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 coaly shales or deep oil shales, OBM often worse than WBMOBM is usually much more expensiveIt is not the answer to all shale problemsWhen it works (mainly in intact reactive shales), it is absolutely fabulousHole ImprovementMBDCIWBM and ShalesWe must cope with fractures and fissuresWe must cope with high wall stressesWe must cope with reactive shalesAgents to block fissures are usefulGilsonite, LCM graded mud if large fissuresAgents that are chemically beneficialReduce clay reactivityAgents that lead to shrinkageAgents that reduce permeation and diffusionNaCl-saturated mud is almost always good for reactive ductile shales (but: slow ROP!)Hole ImprovementMBDCIKCl-Glycol MudsPotassium ion displaces Na+ in clay mineralsK+ fits well into lattice = shale shrinkageHigher concentration of K+ = more shrinkageThis shrinkage leads to reduced near the wall = better stability!Glycol in suspension used. Why?Glycol particles block microfissures = less flowThis tends to reduce and delay sloughingAlso, glycol tends to adsorb on clay particlesAll three effects are generally beneficial for borehole stabilityHole ImprovementMBDCIGlycol EffectsProof of beneficial reduction of hydration

This reduces swelling and borehole wall stress

Hole ImprovementMBDCIboreholeEffect of K+-Induced ShrinkageKirsch elastic solution (expected)Effect of shale shrinkagesqtangential stress -sq radius - rsq]maxShrinkage reduces sq]max !Highly positive shrinkage effect!Far-field stressesHole ImprovementMBDCIRole of Glycol in K+-Glycol MudIt has a chemical effect, reducing swellingIt has a mechanical effect, blocks cracksThis helps maintain the support pressureGlycol concentration must be kept above the cloud point (solubility limit) so that free droplets are throughout the mudchipF ~ A(pw pi)pwpiglycol plugs microfissurespwchip support F:A = chip areaWBMFpiHole ImprovementMBDCICa++-Based MudsGyp muds (low pH, CaSO4)Lime muds (high pH CaO, Ca(OH)2)Lime muds seem particularly effective in controlling geochemically sensitive shalesCation exchange and shrinkageIncreased interparticle bondingBUT! These high solids muds tend to reduce penetration rate = longer exposureHowever, they are inexpensive and thus expendable, + less environmental impactHole ImprovementMBDCITrajectory Choice (Avoidance?)Drilling to avoid crossingfault in fractured shaleDrilling to cross fault and fissile shale close to 90reservoirtroublesome fractured shalefaultSelected trajectories and careful choice of drill site can easily be applied to drilling on land.

tight radius wellextended reachsHMAX = s1sv = s3deviatedwellmudlineDrill through faults and fissile shales at 90 25 Hole ImprovementMBDCINew ProductsOlefin & ester-based drilling fluidsEliminate swelling of reactive claysChips and sloughed shale remain intactLowered ECD compared to WBMClay-free WBM (up to 4100 $/b!)Cesium sodium and potassium formatesExtremely inhibitive on shale swellingNon-corrosive, compared to Cl- brines Lower torque, ECD, less barite sagEnvironmentally more acceptable (no Cl)Clay-free synthetics (Baroids Accolade)Hole ImprovementMBDCINew ProductsStarch/polyglycerol WBMPolyglycerols reduce swelling (i.e. glycols)Starch gives viscosity, water-loss controlEnvironmentally better (cuttings discharge)Lower ECD than many WBMAppears good in ballooning casesShould also use graded LCM in these casesFoamsUsed in underbalanced drilling (MW < po)Very high ROP, less exposure timeSilicates (blend of Na & K silicates)Gelation occurs in low pH fmn, sealing poresHole ImprovementMBDCIOther SystemsSilicates (blend of Na & K silicates)Gelation occurs in low pH fmn, sealing porespH > ~9.5, no precipitationALPLEX - Aluminum-based (Al(OH)3) invert emulsion Asphasol air-blown asphaltSpecially treated Fills fissures, better cake, similar to gilsoniteAnd many other polymers, particles, latex, chemicals, inhibitors, etc., etcHole ImprovementMBDCIStrength Reduction in ShalesExperiments show that shales almost always weaken when geochemical processes actchanges in concentrationchanges in ionic make-upshrinkage or expansionMicromechanical reasons:reduced interparticle electrostatic bondingmicrostrains deteriorate mineral cementationconcentration changes alter stiffnessBut, if shale is supported, these effects can be considered secondary

Hole ImprovementMBDCIDrilling-Induced Damage, Fracturessqsrporadiusstressreduction in sq]mindamaged zoneborehole,pwlimited depth fracturessq, intactsq, damagedshift of peak stress sitefractures are propagatedduring drilling and tripswhen effective mudpressures exceed sqhminHMAXHole ImprovementMBDCICoupling of Diffusion ProcessesExample: DT affects viscosity, therefore the flow rate into the shale is changedExample: C affects electrostatic attraction and adsorbed water content can changeExample: Adsorbed water content changes affect effective stressesExample: Advective transport in micro-fissures can affect T, C Clearly, a highly complex set of issues!Dont worry about these details too muchHole ImprovementMBDCIPermeability Control in ShalesNatural k very low, microfissures increase k (fractured shales have intrinsic high k)Two options: reduce microfissure intensity or block the microfissures somehowReducing microfissure intensity is difficult and causes slower drilling (high MW )Blocking microfissuresStarch in NaCl mudsGlycol in KCl-Glycol mudsGilsonite and other deformable asphaltsGraded LCM for larger fissuresHole ImprovementMBDCIIs Faster Drilling the Best Cure?Shorter exposure = less t for diffusion processes to act on the shale, therefore less deterioration developsApproaches to achieve more rapid drillingSmaller holes (e.g. slim exploration holes)Reduce or eliminate logging if possibleLonger bit runs = fewer tripsImproved hole hydraulicsUnderbalanced or near-balance drillingSmall holes also are relatively strongerExpandable liners + bicentre bitsHole ImprovementMBDCIDrilling Faster Reduces ProblemsDrilling exploration or production wells?Exploration wells can be slim hole, fast wellsReduced exposure time for troublesome shalesSmaller diameter leads to greater stabilityLower mud costs, etc.Fewer casing strings!Watch your ECD!10 days drlg. 30 days drlg.12 hole, 10 casing

9 hole7 casing

6 slim hole, exploration!Same depth, 5 casing stringsDiscuss UNOCAL experience in IndonesiaHole ImprovementMBDCIPressure Managed DrillingA new concept, based on new understandingBHP kept constant using special equipment, ECD is maintained close to zero as possibleEliminate sudden p changesCyclic loading of fractured material degrades strength; if it is reduced, shale stays strongerReduces filtrate and pressure invasionReduces (or eliminates) breathing on connections, reduces uphole ballooningAllows one to operate closer to MW ~ po at hole bottom by eliminating ECD, hence faster drlg.Etc.

Hole ImprovementMBDCIAlberta ExampleDeep Basin, upper soft, lower fracturedOBM best higher up, WBM lower down!WBMcalipercaliperOBMtargetfractured shalesloughingin gaugemainly in-gaugemassive sloughingductile shalesHole ImprovementMBDCIScale Effect in Fractured Shalesunstable shaleblocksnounstableblocksLL = characteristic length of shale fabricL/DincreasingstabilityD = borehole diameterIf L/D is very small, materialacts granular; if very large,blocks do not affect stability scale effectintactgranularAlso in coals!Smaller holes arte always more stableHole ImprovementMBDCIGilsonite, LCM for Fractured ShaleFractured shales slough because of nopMaintaining some Dp is a major benefitIn OBM, solids alone do not seem to bridgeIn WBM, yes, but gilsonite helps greatly (available for OBM as well)Gilsonite + solids plug the natural fissures, reduce k, help Dp, improve cake efficiencyAdd designed LCM material to mud to help the fracture pluggingReduce MW rather than increase itCareful trip and connection policyHole ImprovementMBDCIGilsonite Plugs FissuresNatural asphaltGilsonite is flexible at the T at depth in holesIt can plug induced microfissuresIt also helps plug in fractured shalesThis helps sustain support pressureDiffusion of p, C into shale is retardedShales stay intact longer

chipFF ~ A(pw pi)pwpigilsonite plugs microfissurespwpichip support F:A = chip areaWBMAlso available for OBM and synthetic mudsHole ImprovementMBDCISpecial Pills for Fractured ShalesPump down a polymer slug, close BOP, squeeze polymer through the drill bit, raise bit, Allow to set, drill through the plugged zone

Photos courtesy Doug Coughron (BP) IADC/SPE 74518Hole ImprovementMBDCIThermal Destabilizationshear stressnormalstressDsqsqsrsqsrT + DT

posq + DsqTomudsupportShear strength criterion for the rock around the boreholeinitialconditionsi,jheating leads to borehole destabilizationWhen the stress state semicircle touches the strength criterion, it is assumed that this is the onset of rock deterioration (not necessarily borehole collapse)YHole ImprovementMBDCIThermal Alterations of Kirsch elastic solutionthermoelastic heating (convection)thermoelastic cooling (convection)sq (r) for coolingradiussq]maxTwExcept for heating, mostprocesses reduce the sq]maxvalue at the borehole wallThese curves show hoop stress calculated using an assumption of heating and an assumption of cooling. Heating a borehole increases , and leads to hole problems. Cooling the borehole is always beneficial to stability.tangential stress - sqsq (r) for heatingToInitial shboreholeHole ImprovementMBDCIWhat Happens with Hot Mud?The rock in the borehole wall is heatedThermal expansion takes placeThis attracts stress to the expanding zone around the wellThe peak stress rises right at the borehole wall, and yield and sloughing is likelyFor cooling, the rock shrinks; this allows the stress concentration to be displaced away from the borehole, helping stabilityCooling occurs at and above the bitHeating occurs farther upholeHole ImprovementMBDCIHeating and Cooling in the HoledepthTcasinggeothermaltemperaturebitcoolingheatingmudtemperatureshoe+T-Tmuddownpipemud upannuluscoolingin tanksBHAdrillpipeopenholeHeating occurs uphole, cooling downhole. The heating effect can be large, exceptionally 30-35C in long open-hole sections in areas with high T gradients.

Heating is most serious at the last shoe. The shale expands, and this increases sq, often promoting failure and sloughing.

At the bit, cooling, shrinkage, both of which enhance stability.

Commercial software exists to draw these curves

Hole ImprovementMBDCIExpansion and Borehole Stresseslost selastic rocks resistribute the lost stressDelastic rocks redistribute thermal stresses as wellexpanding rocksHigh sq near the holeThis is the standard elastic case of borehole stress redistribution This is the case of rock heating when the mud is hotter than the formationDSee Module CHole ImprovementMBDCICooling the Drilling FluidReduces the stresses on the borehole wallImproves the stability of the mud systemImproves safety on the rig, especially offshore in hot drilling areasReduces the rate of diffusion of chemicals into the shale, slowing down deteriorationHelps protect the drill bit and the bottom-hole assemblyShould be more widely usedHole ImprovementMBDCISummaryThere are many reasons for borehole instabilityYou must assess the reason for instabilityThen, solutions will be possibleMud weight and hydraulics controlOil based mud for ductile shale is very goodChemical solutions help as wellPlugging the wall for fractured shaleCooling the mudAnd so onHole ImprovementMBDCI