Borehole Stability

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Borehole stability

Text of Borehole Stability

Flow equations in various cases

Borehole StabilityOverview of Methods of Hole Improvement

Maurice B. Dusseault

Hole ImprovementMBDCIMajor Drilling Problems Blowout of fluids under high pressureLost circulation (often leading to blowouts)Massive shale instabilityChemical reasons ductile unfractured shales, incompatibility with oil-base mudMechanical reasons highly fractured shale or coal, fissile shale and horizontal wellsStress reasons high compressive stress fieldsSqueeze while drilling saltInduced slip of high angle joints

Hole ImprovementMBDCI

OilTurkmenistanNatural gasCanadaCoiled tubing drillingHole ImprovementMBDCI Well Blowout in California

September 1968, Union OilSanta Barbara Channel, CAThere has been no drilling offshore California since this blowoutHole ImprovementMBDCIBlowout ConditionsWhen mud pressure is no longer enough to balance a high-pressure fluid (oil, gas, water) that flows at a high rate po > pwHowever, we now use underbalanced drilling where po > pw all the time! ButOnly if there is no large amount of oil or gas in a high permeability zoneOnly if the shale is strong enough to stand without wall supportGenerally limited to relatively shallow drilling

Hole ImprovementMBDCIUnderbalanced Drilling: pw < po

Source: Air Drilling Associates Inc. popo> pwpwpwpoHole ImprovementMBDCILC - Lost CirculationOccurs in two ways:In zones with large diameter pores or channels (vuggy carbonates, rubble zones, fractured zones, fault zones)In conditions where pw > hmin and a hydraulic fracture propagates beyond the borehole regionPressure controlled drilling helps avoid LC/BOStrengthening the borehole wallIncreasing the stress around the borehole wallPlugging initiating fractures with solidsHole ImprovementMBDCILC in Depleted Zones

Depleted zoneCourtesy: At Balance Americas LLCHole ImprovementMBDCIUsing Pressure Control Drilling

Depleted zoneCourtesy: At Balance Americas LLCHole ImprovementMBDCIChemical Shale InstabilityChemically sensitive ductile shalesSmectite-rich clays that swell with (chemistry)Younger and shallower shales (> sHMAX > shminshminsHMAXsvsHMAX >> sv > shminsHMAX ~ sv >> shminsvshminsHMAXsvshminsHMAXTo increase hole stability, thebest orientation is that whichminimizes the principal stressdifference normal to the axis60-90 coneDrill within a 60cone (30) from the mostfavored directionFavored holeorientationHole ImprovementMBDCISalt SqueezeSalt is a viscoplastic materialIt flows (creeps) under differential stressThe higher the stress, the faster the creepThe higher the temperature, the faster the creepAlso, there are other issuesIt is highly soluble (washouts possible)Low density means different stress gradientsThe presence of dolomite or limestone beds can impact drillingRubble zones under thick salt bodies (GoM)Hole ImprovementMBDCISalt in the United States

Williston BasinMichigan BasinAppalachian BasinPermian BasinGulf Coast BasinSource: National Petroleum Technology OfficeParadox BasinHole ImprovementMBDCIZechstein Salt, North Sea Basin300 km0 HamburgBerlinZECHSTEIN BASINLondonOsloDiapir structuresMajor fields: Ekofisk, Valhall, DanHole ImprovementMBDCISaltDrilling Problems in Salt Rock Ledgesandblocks Large dissolved zone washout SaltpinchLimestoneordolomitebitBHADrillpipeSqueeze Washouts Ledges & BlocksRubble zoneHole ImprovementMBDCI Slip of High-Angle JointsHigh angle joints can slip and decrease the borehole diameter, giving trip problems High mud weight high in an open-hole sectionPore pressure permeates into the jointSlip happens, pinching the hole diameterRaising MW makes it worseBlocking flow into the joint helps prevent itBack reaming with a top-drive system makes it possible to drill out of the hole Hole ImprovementMBDCISlip of a High-Angle Fault Planehigh pressuretransmissionslip of jointslip of joint surfaceboreholecasing bendingand pinching in completed holes(after Maury, 1994)sv = s1 sh = s3pipe stuck on tripsHole ImprovementMBDCISlip Affected by Hole Orientation!OFFSET ALONG PRE-EXISTING DISCONTINUITIESFILTRATE

TYPICALMUDOVER-PRESSURECourtesy Geomec a.s.Hole ImprovementMBDCISome Diagnostic Hole Geometriesa.drillpipeb.f.e.sHMAXshminInduced by high stress differencesGeneral sloughing and washoutKeyseatingFissility sloughingSwelling, squeezeBreakoutsOnly breakouts are symmetric in one direction with an enlarged major axisc.d.c.d.Hole ImprovementMBDCISolutions to Borehole Instability ProblemsHole ImprovementMBDCIWhat is Your Problem?Correct identification of the problem is essential to find a good cureHigh differential stresses?Swelling shale? Chemically sensitive.Fractured shale?Slip of joints, or fracture planes?Squeeze of salt?Fissile or carbonaceous shale sloughing? Heating of the borehole causing sloughing?You must establish the reason first

Hole ImprovementMBDCIWhat Keeps a Borehole Stable?Favorable natural conditionsStrong rock (carbonates, anhydrite)Low stresses and small stress differencesGood mud propertiesGood support of the wall (good filter cake)Proper hole cleaning, viscosity, gel strengthProper MW programs, lower T, other effectsGood drilling practices & trajectoryTripping and connections practicesEarly identification of trouble (cavings volumes)Proper choice of well trajectoryHole ImprovementMBDCIExample: Drilling near SaltInstability of surrounding shalesLost circulation in residual rocksSqueeze in saltSheared zone on the flanks of the domeLow hmin near flanks of salt domeWhat is the most important problem?saltdomegasoilresidual rocksmother saltHole ImprovementMBDCISolutions to Salt Drilling ProblemsMaintain a salt-saturated drilling mud with a small amount of free salt in the fluidTo slow down squeeze of boreholes in salt, there are only two possibilities:Reduce the temperature to reduce the salt creep rate around the boreholeIncrease MW so that there is less differential stress: that is - minimize ( pw) Increase your drilling rate! This means that salt has less time to squeezeHole ImprovementMBDCIMud Weight vs. Hole Closure Rate-5051015202514151617181920Mud Weight - #/galClosure rate, %/dayConditions:11,000 depth, North Sea CaseT @ 11000 ~ 95CStress in salt at 11000 = 19.7#/gal MWSalt type: Fast-creeping salt (highinterstitial H2O content)Hole size: 8.5

overburdenHole ImprovementMBDCIMud Cooling Effect on Hole Closure02468101214-35-30-25-20-15-10-50510Cooling Amount (deg C)Closure Rate (%/day)Conditions:11,000 depth, MW is 16 #/galBase case (x = 0) is at 95C temp.Stress in salt at 11000 = 19.7#/gal MWSalt type: Fast-creeping salt (highinterstitial H2O content)Hole size: 8.5

coolingheatingNorth Sea, Zechstein SaltsHole ImprovementMBDCIShale Problems in DrillingContinued sloughing and hole enlargementHole cleaning difficulties, hole fill on tripsMud rings and blockagesSwabbing pressures on trips to change the bitDifficulty in controlling drilling mud properties Sudden collapse (usually when po > pmud)Instability in shale also increases the risk of blowouts and lost circulationIncreased torque, overpull on tripsHole ImprovementMBDCICircumferential Fissuring in ShalesHMAXshminAny hole in a naturallyanisotropic stress field haslarge tangential stresses,low radial stresses.Extensional microfissuringfrom high compressionShearing and dilation, high tangential stress,normal to sHMAXAll types of brittle damage make pressure penetration easier!mud-filledboreholelow sqhigh sqAnisotropic stresses usually exist in the borehole planeSingle extensional axial fractures develop parallel to sHMAX, region of low sqUnderstanding stresses and shale damage is vital!Hole ImprovementMBDCIMudcake and p Supportboreholep(r), steady-state, no mud-cake mudcakelimited solidsinvasion depthDp across mudcakep(r) with mudcakepressurepopwdistance (r)sandstoneUnderstanding the pressure support effect is vital!Hole ImprovementMBDCIChip Support by p Across WallBorehole filledwith mud at ahigher pressurethan the shaleshale chipPressure gradient directionOutward force on the shale chip(the seepage or hydrodynamic force)pp - DpSupport force on the chip is proportionalto the pressure drop across the chip, indirection of maximum pressure gradient.FThe pressure drop across the chip is relatedto time (transient effect) and the materialpermeability (increased by damage).borehole wallThis is why damaged shale can stay in place for some timeHole ImprovementMBDCIDamage Effect on p Supportpressurepopwdistance (r)pressure gradient drops with timelow permeability shale, no mudcake!A(intact borehole)B(damaged borehole)no Dp for wall supportshaletransientpressurecurvesmud pressureformation pressurep(r) curves with timeHigh sq leads to rock damage. This permits pressure penetration, loss of radial mud support. It is time-dependent, and reduces stability.boreholehighly damaged zoneHole ImprovementMBDCIHow Oil Base Muds WorkIntact shales have tiny, water-wet pores: a high capillary entry pressure exists, therefore the pw - po acts very efficiently, right on hole wall, giving good supportNo filtrate invasion = little shale deterioration by geochemistryShales shrink (-DV) by dewatering because of high salinity of the aqueous phase in OBMUndrained behavior (-Dp) maintained longer because of low k in shales, little H2O transferAll of these are beneficial in general Hole ImprovementMBDCIThe Capillary FringepwH2Ooilpw po = Dp = gow/2rpooil-basemudshale, water-wetDp capacitygow = oil-water surface tensionrr = curvature radiusshaleboreholeCapillary fringeThe major OBM effect is the capillary fringe support, which is why they work so well in intact shaleshigh pHole ImprovementMBDCIOBM HoweverThe capillary effect is lost in fractured shales - poor support, add a plugging agentThe salinity effect is irrelevant in non-reactive (Quartz-Illite) shalesIf fissility planes exist in co