsidence problems, deformation orshearing of wellbore tubulars and buckling ofcompletion components. Other effects range from reduction of porosity and permeability tofault reactivation, formation fracturing, sandproduction or loss of reservoir seal.

The effects of geomechanics are especiallypronounced in gas storage operations, where thecyclic process of injecting and withdrawing gas toor from a reservoir provokes changes in fluidpressures within reservoir pore spaces. Thesepressures cushion the stresses acting on the rockmass, but the pressures increase or decreasewith injection and withdrawal. The loads actingon the rock matrix thereby decrease andincrease in response to these cycles. Althoughtotal overburden stress may remain constantthroughout these cycles, the total horizontalstresses acting throughout the reservoir can vary

with pressure, generally decreasing as the gas iswithdrawn. If induced stresses exceed the elasticlimits of the rock, porosity and permeability maybe permanently reduced, along with reductionsin overall storage capacity. Furthermore, as thesurrounding rock adjusts to the isostaticimbalance caused by pressure cycling and stresschanges, nearby faults may be reactivated.17

Production-induced changes can also affectthe rock beyond the productive areas of areservoir. Even in producing formations, reser -voir attributes such as porosity and permeabilitycan vary, giving rise to nonuniform drainage anddepletion. As a reservoir is produced, the rockmay eventually compact, leaving surrounding,undrained areas of the formation to compensatefor changes in pressure and displacement of theadjacent rock. Above the productive formation,compaction will lead to changes in the over -burden, as described later in this article.

Changes in stress imposed on a producinghorizon can put the rock out of equilibrium withits surroundings. The result is a correspondingtransfer of stress between the depletingreservoir or injection interval and the rockimmediately surrounding the reservoir. Ensuingrock deformations may compromise theintegrity of existing completions within thereservoir and overburden (above left). Thesignificance of production-induced stresschanges and their potential to adverselyinfluence field operations, production andeconomics will depend on mechanicalproperties of the rocks, natural fractures andfaults.18 To understand and anticipate suchchanges in the wellbore and beyond, operatorsare increasingly turning to advanced geome -chanical testing and modeling techniques.

42 Oilfield Review

13. Cook J, Fuller J and Marsden JR: “GeomechanicsChallenges in Gas Storage and Production,” presentedat the United Nations Economic and Social Council:Economic Commission for Europe: Working Party onGas: Proceedings of 3rd Workshop on Geodynamic and Environmental Safety in the Development, Storage and Transport of Gas, St. Petersburg, Russia,June 27–29, 2001.

14. Sayers CM and Schutjens PMTM: “An Introduction toReservoir Geomechanics,” The Leading Edge 26, no. 5(May 2007): 597–601.

15. Doornhof et al, reference 5. Sayers C, den Boer L, Lee D, Hooyman P andLawrence R: “Predicting Reservoir Compaction andCasing Deformation in Deepwater Turbidites Using a 3DMechanical Earth Model,” paper SPE 103926, presentedat the First International Oil Conference and Exhibition,Cancun, Mexico, August 31–September 2, 2006.

16. Andersen MA: Petroleum Research in North Sea Chalk,Joint Chalk Research Monograph, RF-RogalandResearch, Stavanger, 1995.

17. Cook et al, reference 13.18. Marsden R: “Geomechanics for Reservoir

Management,” in Sonatrach-Schlumberger WellEvaluation Conference – Algeria 2007. Houston:Schlumberger (2007): 4.86–4.91.

> Stress changes induced by production. As a field depletes, the magnitudeof stresses may alter drastically. Under such conditions, a completion orperforation originally oriented in the most stable direction at the onset ofproduction may subsequently become unstable and fail as productionproceeds. Here, the horizontal perforation will permit the greatest safedrawdown (blue curve) and solids-free production. However, as the fielddepletes and stresses change, this previously stable perforation willcollapse and the vertical perforation will assume a greater role inproduction, though safe drawdown pressure has decreased (red curve).(Adapted from Marsden, reference 18.)

Safedrawdown

Depletion

Wel

l pre

ssur

e, p

si

Reservoir pressure, psi

0 3,000 6,000 9,000 12,000 15,000

15,000

12,000

9,000

6,000

3,000

0

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 42

Autumn 2007 43

Measuring Ground TruthDespite years of geomechanical analysis, manyE&P companies continue to experience drilling-or production-induced problems. However, thefield of geomechanics involves much more thananalysis of stress. Though changing stress fieldscan wreak havoc on drilling and productionplans, the orientation or magnitude of stressesand strains have little significance withoutframing such measurements in the context of therock itself. And rocks are highly variable. Otherproblems are caused, in part, by oversimplifiedcharacterization of rock behavior, and by limitedmodeling and analysis capabilities compoundedby a lack of comprehensive rock property data.

These issues are being addressed by theTerraTek Geomechanics Laboratory Center ofExcellence in Salt Lake City, Utah. TerraTek, Inc.was acquired by Schlumberger in July 2006 (see“Geomechanics Laboratory: Testing UnderExtreme Conditions,” page 44). The modernhigh-pressure testing systems and techniquesdeveloped at the TerraTek facility evolved froman effort to characterize and predict groundmotion and crater development in response tonuclear tests. Evaluation of these tests could notbe performed without rock property measure -ments obtained under high pressure. Measuringthese properties was very difficult, and spawned a number of technical breakthroughs by TerraTek.

Highly accurate load-deformation measure -ments were essential, requiring measurementsinside test vessels under extreme pressures.TerraTek scientists conducted research tomeasure rock properties to pressures of150,000 psi [1,034 MPa]. The TerraTek high-pressure rock property data enabled analysis ofthe magnitude of ground motions caused by anuclear event.

TerraTek researchers carried out tens ofthousands of tests on rocks under high pressure.Their testing capabilities were subsequentlyapplied to other geomechanics investigations,including geother mal energy recovery, coalmining, deep geologic nuclear waste storage,underground energy storage, as well as oil andgas recovery. Today, the TerraTek GeomechanicsLaboratory Center of Excellence regularlyconducts rock tests for deep wells, achievingpressures of 30,000 psi [207 MPa], or to higherpressures of 50,000 to 60,000 psi [345 to 414 MPa]when required for drilling-rock destruction orperforation analysis. In addition to high-pressuregeomechanics testing capabilities, the TerraTekfacility conducts large-scale drilling andcompletions performance testing.

Specialized geomechanics laboratory testingprovides crucial data for wellbore and comple -tion design and for reservoir management. Thiswas not always the case. Traditional engineeringanalysis of reservoir potential and productivitytended to overlook the heterogeneity of reservoirrock. Although heterogeneity may have beencaptured in mud logs and core photographs, orinferred from logs of various petrophysicalproperties, these characteristics were notreflected in simplified homogeneous systemscreated for reservoir and geomechanical models.

Properties related to reservoir rockmechanics were often characterized as uniformthroughout all locations and for all orientationswithin a particular geological unit. This approachinevitably led to underestimations of the role ofmaterial properties in geomechanics. Theindustry, however, is coming to realize that therock matters, and that its varying propertiescannot be ignored in geomechanical analysis.

Further complicating the evaluation processis the fact that each stage of reservoir analysis—from predrill geological studies, throughexploration, to reservoir modeling andproduction—tends to be evaluated in isolation,

and without a common reference for scale. Untilrecently, there has been no framework to makethe process consistent for every stage. However,the development of continuous property profilingand multidimensional cluster analysis of welllogs now provides a uniform scale of referencefor incorporating heterogeneity during allaspects of reservoir analysis and evaluation.

Continuous Profiling—Scratch testing,known formally as continuous profiling ofunconfined compressive strength, provides aquantitative means of evaluating variability instrength, texture and composition of coresamples. By association, this variability may berelated to other rock properties. Scratch testinghas become critical in correctly defining faciesand heterogeneities that would be difficult orimpossible to observe by geologic description orlog characteristics alone. Digital photographs ofthe core, in conjunction with scratch testing,allow visualization of textural heterogeneity andassociated strength heterogeneity (below).

When continuous-strength profiling iscombined with cluster analysis of well logs, it

> Overlay of a core photograph and scratch test results. A scratch test uses a sharp point that is pulledalong the core with a fixed force to press it into the core’s surface. The depth of the scratch, as anindicator of rock strength (red curves), can be correlated to mechanical properties of the rock. Coredintervals exhibiting visually similar properties (same shades of gray, points A and A’) may have differentstrengths, while other intervals exhibiting different visual properties (lighter and darker shades of gray,points B and B’) have equal strengths. Variability in mechanical strength along the length of the core ishigh, ranging from 8,000 psi to 23,000 psi [55 to 159 MPa] within just 8 contiguous feet [3.6 m] of core.

0 ft 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

ft 1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

ft

AA'

B B'

0 psi

50,000 psi

0 psi

50,000 psi

0 psi

50,000 psi

0 psi

50,000 psi

(continued on page 48)

60527schD6R1.qxp:60527schD5R1 1/8/08 9:21 AM Page 43

44 Oilfield Review

The TerraTek facility in Salt Lake City, desig-nated as the Schlumberger GeomechanicsLaboratory Center of Excellence, investigatesthe impact of geomechanics on a wide rangeof exploration and production applications.The range of applications also provides insightinto the kinds of problems that operatorsmust try to circumvent: • Well construction and completion: evaluate

wellbore stability and the potential for sandproduction and perforation collapse; analyzemultilateral junctions and evaluate stabilityof conventional and expandable liners.

• Completion and stimulation design: deter-mine optimal completion alternatives basedon rock mechanical and physical properties;investigate options for delayed gravel pack-ing and oriented perforating; optimizestimulation treatment design.

• Long-term production behavior: investigatestress regimes contributing to reservoir compaction during production; predict surface subsidence and subsequent loss ofreservoir permeability; analyze fines gener-ated during the compaction process, alongwith associated skin damage; evaluatepotential for casing collapse.

• Overburden rocks: test for compatibilitybetween drilling fluids and shales; optimizeselection of drilling fluids; evaluate poten-tial for delayed shale failure caused bymud-shale interactions; analyze thermaleffects that cause delayed shale failure.

• Exploration and frontier drilling operations:develop field and laboratory correlations forpredicting mechanical properties and in-situstress prior to and concurrent withexploratory drilling activity.Testing is conducted in different specialized

laboratories, depending on available test mate-rial, client specifications and research efforts.Many large-scale tests are carried out in thecompletions laboratory. One of the moreprominent features of this facility is its large-block polyaxial stress frame. The stress frameprovides a controlled environment for monitor-ing rock responses during pseudostatic testing.

In this setting, researchers can measure defor-mation parameters while simultaneouslymeasuring dynamic responses of rock samplesto different load rates and magnitudes. Thelarge-block stress frame can be configured tosimulate a variety of downhole pressures andconditions. Large-block testing applicationsrange from wellbore stability analysis to evalu-ating sanding potential, liner and screenloading, perforating effectiveness and hydraulicfracturing simulations.

Located inside a pit, the exterior of thestress frame is formed by a series of steelrings. These rings are stacked to encase an

internal chamber that can accommodateblocks of rock measuring up to 30 x 30 x 36 in.[76 x 76 x 91 cm]. The chamber is sealed withsteel platens that are bolted over 12 large tierods (above).

Pairs of bladder-like devices, called flatjacks,are placed on opposite sides of the sample toapply independent triaxial loading in each ofthe three principal stress directions. The threepairs of flatjacks are internally pressurized,with one surface of the flatjack reacting againstthe face of the rock, and the other surfacereacting against the wall of the internal cham-ber of the stress frame, or its platen.

Geomechanics Laboratory: Testing Under Extreme Conditions

> Large-block polyaxial stress frame for simulating downhole conditions.Here, a worker lowers a steel platen while preparing to seal the test chamber.

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 44

Autumn 2007 45

A maximum stress of 8,000 psi [55 MPa] can beapplied in all three directions, with a maximumdifference of 2,000 psi [13.8 MPa] between thetwo horizontal stresses. Each stress can be con-trolled independently.

The stress frame also has the capability tocontrol pore pressure within a sample. In suchtests, the rock sample is encased in a thinsteel canister. Thick rubber sheets are placedat the top and bottom surfaces of the rock toact as pore-pressure fluid seals. A porousproppant pack placed around the block estab-lishes a constant pressure boundarycondition. Custom software controls each ofthe three principal stresses, along with porepressure and wellbore pressure. The softwarecan be programmed to keep a constant effec-tive stress on the sample block at all times.

Some experiments require a simulated permeable zone bounded above and below byimpermeable formations. In these kinds of

tests, a servo-controlled injector is used tosupply the fluid at either a constant rate or aconstant pressure. The injected fluids canrange from brine to drilling mud to variouscompletion fluids. The injection can simulatea scaled or actual-sized wellbore.

For smaller samples, a medium-sizedpolyaxial stress frame is used (above left).This device is often used for studying acidfracturing and other stimulation techniques,providing a wide range of testing capabilities.

Another unique testing facility is the rockmechanics laboratory, where 14 stress framesare used to test cylindrical samples with diam-

eters ranging from 0.5 inches [12.7 mm] to6 inches [152.4 mm]. Testing on a smallerscale can also provide valuable insights intorock characteristics.1 A special triaxial testframe has been designed to measure rockstrain as well as its effects on seismic veloci-ties (above right). Ultrasonic velocities,

1. Laboratory capabilities include an extensive variety of tests—unconfined compression, uniaxial-straincompression, triaxial compression, multistage triaxialcompression, controlled constant stress path, thick-walled cylinder (with and without radial fluid flow andmeasurements of produced sand), and tensile strengthtests, as well as testing with concurrent ultrasonicvelocity and acoustic emissions measurements—along with many customized test programs andresearch efforts.

> Polyaxial stress frame. This device canaccommodate rock samples measuring up to12 x 12 x 16 in. [30 x 30 x 41 cm].

> Instrumented sample for triaxial testing. This test-frame assembly is used to measure radialand axial strains, along with compressional and shear wave velocities. In this configuration, bothpseudostatic and dynamic elastic properties are determined concurrently under simulated in-situstress conditions. Here, a core consisting of alternating light and dark layers of siltstone andmudstone is subjected to ultrasonic pulses to test seismic responses in the rock. The sample issealed with a clear polyurethane jacket that prevents fluid communication between the confiningfluid pressure and the pore pressure. These test frames can also perform uniaxial straincompaction testing, thick-walled cylinder testing and other specialized stress paths up totemperatures of 200°C [392°F]. Axial force up to 1.5 x 106 lbf [6.7 MN] can be applied to samplesup to 6 in. [15 cm] in diameter. Confining pressure and pore pressure are monitored withconventional pressure transducers with pressure limits of 30,000 psi [207 MPa]. Another systemin this laboratory can attain 60,000 psi [414 MPa].

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 45

46 Oilfield Review

obtained in combination with deformationmeasurements of axial and radial strain, pro-vide information on static and dynamicmechanical properties that can be correlatedto well-log data.

The triaxial test frame holds a core samplebetween polished, hardened-steel end-caps. The sample, measuring 1 in. [2.5 cm] in diame-ter by 2 in. [5 cm] in length, is jacketed by animpermeable membrane. Axial and radial can-tilever-beam sets are mounted to the sample tomeasure displacements when the sample issubjected to stress and pressure. The axialstrain cantilever set is attached to the upperend-cap and measures axial displacementthrough deflection on the base cone attachedto the bottom end-cap. The radial-strain can-tilever set consists of a ring with fourstrain-gauge arms, which measure radial dis-placement at four points, forming twoperpendicular directions at the midpoint of thesample. The bottom end-cap rests on an inter-nal load cell, and the axial stress is calculatedfrom measurements of force on the internalload cell. During testing, data are corrected forelastic distortion of the end-caps and forstrains associated with the jacketing material.

The end-caps also contain ultrasonic trans-ducers. Ultrasonic velocity measurements areperformed with piezoelectric transducers thattransform electrical pulses into mechanicalpulses and vice-versa. Compressional andshear pulses are generated by a pulse genera-tor that applies a high-voltage, short-durationelectrical pulse at an ultrasonic frequency toone of the piezoelectric transducers. Thispulse is transmitted through the rock samplein the form of an elastic wave. The receivingtransducer at the opposite end of the rocksample transforms this elastic wave into anelectric signal, which is captured on a digitaloscilloscope. The P-wave and S-wave velocitiesare calculated on the basis of the time requiredfor the compressional or shear pulses to travelthrough the specimen.

The instrumented test sample is nextplaced inside a pressure vessel. The pressurevessel is then filled with either mineral spir-its or oil to apply confining pressure. Axialstress, axial strain, radial strain and confiningpressure are all measured and controlled

during each test. Depending on testing objec-tives, these tests may be performed with porefluids drained to atmospheric pressure, orwith pore fluids undrained. Temperatures canalso be increased to better approximateactual in-situ conditions.

The triaxial test frame permits measure-ments to be taken at different orientationswith respect to bedding planes. Using thesemeasurements, the failure envelope of therock sample can be defined as a function ofstress orientation to bedding; in addition,anisotropic properties of the rock can bedefined. This information is essential for pre-dicting wellbore stability, evaluating in-situstress and designing hydraulic fracture pro-grams for strongly anisotropic formationssuch as those found in unconventional tightgas shales.

Ultrasonic velocities, obtained in combina-tion with deformation measurements of axialand radial strain, provide information onstatic and dynamic mechanical propertiesthat can be correlated to well-log data. Ultra-sonic wave velocities in sandstones, particu-larly those that are poorly consolidated, arestrongly dependent on stress; thus, stresschanges can be calibrated to seismic velocitymeasurements. Other, more consolidatedrocks, such as tight sands and tight shales,exhibit an entirely different behavior. Wavevelocities in these rocks are virtually inde-pendent of stress, so changes in measuredseismic velocities can be attributed to otherphenomena such as anisotropy.

Early knowledge of rock behavior was basedon testing of homogeneous and isotropicmaterials; early models reflected this simplic-ity. New opportunities, such as unconventionalhydrocarbon plays, are emerging, and callattention to the true nature of the rocks inwhich they are based. Platforms such as thetriaxial test frame provide data that are fun-damental for developing new models to honorthe heterogeneous, anisotropic nature of com-plex formations.

The TerraTek facility is also called upon totest new drilling, completion and stimulationtechnologies, including evaluation of drillingfluids and bits at high pressures. Althoughcapabilities exist for measuring individual rock

properties or fluid properties at extreme tem-peratures and pressures, determining themanner in which complex rock cutting andbreakage mechanisms interact in the presenceof drilling fluids at great depth is much moredifficult. To accommodate large-scale geome-chanics testing, the drilling laboratory isequipped with a wellbore simulator capable ofreproducing pressure conditions at reservoirdepth while also accommodating the flow ratestypically required to drill in extreme environ-ments (above).

> TerraTek wellbore simulator. The full-scaledrilling rig and wellbore simulator can beconfigured to test the performance, wear,deviation and dynamics of full-size drill bits inoverbalanced or underbalanced conditionsand at simulated depths. A triplex mud pump,equipped with a special high-pressure fluidmanifold, can produce wellbore pressures upto 11,000 psi [75.8 MPa] to simulate high-pressure drilling conditions. Effects of variousfluids on drilling performance, bit balling,formation damage, coring and core invasionare also investigated here.

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 46

Autumn 2007 47

The TerraTek wellbore simulator was cen-tral to a recent high-pressure drilling studysponsored by the US Department of Energy(DOE) joint industry program, called DeepTrek. The facility was contracted to providefull-scale laboratory tests of drill bits and pro-totype drilling fluids at 10,000-psi [68.9-MPa]borehole pressure—substantially higher pres-sures than any previously studied. Resultsfrom these tests may influence the economicsof deep drilling.

The study demonstrated that drilling ratesof penetration (ROPs) can be increased indeep-well applications using advanced bit anddrilling fluid designs. Although previous stud-ies have shown that ROP usually falls withincreasing borehole pressure, these earlierstudies did not account for certain mecha-nisms that affect ROP at great depth, such astype of drilling fluid, weighting material andspurt loss.2

Another common wellbore stability probleminvolves borehole breakouts. Although break-outs often occur during drilling, they can alsoaffect the completion process. In one break-out investigation, TerraTek engineers drilledan 81⁄2-inch [21.6-cm] borehole in a large sandstone core. The core was subjected toincreasing rates of confining pressure in thelaboratory. The resulting borehole breakoutwas similar to that produced in actual well-bores when drilling fluid weights are too low(above left).

The sample was subsequently used for anexpandable sand screen (ESS) mechanicalintegrity test. The screen and basepipe assem-bly was compliantly expanded to the boreholewall and partially into the breakout zone.Results from this test showed how far thescreen could be expanded into the boreholebreakout, in addition to determining the col-lapse load resistance of the ESS product.

Other problems that adversely impactdrilling performance, such as vibration orborehole spiraling, are identified throughexamination of drilling patterns (left).Through the aid of the borehole simulator,researchers have an opportunity to closelystudy bottomhole patterns that would other-wise not be accessible.

2. Spurt loss is an instantaneous loss of a volume of the liquid component of drilling fluid as it passesthrough the borehole wall prior to deposition of competent filtercake.For more on ROP testing: Judzis A, Bland R, Curry D,Black A, Robertson H, Meiners M and Grant T:

“Optimization of Deep Drilling Performance;Benchmark Testing Drives ROP Improvements for Bits and Drilling Fluids,” paper SPE/IADC 105885,presented at the SPE/IADC Drilling Conference,Amsterdam, February 20–22, 2007.

> Breakout simulation. With no drilling mud used to drill thissandstone subjected to increasing confining pressure, thissimulated wellbore progressively broke down, producing a classicborehole breakout pattern.

> Bottomhole drillbit patterns. Bottomhole impressions trackperformance of a bit as it drills a borehole through high-strengthsandstone. In this case, a polycrystalline diamond compact bit wasdrilling with a 16-lbm/gal (ppg) [1.9-g/cm3] oil-base mud at 10,000-psi[68.9-MPa] wellbore pressure. The patterns on the bottom weresubsequently studied to determine how various drilling conditionsaffected drilling performance. As depth of the rings decreases, sodoes the cutting efficiency of the bit, and hence the ROPdecreases. With different drilling fluids, the patterns sometimesdisappear altogether.

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 47

provides fundamental relationships for upscalingor downscaling, and so is a powerful tool for core-log integration.

Cluster Analysis—Cluster analysis defineslog-scale heterogeneity, based on multidimen -sional analysis of log responses (above left). Thistechnique uses detailed algorithms to distinguishsimilar and dissimilar patterns of log responses.Because this technique interprets the combined

effect on all measurements, it is able torecognize small but consistent variations incombined log responses. As applied to heteroge -neous distributions of material properties,cluster analysis also provides a relevant scale formanipulating property variability in subsequentevaluation steps throughout a project.

Cluster Tagging—The application of clusteranalysis can be extended to multiple wells,providing comparisons between the cored, orreference, well and other wells in a field. Detailsobtained through cluster analysis of one well canbe used to recognize similar traits in adjacentwells through a process known as cluster tagging.

Cluster tagging begins with log-responseclusters defined over discrete cored intervals in areference well, then compares these clusterswith log responses from a noncored well. Usingdefinitions established from core-log responses

in the reference well, the technique assignsclusters to logs from the noncored well and thenoutputs an error curve to help evaluatecompliance between two correlative zones.Clusters showing poor compliance, where errorexceeds 40%, indicate a log response that is notrepresented in the defined clusters, and thus anew facies. These clusters are candidates fordetailed core sampling to provide new clusterdefinitions and further characterize the range offacies in a prospect (above right).

Cluster analysis is also used for optimalselection of core samples. In reservoir studies,both the strongest and weakest core samplesmeasured by continuous profiling must be testedin proportion to their relative abundance in areservoir. Improper sampling in heterogeneousor thinly interbedded formation cores can resultin biased representation of the formation.Cluster analysis can help operators tie log

48 Oilfield Review

> Cluster analysis of well logs. A multidimensionalstatistical algorithm is applied to the well-logmeasurements to identify similar and dissimilarcombined log responses, enabling users toidentify rock units with similar and dissimilarmaterial properties. The output is displayed as acolor-coded representation of clusters for visualinterpretation of rock units with distinct propertiesalong the interval of interest (Track 4).

Caliperin.5 15

Resistivityohm.m0 1,000

Neutron PorosityCluster Tag

vol/vol0.45 –0.15

Bulk Densityg/cm32 3

PEbarn/e-1 6

Gamma RaygAPI0 150

> Cluster tagging between two wells. Color-coding of log responses from each well, combined withanalysis of compliance in the Error track, is useful in identifying changes in thickness and location ofpreviously defined cluster units between wells. Here, the red-blue-yellow sequences are significantlyhigher and thicker in Well 1 than in Well 2. Three excursions above 40% error (red line) indicatecandidate zones for further sampling to better describe the range of facies encountered.

Y,500

Y,400

Y,300

Y,200

Y,100

Depth, ft

Y,000

X,900

X,800

X,700

X,600

X,500Depth, ft

Y,500

0 50Percent

100

Y,400

Y,300

Y,200

Y,100

Y,000

X,900

X,800

X,700

X,600

X,500

Y,500

Y,400

Y,300

Y,200

Y,100

Y,000

X,900

X,800

X,700

X,600

X,500Depth, ft

Well 1 Well 2 Error

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 48

Autumn 2007 49

properties to core properties throughout thereservoir, and thereby recognize which parts of acore merit additional plug-sample analysis(below). With cluster-analysis measurements oflog-scale heterogeneity and core-scale hetero -geneity measurements obtained through scratchtesting, the operator can determine the locationand number of samples required to adequatelycharacterize the core.

Cluster-Level Property Predictions—Sincemodels are traditionally built around thestructure and stratigraphic layout of a basin, thediscontinuous and heterogeneous distribution ofreservoir and nonreservoir lithological unitswithin a single stratigraphic section is oftenpoorly represented across the basin. Cluster

analysis identifies units by their materialproperties and maps their distribution along thelength of a well. By relating laboratory measure -ments of these units to their combined logresponses, core-log relationships are developedfor each cluster. Since the method is unaffectedby variability in thickness or stacking arrange -ments of the various cluster units, it allowsprediction of properties along the length of thelogged section in a well.

Multiwell Analysis—For basin-wide analysis,cluster tags of multiple wells are tied to a singlereference model containing definitions ofmaterial properties across the basin. The resultscan be used for 3D visualization of lateralvariability in reservoir and nonreservoir units.

Cluster tag analysis was instrumental ingenerating a regional study for a client who waspursuing an unconventional gas play. The goalwas to model the vertical and lateral discon -tinuity of principal reservoir units in a tightgas-shale reservoir. These reservoirs are highlyheterogeneous both vertically and laterally, withlocalized diagenetic alterations that create greatvariability in material properties. As a result,reservoir and mechanical properties changesignificantly from location to location betweenwells, and production performance often varies,even between wells drilled in close proximity toeach other.

The client ordered a study to understand thevariability in permeability, gas-filled porosity and

> Using rock heterogeneity to select laboratory samples. Log-scale heterogeneity, indicated by cluster colors (left), is compared against core-scaleheterogeneity data obtained through scratch testing (red curves) superimposed onto core photographs (middle). In the log-scale heterogeneity plot, color isused to differentiate between zones of similar or dissimilar material properties as a function of unconfined compressive strength measurements. Here theyellow clusters are the weakest units and brown clusters are strongest. Progressing from region 1 (yellow cluster), region 2 (yellow cluster transitioning todark blue), region 3 (dark blue transitioning to brown), and region 4 (brown cluster), the rock strength varies by more than 400%. Core photographs (middle)show a corresponding transition in unconfined compressive strength from 10,000 psi [68.9 MPa] in the argillaceous mudstone (core section 1) to 40,000 psi[275.8 MPa] in the basal carbonate (core section 4) within this 40-ft [12-m] interval. Sample plugs (right) are taken from the whole core for detailed analysisand testing. This methodology helps operators ensure that their 2-in. sample plugs account for the variability present in the whole core.

Core-Scale Heterogeneity Sample-Scale

Heterogeneity

Log-Scale

Heterogeneity 10 k

50 k

10 k

50 k

10 k

50 k

0 ft

1 ft

2 ft

2 in

.

0 ft

1 ft

2 ft

0 ft

10 k 40 k

1 ft

2 ft

0 ft

1 ft

2 ft

10 k

50 k

1

1 2

3 4

2

3

4

1

4

2

3

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 49

total organic content as they relate to reservoirquality. It was also important to understand thevariability in the conditions of hydraulic fracturecontainment along the various wells containingunits with best reservoir quality. For optimal wellproductivity, reservoir quality must be coupledwith completion quality. In this field, reservoirquality alone, without successful fracturing andfracture-height containment, would result inpoor well productivity. By mapping locationsthroughout the field where both conditions ofreservoir quality and completion quality existsimultaneously, the client could identify sweetspots in the reservoir (above left). The results ofthis field study would also help improvevisualization of production distribution acrossthe basin.

TerraTek geoscientists used cluster analysisand cluster tagging to evaluate the field.Understanding the vertical stacking patterns ofcluster units on a well helped the client definethe location and thickness of clusters with thebest reservoir-quality properties.

Once these parameters were defined, theclient could select the best geometry ofhorizontal wells and the best locations forperforating. Understanding the properties ofcluster units immediately above and below thebest reservoir-quality units also helped identifymechanical properties and conditions forhydraulic fracture containment.

Modeling Geomechanical PropertiesThe interaction between geology, wellboreorientation and stress changes caused by drillingor production is a complex 3D process. Thisinteraction continually changes over time,adding yet another dimension of complexity.Over the life of any productive field, innumerableevents take place that alter the geomechanicalframework between the reservoir and thesurface. Exploration wells are drilled and tested;additional wells are drilled and produced; somemay be turned into injectors, some are workedover, while others are plugged and abandoned.Each activity causes changes in stress—someephemeral, others more enduring. And thesechanges can be costly, with potential to affectformation integrity, porosity and permeability;reservoir compaction and subsidence; and welland completion integrity.

The movement to understand such changeswas spurred in part by recognition thatsubsidence in certain fields was directly relatedto production. Basic mathematical models weredeveloped by the early 1950s to understand andpredict subsidence in Wilmington field,California.19 Later, subsidence of the North SeaEkofisk field, discovered in the early 1980s,prompted development of more comprehensivecomputer models, based on finite-elementanalysis. These models linked hydrocarbonproduction to changes in reservoir properties anddeformation and, in turn, to seabed movementand faulting in the overburden.

E&P companies became interested inlearning how stress evolves as reservoirs become

50 Oilfield Review

19. McCann GD and Wilts CH: “A Mathematical Analysis ofthe Subsidence in the Long Beach-San Pedro Area,”technical report, California Institute of Technology,Pasadena (November 1951), in Geertsma, reference 2.

20. Ali AHA, Brown T, Delgado R, Lee D, Plumb D,Smirnov N, Marsden R, Prado-Velarde E, Ramsey L,Spooner D, Stone T and Stouffer T: “Watching RocksChange—Mechanical Earth Modeling,” OilfieldReview 15, no. 2 (Summer 2003): 22–39.

> Basin-wide multiwell cluster analysis. This presentation uses Petrel seismic-to-simulation softwareto help operators visualize the cluster-analysis results and track reservoir quality throughout the field.Different cluster units are associated with distinct reservoir qualities. They are also associated withdifferent values of fracture containment potential. Once the reservoir quality and fracture containmentpotential are identified in detail by laboratory testing, they can be tracked laterally across the basin.Surfaces identifying the intervals of best reservoir quality have been delineated. Cluster analysis in this case identifies the heterogeneity inherent in any of these units that otherwise might be considered homogeneous.

1 2 3 4 5 6 7 8 9 10 11 12

Cluster tag

> Array of input parameters for a mechanicalearth model.

• Regional tectonic framework• Structure depth maps• Lithostratigraphic column• Regional compaction trends• Basin analysis• Earthquake fault-plane solutions• Tiltmeter surveys• Core tests and descriptions – Rock composition and texture – Core-log integration – Heterogeneity and anisotropy – Petrophysical and mechanical characterization

Geologic Data

• 3D seismic cube• 2D seismic profiles• Tomographic velocity• Vertical seismic profiles and checkshot data• P-wave velocity profiles

Seismic Data

p

• Daily drilling reports• End of well reports• Mud weight profile• Leakoff tests, extended leakoff tests, formation integrity tests, minifrac tests• Directional surveys• Mud logs

Drilling Data

• Laboratory measurements on cores• In-situ stress measurements from hydro- fracturing tests• Observed breakouts and stress-induced features• Field and production observations

Calibration Data

• Wireline and LWD logs – Gamma ray, resistivity, density, sonic, caliper – Acoustic scanning tool – Borehole imaging• Well test and production pressure measurements – Formation tests and drillstem tests

Formation Evaluation Data

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 50

Autumn 2007 51

depleted. If stress changes could be modeledover the life of a field, operators could predictproblems during the life of a well or anticipatethe need for infill drilling. With a steady growthof computational capacity, geomechanicsprograms acquired increasingly sophisticatedmodeling capabilities. Rock mechanical modelsdeveloped to analyze stress changes in reservoirsincluded the VISAGE stress analysis simulator.This advanced geomechanics modeling systememerged from waterflood directionality studiesin the North Sea and elsewhere.

Developed in 1993 by V.I.P.S. (VectorInternational Processing Systems) of Bracknell,England, VISAGE geomechanics software solvesequations that relate rock stress and porepressure to deformation and reservoir proper ties.By integrating geomechanics and rock mechanicswith reservoir engineering, V.I.P.S. developed theworld’s first coupled geome chanics stress-dependent reservoir simulator. With acquisitionof V.I.P.S. by Schlumberger in April 2007, theBracknell facility was designated as the ReservoirGeomechanics Center of Excellence.

Finite-element modeling is widely used forstress analysis in both conventional engineeringand geomechanics. Finite-difference modeling isused to analyze fluid flow. The advantage of theVISAGE simulator is its capability to describeand simulate the coupled nature of geomechanicalstresses and fluid flow as they change over timeby linking these two analyses. This capability iskey to development of 3D and time-sequenced 4Dmechanical earth models.

Unlike reservoir production models,mechanical earth models (MEMs) must take intoaccount not only the reservoir, but also theoverburden, seabed, the underburden, or rockbeneath the reservoir, and sideburden, oradjacent rock, which often provides stressboundary conditions.20 The MEMs are usuallymuch larger than ordinary reservoir models. Assuch, they have substantial data requirementsthat may be difficult to satisfy.

Complex rock behavior, varying rockproperties and large-scale simulations requirebetter software and better data, especially with

regard to cores. Basic models of the past enabledthe industry to opt for simplified assumptions,using homogeneous formation propertiesthroughout their models. Today’s sophisticatednumerical simulators inevitably dictate a widerarray of data. The MEM is built to honor this widearray of data (previous page, top right).

A geomechanical simulation might begin withconstruction of a 3D structural model. Next, themodel is populated with mechanical propertiesof each formation and fault. The properties arederived from seismic data, logs, cores, geostatis -tical projections and inversion of breakout anddrilling data for individual wells. Boundaryconditions, simulating the expected stressprofiles at the sides of the model, are then added.This populated model is imported into theVISAGE system to calculate the evolution ofstresses throughout the model (above).

The driving mechanism of the modeling ismainly pressure changes induced by fluidextraction from the reservoir, or by injection into

>Workflow for 4D coupled reservoir geomechanics modeling. Formation and structural data form the framework for the initial reservoir model, thencharacteristics from surrounding rock bodies are added. Stress and strain are modeled throughout the reservoir and adjacent rock to understand changesover time.

Importing from ECLIPSEor Petrel software, or both

Importing fault surfaces Embedding in overburden,underburden and sideburden

Population with properties andassign behavioral models

Initialization and coupledsimulation (parallelization)

Data and results utilizedin engineering designs

and planning

VISAGEsimulation

ECLIPSEsimulation

Δp, ΔT

Δkij, ΔVpore

60527schD6R1.qxp:60527schD5R1 1/8/08 9:21 AM Page 51

the reservoir. Fluid flow is modeled using areservoir simulator, such as the ECLIPSEreservoir simulation package. By accounting forthese pressure changes in the stress calculationsusing VISAGE software, it is possible to accuratelypredict subsurface deformations and stresschanges, and evaluate their influence on materialproperties such as permeability and porosity.

The resulting model can be used as a sourceof stress data for several key stages:• well planning—wellbore stability and optimum

drilling azimuth• well completions—sand management• formation stimulation—hydraulic fracture

orientation• field management—pressure maintenance

and injection

• well integrity—well design to accommodatecompac tion and subsidence as the well is produced.

This coupled approach was recently used in aNorth Sea field study. The South Arne field,located in the Danish sector of the North Sea,produces from the Maastrichtian Tor and DanianEkofisk chalk formations. Oil production fromthe low-permeability chalk is driven both bywater injection and by compaction of the chalk.

In 2006, a field study of the South Arne fieldwas conducted to quantify the effects ofproduction from 1999 to 2005, and to assessoutcomes of a proposed development plan. Thefield study was carried out using a history-matched ECLIPSE model and the VISAGEgeomechanical simulator. The geomechanicalstudy comprised four phases.

The goal of the first phase was to enhance anexisting reservoir model by adding more rocklayers and structural detail. First, the reservoirmodel was extended up to the seafloor, adding 20new layers and eight horizons for optimaldescription of the overburden sequence. Tenlayers were added below the reservoir layer toserve as underburden, and eight cells were addedon each of the four vertical boundaries to serveas sideburden. Next, 45 faults and two differentfracture sets were incorporated into theembedded model. The mechanical propertieswere determined based on laboratory tests, corecalibration and literature reviews. A 1D stresscalibration was determined from density logintegration, leakoff tests and pore-pressuremodeling based on wireline log data.

The second phase sought to characterize thestress state prior to production operations. Aninitial effective stress state was computed, basedon properties determined in the first phase. Thestress-state computations accounted for contrastsin deformation and strength properties betweendifferent rock layers, and also considereddiscontinuity within the rock layers themselves(left). The computed initial stress state waschecked to verify agreement with field data andgeological features relating to stress orientations,stress magnitudes and fault orientation.

The goal of the third phase was to determinethe state of present-day stresses. The approachcalled for both flow and stress modeling, startingwith the change in pressure predicted by theECLIPSE reservoir simulator. The changes instress and strain induced by production andinjection operations were then assessed usingthe VISAGE geomechanical simulator. Thecomputed compaction at the top of the reservoirwas in good agreement with the estimated valuefrom 3D seismic inversion.

It was also important to assess the risk of wellfailure. The coupled simulations demonstratedthat pore collapse within the reservoir layerswould cause compaction and subsidence, and thatdifferential pore collapse could result in localizedwell failure (next page).

In the last phase, fluid-flow and stresssimulation was performed in which permeabilitychanged in accord with stress and strainchanges. After history-matching of productionand injection data, the geomechanical modelagreed with production history.

52 Oilfield Review

> Three-dimensional view of a reservoir. The uppermost horizon of an anticlinalreservoir is intersected by numerous faults (inclined planes of semitransparentpurple, red, green and blue). The axis of the anticline is aligned with the longaxis of this figure. Colors on the reservoir surface represent the computed stateof initial maximum principal stress acting on this horizon. In regions remote fromand unaffected by the presence of faults, the maximum principal stresses(green) correspond closely to the magnitude of the vertical or overburden stress,meaning that the principal stresses are near-horizontal and near-vertical. Theregions of reduced stress (blue) are the result of stress-arching in areas wherethe structural geometry and the stiffness of overburden layers create anincomplete transmission of overburden weight onto the underlying reservoir. Thehigh maximum stress concentrations (yellows and reds) near the faults coincidewith inclined principal stresses, causing the magnitudes of the maximumprincipal stresses to exceed lithostatic or overburden stresses generated bygravity and the weight of the overlying rock mass. The black box in the upperquadrant represents the area of study shown in the following figure (next page).

0 Maximum

Stress

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 52

Autumn 2007 53

Monitoring: Geomechanics and 4D Seismic DataOnce a field model is developed, it should beperiodically updated with data obtained throughmonitoring. A variety of techniques have beendevised for monitoring field-scale geomechanicaleffects. For example, global positioning systems,bathymetry and borehole tiltmeter surveys havebeen used to measure surface subsidence.Reservoir compaction can be detected by moni -toring casing collar movement, although thismethod is not precise. Microseismic tech niqueshave been used to detect regions of movementand rock failure during depletion, and areparticularly useful for identifying faultmovements and monitoring fracture creationduring injection and thermal recovery processes.21

Time-lapse, or 4D, seismic surveys are also beingused for geomechanical monitoring.22

Both seismic compressional and shear wavesare influenced by production-induced stresschanges inside and around a reservoir. Time-lapse seismic surveys, which predominantly usecompressional waves, have long been used tomonitor reservoir changes caused by production.Repeatedly surveying a reservoir over timeenables geophysicists to compare differences inseismic attributes, such as reflection amplitudesand traveltimes, between the initial baselinesurvey and subsequent monitor surveys. Thesedifferences are particularly useful in detectingmovements of gas/liquid contacts that occur asreservoirs are produced. In recent years, 4Dseismic techniques have also been used tomonitor production-induced changes in reservoirgeomechanical properties.

Among the differences between baseline andmonitor surveys, geophysicists sometimesobserved shifts in seismic traveltimes to aspecific horizon. Initially, these discrepancieswere attributed to logistical problems associatedwith repeating surveys over a reservoir: namely,the difficulty in placing seismic sources andreceivers in exactly the same position for everysurvey. The slightest mispositioning of sources orreceivers could lead to modified raypaths thattraveled through slightly different parts of thesubsurface, generating perturbations in observedtraveltimes. In the past, discrepancies in seismic

> Production-induced compaction. These figures correspond to the boxed area shown in the previous figure (page 52). Production-induced time shifts seenfrom the 4D seismic response (left) closely match the pattern of computed plastic strains obtained through coupled numerical simulation (right). Maximumcompaction (red) follows the NW trend of horizontal wellbores (dark blue lines) in the upper part of this figure. As expected, the area of greatest compactioncorresponds to that part of the reservoir experiencing the greatest production and consequently the greatest depletion. The computed maximum compactionof 1.45 m [4.76 ft] at the top of the reservoir was in good agreement with the estimated value of 1.4 m [4.59 ft] from 3D seismic inversion. The absence of 4Dseismic data (white zone) is caused by a gas cloud. Close agreement between the 4D seismic data and the numerical model reinforces confidence in modelresults over the area where seismic data were not available.

0 Maximum

Compaction

21. For more on microseismic applications: Bennett L, Le Calvez J, Sarver DR, Tanner K, Birk WS, Waters G,Drew J, Michaud G, Primiero P, Eisner L, Jones R,Leslie D, Williams MJ, Govenlock J, Klem RC andTezuko K: “The Source for Hydraulic FractureCharacterization,” Oilfield Review 17, no. 4 (Winter2005/2006): 42–57.

22. Doornhof et al, reference 5.

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 53

traveltimes were frequently attributed todifferences in acquisition geometry or toprocessing artifacts.

However, seismic acquisition and processingtechnology has steadily improved, so that sourcesand receivers can now be repeatedly positioned

with high accuracy, allowing reliable measures oftraveltime changes as small as 1 millisecond.With this level of accuracy, geophysicists are ableto use time-lapse seismic techniques for observ -ing depletion-induced traveltime changes for a

growing number of fields. In the North Sea’sEkofisk and Valhall fields, combined observationsby reservoir engineers, geophysicists and geome -chanics specialists have led them to concludethat the soft chalk of the reservoir rock wasundergoing substantial reservoir compaction,

54 Oilfield Review

> Changing seismic characteristics. Both change in geometry (top left) and change in seismic velocity (bottom left) influence seismic reflection traveltimes.The seismic two-way traveltime (TWT) (right) gradually increases toward the top of the reservoir due to overburden stretching and associated velocitydecrease. The largest time shifts are observed around the producing wells. Inside the reservoir, the seismic velocity increases because of increased stress, so the time shifts become smaller.

Dept

h, m

1,500

2,000

2,500

3,000

3,500–4,000 –3,000 –2,000 –1,000 0

Distance, m1,000 2,000 3,000

Dept

h, m

Decr

ease

in T

WT

Incr

ease

in T

WT1,500

2,000

2,500

3,000

3,500–4,000 –3,000 –2,000 –1,000 0

Distance, m1,000 2,000 3,000

Dept

h, m

1,500

2,000

2,500

3,000

3,500–4,000 –3,000 –2,000 –1,000 0

Distance, m

Vertical displacement, Δz

Time-lapse time shift for vertical P-waves, Δt

Change in vertical P-velocity, ΔVp

Δz, m

Δt, ms

ΔVp, m/s

1,000 2,000 3,000

–2

0

2

4

6

8

–0.05

0.00

0.05

0.10

0.15

0.20

0.25

23. Barkved O, Heavey P, Kleppan T and Kristiansen TG:“Valhall Field—Still on Plateau After 20 Years ofProduction,” paper SPE 83957, presented at SPEOffshore Europe, Aberdeen, September 2–5, 2003.

24. Guilbot J and Smith B: “4-D Constrained DepthConversion for Reservoir Compaction Estimation:Application to Ekofisk Field,” The Leading Edge 21, no. 3(March 2002): 302–308.Nickel M, Schlaf J and Sønneland L: “New Tools for 4DSeismic Analysis in Compacting Reservoirs,” PetroleumGeoscience 9, no. 1 (2003): 53–59.Hall SA, MacBeth C, Barkved OI and Wild P: “Time-Lapse Seismic Monitoring of Compaction andSubsidence at Valhall Through Cross-Matching andInterpreted Warping of 3D Streamer and OBC Data,”presented at the 72nd SEG International Exposition andAnnual Meeting, Salt Lake City, Utah, October 6–11, 2002.

25. Hatchell PJ, van den Beukel A, Molenaar MM,Maron KP, Kenter CJ, Stammeijer JGF, van der Velde JJand Sayers CM:“Whole Earth 4D: Monitoring

Herwanger JV and Horne SA: “Linking Geomechanicsand Seismics: Stress Effects on Time-Lapse Multi-Component Seismic Data,” presented at the 67th EAGE Conference and Exhibition, Madrid, Spain, June 13–16, 2005.Sayers CM: “Asymmetry in the Time-Lapse SeismicResponse to Injection and Depletion,” GeophysicalProspecting 55 (September 2007): 699–705.Sayers CM: “Sensitivity of Time-Lapse Seismic toReservoir Stress Path,” Geophysical Prospecting 54(September 2006): 369–380.Sayers CM: “Sensitivity of Elastic Wave Velocities toReservoir Stress Changes Caused By Production,” paper ARMA/USRMS 06-1048, presented at the 41st USSymposium on Rock Mechanics, Golden, Colorado,June 17–21, 2006.Sayers CM: “Sensitivity of Elastic-Wave Velocities toStress Changes in Sandstones,” The Leading Edge 24,no. 12 (December 2005): 1262–1267.

Geomechanics,” Expanded Abstracts, 73rd SEG AnnualInternational Meeting, Dallas (October 26–31, 2003):1330–1333.Hatchell P and Bourne S: “Rocks Under Strain: Strain-Induced Time-Lapse Time-Shifts Are Observed forDepleting Reservoirs,” The Leading Edge 24, no. 12(December 2005): 1222–1225.

26. Hatchell et al, reference 25.Hatchell and Bourne, reference 25.Herwanger JV, Palmer E and Schiøtt CR: “FieldObservations and Modeling Production-Induced Time-Shifts in 4D Seismic Data at South Arne, DanishNorth Sea,” presented at the 69th EAGE Conference and Exhibition, London, June 11–14, 2007.

27. Herwanger et al, reference 26.Sayers C: “Monitoring Production Induced Stress-Changes Using Seismic Waves,” presented at the SEGInternational Exposition and 74th Annual Meeting,Denver, October 10–14, 2004.

60527schD6R1.qxp:60527schD5R1 1/8/08 9:21 AM Page 54

Autumn 2007 55

accompanied by another significantphenomenon—that of overburden stretching.23

Resulting traveltime changes are significant andof a magnitude that could not be explained bynonrepeatability of survey acquisition geometry.24

Seismic data confirmed that the reservoirrock did not deform uniformly, and deformationin the reservoir rock caused the surrounding rockto deform. In this case, the differential deforma -tion associated with reservoir compac tion and anarching effect in the overburden resulted incompressive stress relaxation and correspondingstretching in the overburden. Similar overburdentime shifts were subsequently reported abovehigh-pressure, high-temperature reservoirs andcertain deepwater-turbidite fields.25

The geomechanical implications of time-lapse time shifts are evaluated with reservoirgeomechanical models to characterizeproduction-induced subsurface deformation andto predict associated stress changes. Establishedworkflows allow geophysicists to compareobserved time-lapse time shifts against timeshifts predicted by the reservoir geomechanicalmodels.26 Both subsurface deformation and stresschanges influence the seismic traveltime, eitherby changing the length of the path that a seismicwave must travel or by altering the propagationvelocity of the seismic wave, respectively(previous page). The workflows allow predictionsof traveltime changes to any point in a three-dimensional subsurface model.

Traveltime changes can also be observed from4D seismic field experiments (left). Theprediction and observation of 4D traveltimechanges may be used to validate and calibratereservoir geomechanical models and therebyimprove their capability to predict stresschanges for a variety of projected productionscenarios. Furthermore, laboratory measure -ments conducted on rock cores are helping E&Pcompanies learn more about changes inultrasonic velocities under various stressconditions and saturation states. This allowsoperators to better manage reservoir stress andoptimize the trade-off between compaction driveof hydrocarbon production and unwantedcompaction problems such as wellbore failureand reduced permeability.

At present, the observation of changes invertical traveltime is a common practice formonitoring geomechanical changes such asvertical stress and strain. This techniqueprovides useful information, and allows geophysi -cists to identify compacting and noncompacting

reservoir compartments. However, to understandand predict other geomechanical factors, such aswellbore stability or rock failure, the triaxialstress state must be known. Recognizing thisneed, Schlumberger and WesternGeco scientistsare exploring the use of surface-seismic 4Dmeasurements to characterize the change intensor-stress over time.27

Future DevelopmentsThe industry is striving to develop furthercapabilities for integrating rock fabric intogeomechanics analysis, with the vision ofenabling operators to extrapolate informationfrom rock microstructure observations to thecore-sample scale, through the well-log scale andeventually up to the seismic scale. This capabilitywill let operators track reservoir characteristicsalong the extent of a play and beyond, tolocations where no well control exists. In doingso, geomechanics may change not only the waythat fields are drilled and produced, but also theway in which they are explored. To this end,researchers at Schlumberger are activelyinvestigating new laboratory measurementtechniques, wellbore logging methods, seismicmeasurements and modeling programs. Indeed,computational capabilities already exist; it is theactual rock, its fabric, and the relation of fabricto rock behavior that must be furthercharacterized. —MV

>Monitoring compaction over time. Acomparison of traces using the same source andreceiver position between the baseline (green)and monitor surveys (blue) shows the effect ofoverburden stretching on the arrival time of theseismic signal. Note the consistent shift towardlater arrival times of the monitor surveycompared with the baseline survey.

Top reservoir reflectionshifts toward later arrivaltime and brightens

Bottom reservoir reflectionshifts toward later arrivaltime and dims

Baseline surveyMonitor survey

60336schD5R1.qxp:60336schD5R1 11/29/07 4:13 PM Page 55