geomechanics

Geo-mechanical study for stable and safe well bore

GeomechanicsGeomechanics involves study of the behavior of soil and rock.

The two main disciplines of Geomechanics are Soil mechanics and Rock mechanics

Rock mechanics deals with rock mass characterization and rock mass mechanics, such as applied to petroleum industry or high depths, tunnel design, rock breakage, and rock drilling.

GeomechanicsGeomechanics related issues are thought to cause almost half of drilling-related NPT in HPHT, Deepwater, and other challenging environments.

Without a strategy for avoiding or minimizing potential geomechanical problems, the project may cost millions more than budgeted.

Today Geomechanics analysis and planning are considered a necessary strategic component of exploration and field development activities.

GeomechanicsThe first consideration of Geomechanics is often for drilling design.

Reduce drilling cost and duration

Predict well bore instability prior to drilling and reduce or eliminate stuck pipe, formation collapse, formation fracture, lost circulation etc

Predict Wellbore trajectory for complication free drilling Establish the mud weight boundaries for safe and stable drilling

Geomechanical Studies done by ONGC

Gandhar Field Geomechanics study by Schlumberger

Geomechanical Study to evaluate wellbore instability in ERD wells in Western Offshore, Mumbai by Baker- Hughes

Wellbore stability and Formation damage study in Southern Cambay basin by UNSW, Sydney, Australia

In-house beginning by IDT

IN-SITU STRESSES

WELL BORE STRESSES

Approach For Stable Well

Mud weight and Mud type/ design play major role.Improvement in well bore stability achieved by use of High performance mud and Advanced drilling technology in ONGC.Still fails to reduce NPT to barest minimum

Missing Link

Selection of Mud weight is based on Formation pressure (Pore pressure) given in GTO.Pore pressure is not meant for stabilising well bore.It now becomes imperative to understand what is required for Wellbore stability

Missing Link

S= + Pf

S= Overbureden stress. = Intergranular stress/ Matrix stress (to control caving/collapse of borehole wall.Pf= Formation pressure or Pore pressure (to control influx of fluid; gas or oil.

Mud Weight Selection

It is based onPore Pressure GradientCollapse GradientFracture Gradient

Mud weight > Pore Pressure Gradient ~ No Kick.Mud weight > Collapse Gradient ~ Borehole stable (no caving). Mud weight < Fracture Gradient ~ No lost circulation.

Background

Cauvery asset, Karaikal proposed following work to IDT under AWP 2009-10

Formulation of very low weight (close to hydrostati ~1.05) Drilling fluid system for drilling large section of shale and sand alternation without isolation by casing

Fluid Design: Chemical PerspectiveShale cutting collection of subject fieldCationic Exchange Capacity testsCapillary Suction Time testsLinear Swellmeter testDispersibility test (Temperature~1100C; SG~1.05)

Fluid Design: Chemical Perspective

Following fluids provided sufficient inhibition to shaleof subject field KCl-PHPA-Polyol fluidAmine-PHPA fluidCationics-O fluids

Point of Discussion: Whether these fluids would beable to drill the well with SG~1.05

Fluid Design: Mud Weight Perspective

As per GTO (#MDMA;Cauvery asset, Karaikal) Expected formation pressure: Hydrostatic + 5%Interval: 1150-1800 M covering shale and reservoirsandsMW corresponding to this interval: 1.10-1.22On higher side vis--vis Expected formation pressureWhy is it so?

Case Study

Patan area of Mehsana asset was taken for bringing the point home

Wellbore instability can occur as a result ofChemical effects Mechanical effectsor Combination of both

Case Study: Mehsana Asset#KAAF: Exploratory

LithologyInterval(meter)FormationPressureMW as per GTOMW actualPhase-I (Prehydrated Bentonite Suspension)Guj alluvium00-540Hydrostatic1.06-1.101.05-1.10Phase-II; KCl-K-Lignite System;KCl-12%, S.Asphalt-2%Jhagadia540-740Hydrostatic1.10-1.151.1.-1.12Kand740-1020Hydrostatic1.10-1.151.12-1.15Babaguru1020-1120Hydrostatic1.10-1.151.15-1.22Tarapur1120-1130Hydostatic1.10-1.15Kalol & Kadi110-1140Hydrostatic1.10-1.15OCS1140-1400Hyd+5%1.15-1.201.22-1.40Olpad1400-1600Hyd-10+15%1.20-1.261.40Sidetrack-I & II---1.50


OBSERVATIONSPhase-II; KCl-K-Lignite System;KCl-12%, S.Asphalt-2%Drilled down from 400m to 1378m; SG~1.25; Observed held up at 570m while R/I 12-1/4 bit . Reamed 570-1378 m. Increased SG~1.40Drilled down 1378-1603m (Olpad) with SG~1.40; During P/O for W/T- Observed tight holeR/I bottom to stabilise hole; R/I 1585m; String got stuck while trying to establish circulation. No mud returnString released while working on it; Circulation could not be established; String got stuck at 1499m; F/T at 1394m; Cement plug placed 1392-1202m


OBSERVATIONSSidetrack-I; KCl-K-Lignite System;KCl-12%, S.Asphalt-2%; SG~1.50Sidetracked from 1343- 1394m; Further sidetrack could not be done as it hit the F/Top; Placed Cement plug 1333-1244m.

Sidetrack-II; KCl-K-Lignite System;KCl-12%, S.Asphalt-2%; SG~1.50Sidetracked from 1267m and drilled down to 1502m. Tight hole prevailedString got stuck at 1298mCirculated, WOS, Two oil spotting, One acid job but failedBacked off string from 580m (Fish top), Placed cement plug 350-500m

Combination of 12% KCl & SG~1.50 insufficient to stabilise the hole


LithologyInterval(meter)FormationPressureMW as per GTOMW actualSidetrack-III ; KCl-K-Lignite-Polyol ,KCl-15%, S.Asphalt-2%, Polyol-5%440-9001.30.-1.32900-11001.32-1.431100-13601.43-1.461360-15001.46-1.511500-16001.51-1.57Phase-III; KCl-K-Lignite-Polyol ,KCl-15%, S.Asphalt-2%, Polyol-5%Olpad1600-1800Hyd+10-15%1.20-1.261.57-1.581800-2000Hyd+10-15%1.20-1.261.58-1.592000-2200Hyd+10-15%1.20-1.261.59-1.612200-2350Hyd+10-15%1.20-1.261.61-1.642350-2600Hyd+10-15%1.20-1.261.64-1.68


OBSERVATIONSSidetrack-III; KCl-K-Lignite Polyol;KCl-15%, S.Asphalt-2%, Polyol-5%; SG~1.57Sidetracked from 440m and drilled down to 497m. Incorporated 5% Polyol and Increased KCl upto 15%Recorded logs, Lowered 9-5/8 casing and cementedCombination of 15% KCl, 5% Polyol and SG~1.57 clubbed with controlled drilling stabilised hole in OCS/Olpad

Phase-III; KCl-K-Lignite-Polyol;KCl-15%, S.Asphalt-2%; Polyol-5%SG~1.57-1.68Deeper section (1600-2600m) of Olpad was drilled without complication with SG~ 1.57-1.68 against GTO recommended SG~1.26

Case Study: Mehsana Asset#WPAA: Exploratory

LithologyInterval(meter)FormationPressureMW as per GTOMW actualGuj alluvium00-420Hydrostatic1.06-1.101.05-1.10Jhagadia420-810Hydrostatic1.10-1.151.18-1.25Kand810-1020Hydrostatic1.10-1.151.25-1.33Babaguru1020-1200Hydrostatic1.10-1.151.33-1.45Tarapur1200-1260Hyd+10%1.15-1.201.45-1.50Kalol & Kadi1260-1280Hyd+10%OCS1280-1380Hyd+10%1.15-1.201.50.1.57Olpad1380-2400Hyd+10-15%1.20-1.261.57-1.76

Case Study: Mehsana Asset#WPAB: Exploratory

LithologyInterval(meter)FormationPressureMW as per GTOMW actualGuj alluvium00-400Hydrostatic1.06-1.101.05-1.12Jhagadia400-800Hydrostatic1.10-1.151.12-1.25Kand800-1000Hydrostatic1.10-1.151.25-1.27Babaguru1000-1180Hydrostatic1.10-1.151.27-1.31Tarapur1180-1220Hyd+10%1.15-1.201.31-1.38Kalol & Kadi1220-1320Hyd+10%1.15-1.201.38-1.40OCS1320-1740Hyd+15%1.20-1.261.40-1.58Olpad1740-2600Hyd+15%1.26-1.281.58-1.75

InferencesHydrational stresses due to swelling of shales could be mitigated by increasing the dose of KCl from 12% to 15%

MW to control formation pressure in Phase-III is 1.20-1.26.

MW actually maintained in Phase-III is 1.57-1.68 for borehole stability.

Inferences

This increase in MW, indirectly is required to satisfy collapse needs.Other wells; WPAA & WPAB were drilled without problems by maintaining 15% KCl in mud system and MW in the range 1.58-1.76

Cost Analysis

WellRig daysCost@Rs 0.10/day(Rs in Crore)Cost analysis(Rs in Crore)

PlanActualPlanActualGainOverrun

KAAF59.281375.92813.70-7.772

WPAB63.268.26.326.82-0.50

WPAA55.81555.5815.500.081-

Mud Weight Window

Boundary between Pore pressure/ Collapse pressure and Fracture pressure

Pore pressure/ Collapse pressure constitute Low bound side and Fracture pressure upper bound side of Mud weight window

Minimum Mud weight should be in excess of Pore Pressure or Collapse pressure whichever is higher

Mud system design (High performance inhibited) tends to move mud weight need towards low bound side of Mud weight window

Steps for Stability Analysis

Trajectory AnalysisTVDInclination ()Azimuth ()In-situ StressesField DataVHhCartesian Distributionxyxyzyxzz

Stability AnalysisCylindrical DistributionraxialPrincipal DistributionField Data123Failure AnalysisPw maxPw minPoissons ratioFailure CriteriaSafe Mud Weight Window

Inputs for Stability Analysis

Elements of a Geomechanics ModelRock stressRock strengthStableRock StressVertical stressHorizontal stressesStress directionPore pressureMud pressureActive tectonicRock StrengthTensile strengthShear strengthunstableThe entire drilling rig team is responsible for detecting stability problems. There are many controls to consider to achieve the balance (stable wellbore)

Geomechanics Model ParametersShminThe geomechanical model requires a detailed knowledge of in situ stress orientations, In situ stress magnitudes, Pore pressure, and Effective rock strength

ParameterSourcesVertical stress(Sv, )Integrate density logs along the well vertical depth Least horizontal principal stress, ShminLOT, XLOT, minifrac

Maximum Horizontal stress magnitude, SHmaxModelingMaximum Horizontal stress directionBore hole failure gives azimuth of max hor stressPore Pressure, PpRFT, DST, sonic, seismic, Rock strength, CoLab measurements, logs

Inputs for wellbore stability analysisSP/GR logs (for drawing shale baselines)

RHOB(Density log) (Determination of Overburden gradient)

Resistivity/ Sonic logs (Determination of Pore pressure gradient)

Poissons ratio/ LOT (Determination of Fracture gradient)

DSI log (cross dipole mode) (Determination of Shear failure gradient)

FMI/ or Four arm caliper log (Maximum stress orientation)

Inputs for wellbore stability analysis

Rock Properties (from cores of shale)P-Wave velocity, S-wave velocity, Youngs modulus, Poissons ratio, UCS, Tensile strength, Cohesive strength, Angle of friction

Software Landmark: Geostress module (Mud weight window, Wellbore trajectory,Breakout analysis)

Fundamentals of Solid Mechanics

Stresses: A solid can be subjected to all three stresses simultaneouslyRock Mechanics Terminology

Rock Mechanics TerminologyCompressive Stress: this stress consists of two opposing forces acting on a rock which decreases the volume of the rock per unit area. 'Compressive strength' The stress felt in the rock at the moment it fails is called Uniaxial compressive strengthRock Strength: is a measure of the strength of a rock mass when subjected to any one or a combination of three primary stresses

Rock Mechanics TerminologyTensile Stress: It occurs when material is in tension, like a cable suspending load. Tensile strength for a rock is usually much lower than its compressive strength, i.e. rocks are most likely to fail under tension well before they would fail under compression.

Rock Mechanics TerminologyShear Stress: It induces lateral movement within the material.

Shearing action is caused by two forces acting in opposite directions along a plane of weakness (fracture, fault, bedding plane, etc.) that is inclined at some angle to the forces. The result is a force couple which effectively tears the material.

StressStress is independent of size of the bodyIt is also independent of shape of the bodyNormal stress () acts normal to the plane; Shear stress() acts along the planeNormal stress results in Tensile or Compressive failureShear stress results in Shear failure where material is slipped along a plane

Three Dimensional stress state x xy xz

= xy y yz

xz yz z In the analysis of solid rock;Compressive stresses are defined as positive entitiesTensile stresses are defined as negative entities

Strain: It is related to elasticity.

Materials that are subjected to a load (compressive, tensile, or shear) will deform and either stretch or shrink in length.

This action is referred to as 'strain'

e = DL/L, where L is length and DL is the change in length. This is a dimensionless number.

The ratio between stress and strain is referred to as the 'Modulus of Elasticity', or Young's Modulus and is denoted as E.

E = P/A = s DL/L e

Strain= l (deformed dimension) lo (Initial dimension)

= l2 - lo2 (Almansi formula) 2l2

= l2 - lo2 (Green formula) 2lo 2

ElasticityStressStrainESlope=Youngs modulus= E x

= F/A

= l lo

l = F.l0 E.A

= / x

Elasticity: The ability of rock material to rebound to its original shape after an applied stress is relieved. Elastic deformation: occurs when all of the deformation caused by the stress is restored upon its release.

Plastic deformation: when stress that is below a critical threshold value is released, all of the deformation is restored.

However, if the applied stress exceeds the threshold value (which differs for various materials and rock types), permanent deformation results due to the load.

Stages of deformationBrittle vs Ductile failureRock Mechanics Terminology

Poissons ratioPoissons Ratio = Lateral Strain / Axial Strain.Value of the Poissons ratio varies in natural rock from between 0.1 to 0.5.

Use of Poisson's ratio is in the analysis of the propagation of an energy wave generated by an earthquake.

This wave moves through solid rock and is, therefore, somewhat subjected to rock properties.

The speed of propagation, or wave velocity, is dependent upon the Poisson's ratio of the rock.

As rock type changes, wave velocity changes as a function of rock properties.

P-Wave : Longitudinal sound wave; Can travel in any material

S-Wave: Transverse sound wave; Travel in solids only

Cross plotting of P-wave & S-wave velocities allow determination of

Rock properties such as

Fracture Density & Orientation

Poissons ratio

Rock type

Principal Stresses; Deviatoric Stresses

Principal StressesThe three stresses normal to the principal planes of a three dimensional stressed body in which associated shear stresses are zero A) x xy xz

= xy y yz

xz yz z 1 0 0

= 0 2 0

0 0 3 B)x - xy xz

xy y - yz = 0

xz yz z -

Principal Stresses3 - I12 - I2 - I3 = 0

WhereI1 = x + y + zI2 = xy2 + xz2 + yz2 - xy - xz - yzI3 = x(yz - yz2) - xy(xyz - xzyz) + xz (xyyz -xzy)I1, I2 and I3 are invariants as they remain unchanged for a given stress state regardless of orientation of the coordinate systemEquation C) always has three real rootsThese roots are called Principal stresses; 1, 2 and 3 where 1 > 2 > 3

Average and Deviatoric StressesAverage stress is given byD) m = 1 (x + y + z )

The total stress is sum of Average stress and Deviatoric stress

E) x xy xz

xy y yz

xz yz z m 0 0

= 0 m 0

0 0 m x -m xy xz

+ xy y -m yz

xz yz z -m

Average and Deviatoric StressesThe reason for splitting stress into two components is that many failure mechanisms are caused by deviatoric stress

Deviatoric stress actually reflects shear stress level

The Deviatoric invariants are as follows

J1 = 0

J 2= 1 [(1 - 2 )2 + (1 - 3 )2 + (2 - 3 )2] 6

J3 = I3 + 1 I1I2 + 2 I13 3 27

Average and Deviatoric StressesThe physical interpretation of the above invariants is that any stress state can be decomposed into its hydrostatic and deviatoric stress component

Hydrostatic component causes volume change in the body but not shape change

The Deviatoric component is the reason for shape change , and eventual rise in shear stresses

General Interpretation of Principal StressesIf all three principal stresses are equal, no shear stresses will existThis means that principal stresses exist in all directionsIt can be presented as sphere if plotted in spaceThis is known as Hydrostatic state of stress 0 0

[] = 0 0

0 0

General Interpretation of Principal StressesIn Hydrostatic state of stress, there are no shear stresses. It can be concluded that:Shear stresses arise when the principal stresses are differentA fluid under compression is in hydrostatic equilibriumA fluid at rest can not transmit shear stress Maximum shear stress is equal to one half the difference of the principal stresses 0 0

[] = 0 0

0 0

General Interpretation of Principal StressesComplex case occurs when two of the principal stresses are equal but different from the third. There will be symmetry in the plane which is orthogonal to the third stress componentThis cylindrical stress state is normally used when rock core plugs are tested in laboratoryThis stress state is often used for well bore stability analysis122 1 0 0

[] = 0 2 0

0 0 2

General Interpretation of Principal StressesThe third stress state is Tri axial stress stateAll principal stresses have different values132 1 0 0

[] = 0 2 0

0 0 3

Failure Criteria

Modeling Rock Failure: the Failure CriteriaFailure criteria are means of generalizing results of tests with simple stress conditions to apply to complex, 3-D stress conditions.Simple tests typically conducted in triaxial apparatus in which minimum and intermediate principal stresses are equal.Some use even simpler unconfined compressive strength (UCS) tests with zero confinement and assume the pore pressure = 0.

Examples of failure criteria includeMohr CoulombDrucker Prager

Rock Strength ParametersUCS an index measure of rock strength. Denotes the maximum compressive stress a material can sustain when unconfined.Cohesion (Co)the intercept on the shear stress axis of a straight-line Mohr-Coulomb envelope.In physics, cohesion is described as the force that holds together molecules or like particles within a substance.Friction Angle (FANG)Measure of internal rock friction, controlling intergranular slipTensile StrengthMaximum tensile force rock can sustain

2c211-331-3 2UnstableStableShear stress ()Principal stress ()Failure lineFailure lineFailure CriteriaMohr Coulumb Criterion= c + ; = 1 (1 - 3)cos; = 1 (1 + 3) 1 (1 - 3)sin 2 2 2

Effect of Mud WeightIncreasing mud weight causesIncreased radial stress which support the rock and prevents shear failureMakes the Mohr circle smallerPushes the state of stress to the safe region

Effect of Mud Weight - ContinuedIncreasing mud weight causesReduced hoop stressIncreased radial stressIf we plot Mohr circle with the two stresses as the least and greatest stresses we notice the following:Axial stressHoop stressradial stressAxial stressradial stressHoop stress (greatest)radial Stress (least)Low mud weighthigher mud weight

Effect of Mud WeightExcessive increase in mud weight causesNegative hoop stressIncreased radial stressIf we plot Mohr circle with the two stresses as the least and greatest stresses we notice the following:

Effect of Mud Filter CakeStrong, impermeable, flexible, and thin filter cake helps prevent differential sticking, and stabilize the wellboreCake prevents the penetration of drilling fluid pressure into the near wellbore region. Therefore, the stabilizing effect of mud pressure against the wellbore is preservedIf there is no cake, the drilling fluid pressure equalize the pressure in near wellbore reducing the radial stress to ZERO. Also the hoop stress will decrease

Axial stress

Failure CriteriaVon Mises Criterion

m-P0J2IntactFailureJ2 = 1 (1 - 3) 3

m-P0 = 1 (1+23) P0 3

Failure CriteriaGriffith Criterion

2at = keE a (1 - 3)2 = -8t (1 + 3)

Failure CriteriaHoek-Brown Criterion

1/c3/cIntactFailure1 = 3 + Ifc3 + Iic2

Failure CriteriaMogi-Coulumb Criterion

octoctIntactFailureoct = k + moct

oct = 1 (1-2) 2+(1-2) 2+(1-2) 2 = 2 J2 3 3

oct = 1 (1+2+1) 3k

Petroleum Rock Mechanics:Effective Stress

overburdenEffective StressTotal Stress = Pore Pressure + Effective Stress(S)(Pf)()Grain to grain contactFluid pressureEffective Stresses

Randy Smith

Effective Stress RatioPore Pressure+Effective Stress=Pore PressureTotal StressOverburden StressSvSV = P + sVSh = P + sh = P + K sVPPShsVshK == Effective Stress Ratio; K increases with ductility.

Petroleum Rock Mechanics:In-Situ Stresses

Overburden - Sv

The pressure exerted on a formation due to the total weight of the rocks and fluid above that depth.

As the rock is pushed vertically, it is squeezed horizontally, causing horizontal stresses to build.

Terzaghis Law:Sv = v + Pp

Creation of Horizontal Stresses H and hTectonic Loading is a typical mechanism for creating Horizontal Stresses H and h within the earth.

Cause of In Situ Horizontal Stress

Far Field Stresses v H and hBefore a well is drilled, the rock is in a state of equilibrium, its stresses are known as Far Field Stresses.When a well is drilled it introduces a new set of stresses known as Wellbore Stresses (influenced by Far Field Stresses and MW.)

In-SituStressesVertical StressV = b (h) gdh b = R (1 - ) + F.V = b.g.d = b.dV = 0.434 .d

b Formation bulk density lb/ft3 g Gravitational constant 32.175ft/sec2 h Vertical thickness of rock ftR Rock grain densityF Pore fluid densityvhH=2v=1Hh=3Rock In-situ stressesv >H >hRock In-situ principal stressesFor drilled vertical well

Extended LOT

Horizontal Stress

h = (v - P0) + P0 1-

Pb =3h - H Po + To

H =3h + To Po - Pb

Biot constantb rock formation specific weightPoissons ratioPo Pore pressureTo Rock tensile strengthPb Formation breakdown pressure

Stresses around WellborevHharRock formation with uniformStress stateRock formation with a drilled holeWhere the stress state will not remainunchanged

Petroleum Rock Mechanics:Stress Anisotropy

Stress Anisotropy

Rock Failure Modes

Shear FailureRandy Smith

Rock Failure

Shear Failure ModesFailure ConditionName

>a >rBreakout

a > >rToric Shear

a >r >Helical Shear

r >a >Elongated Shear

Tensile Failure Modes < r - Pp < tHydraulic Fracturer

Rock Failure: Breakout Failure

It is also known as Onion peel failure

Insufficient support to the wellbore wall by drilling fluid

Breakouts are usually oriented towards the axis minimum horizontal stress

Once it is initiated, it continues to grow resulting into irregular variation in diameter

>a >r

Rock Failure: Toric Failure

It is similar to Breakout failure

It results from insufficient support to the wellbore wall by drilling fluid

It is Overburden stress rather than horizontal stresses which causes the failure

Toric failure can extend along the whole circumference of the wellbore

a > >r

Rock Failure: Helical Shear Failure

Vertical stress is balanced by Radial stress exerted by drilling fluid

But the tangential stresses are insufficiently balanced to keep the formation balanced

The situation will cause the formation to shear downwards

The failure can extend along the whole circumference of the wellbore

a >r >

Rock Failure: Elongated Shear Failure

It is a result of excessive mud pressure with insufficient horizontal stresses to hold the wellbore together

It is similar to Breakout failure, but rotated 900 around the longitudinal axis of the wellbore

Due to rotation it is oriented towards maximum horizontal stress direction

The failure plane is further exaggerated in length, hence the name elongated

r>a >

Rock Failure: Tensile Failure: Hydraulic Fracturing

It is a result of mud pressure exceeding tensile strength of rock

The fracture will initiate and propagate along the plane where formation compressive stress is reduced to zero i.e. perpendicular to the minimum principal stress

Orientation of fracture plane and fracture propagation direction depends on Stress regimes

< r - Pp < t


123Normal Fault: v >H >h (1) (2) (3)Fracture propagationdirection


213Strike slip Fault: H >V >h (1) (2) (3)Fracture propagationdirection


312Reverse Fault: H >h >V (1) (2) (3)Fracture propagationdirection

Rock Failure: Tensile Failure: Exfoliation

Exfoliation usually occurs when the pore pressure exceeds the mud pressure and will come as the result of induced matrix deformationsunder predominantly undrained conditions

Wellbore Breakout AnalysisBorehole breakouts occur in the direction of minimum horizontal stress with the maximum horizontal stress orientation being perpendicular to the breakout direction.

Drilling approach: Normal Faulting Stress RegimeThe most stable direction for drilling is along the azimuth of h

The inclination angle should increase as the difference between H and h increases

If the horizontal stresses are equal (H = h) then the inclination angle should be zero

If major horizontal stress is equal to the vertical stress (H and V) then the well should be drilled horizontally (900)

Drilling approach: Strike Slip Faulting Stress RegimeThe most stable direction for drilling is horizontally (900)

The most stable direction to depends on ratio of principal horizontal stress to the vertical stress

Generally as the difference between the horizontal stresses increases, the direction needs to get closer to the direction of H

As the ration between the major stress and the vertical stress increases, the most stable drilling direction gets closer to the direction of H

If minor horizontal stress is equal to the vertical stress (h and V) then the well should be drilled along the azimuth of H

Drilling approach: Reverse Faulting Stress RegimeThe most stable direction for drilling is along the azimuth of H

The inclination angle should increase as the difference between H and h increases

If the horizontal stresses are equal (H = h) then the well should be drilled vertically

If (H = h), then the well should be drilled horizontally

Effect of Depletionpressuredepletionin stratum

lateral stressredistributionstressdepthstressconcentrationDrop in pore pressure in depleted zone.The reservoir shrinks because of drop in pore pressure.Increase in horizontal stresses above and below the zone.Effect of vertical stresses are negligible.

EFFECT OF DEPLETION

Consequences:Slower drilling rate because rock is tougherLost circulation and blow out risks go up substantially.More casing strings and LCM squeezes.Most serious in HTHP wells, Multiple zones.

Hole Instability

Cavings Due to Underbalance vs Mechanical InstabilityDelicate, Spikey ShapesUnderbalanceBlocky, or Angular ShapesMechanical Instability

Cavings Due To Underbalance

Cavings Due To Mechanical InstabilityShPmudPmudHoop StressBlocky ShapeShear PlaneIn Situ Horizontal StressWellboreHoop Stress Squeezes Chips Into Wellbore

Wellbore Stability and GeomechanicsAffecting Parameters While DrillingControllable Factors

Mud Chemistry and PropertiesDensityWhy is it important?Avoiding:Kicks or Blowout MW PP

Mud Chemistry and PropertiesDensityWhy is it important?Avoiding:Borehole collapse MW ShF

Mud Chemistry and PropertiesDensityWhy is it important?Avoiding: Fracturing Borehole MW FG

Mud Chemistry and PropertiesStable mud weight operational window

Constructing a Stable Wellbore

Mud Chemistry and PropertiesWhy is mud rheology important?In narrow windows where lower ECD is required, high shear thinning behavior is crucial

Mud Chemistry and PropertiesWhat about thixotropy?

Mud Chemistry and PropertiesWhy is the muds filter loss important?Micro-fractured Formation

Mud Chemistry and PropertiesWe need to avoid infiltration of fluids using the correct blend of sealing materials

Mud Chemistry (Rock-Fluid Interaction)Clays: What they look like

Mud Chemistry (Rock-Fluid Interaction)

Good Drilling PracticesHydraulic Optimization: Smart use of energy

Good Drilling PracticesHole Cleaning: So difficult in highly inclined wells

Good Drilling PracticesHole Cleaning: Best practicesUse rotary drilling as far as possible than sliding, but if sliding is necessary for trajectory control, put special attention.Use the maximum hydraulic capacity available (pump flow rate), consider wellbore stability issues and optimize mud velocity.Establish a strategy for hole cleaning practices.

Good Drilling PracticesHole Cleaning: Best practicesMuds rheology under bottom conditions has to be optimized.Annular velocity should be lower (big pipes better than small ones).Control ROP and avoid ROP peaks and borehole disturbances ( a few small cuttings is preferred than a large quantity of bigger cuttings).Spend some hours to clean efficiently, than spend days solving problems.Minimize the length of 40-60o deviation is a must.A young cutting bed is easier to remove than old one. So you have to detected it ASAP (permanent monitoring of torque, weight and traction, amount of cuttings returning to surfaces, etc.)

Good Drilling PracticesTripping and Running casingPipe movement into the hole should be gentle and smoothe

Good Drilling PracticesCementing: Poor cementing jobs always cause problems in the futureDeficient BoundCanalizationStress concentrationCasing displacementCasing collapse

Wellbore Stability: SummaryConsider the effect of mud weight but also note the interaction of other factors which control WBS, such as:

Well planning and well trajectoryDrilling practices (swab/surge pressures)Drilling fluids (chemical effects)Non-shear failure (e.g. regions of pre-fractured rock)

Wellbore stability is often, but not always, a mud weight issue.

Mud Weights Must lie within the range bounded by: - Pore Pressure - Fracture GradientCasing Used when mud weights must be increased above shallow fracture gradients to keep pace with deeper pore pressures.

Allowable Mud Weight Range

Shear FailureModified LadeSHg=.5(OBG+FG)Assumes vertical well.Zero breakouts

FLUIDS FOR SHALE CONTROL

Chrome Lignite- Chrome Lignosulphonate SystemInhibition of shale through maintaining CL-CLS in desired ratioAdvantageSolid tolerant Thermally stable (~1700C)Stabilising Borehole in conjunction with weightApplicationExtensively used in Exploratory drilling

KCl-K-Lignite Mud SystemEnhanced Inhibition of shale through Potassium ions AdvantageSolid tolerant Thermally stable (~1700C)Stabilise Borehole with lesser SG of MudApplicationExtensively used in Exploratory drilling

Low Solids Non dispersed Polymer (PHPA) Mud SystemBetter inhibition of shale through encapsulation of cutting and coating of borehole wall by PolymerAdvantageIncreased ROPBorehole stabilityGauged holeImproved hole cleaningFurther cut in SG of mud for borehole stabilityApplicationExtensively used in Development drilling

Polyol SystemBetter inhibition of shale through manipulating clouding of Polyol at required BHT with salt. Able to sustain differential pressure. AdvantageIncreased ROPBorehole stabilityImprovement in lubricityGood control over HTHP fluid lossComplication free drillingApplicationExtensively used in development drilling

Amine based HPWBMAmines derivatives are considered as good a shale inhibitor as Potassium chloride, hence has the potential to replace KCl in drilling fluid.

AdvantageLess dose vis-s-vis KClBorehole stabilityComplication free drillingPreventing Accretion

Bentonite pellet in 2% KCl (LSM study): Bentonite pellet crumbles appreciably

Bentonite pellet in 2% Amine (LSM study): Crumbling of Bentonite pellet is checked

Aftermath of Bit and Collar balling on the rig floor using 20% NaCl in mud in GOM (Left photo)Appearance of BHA when drilled with 20% NaCl+Amine+ROP enhancer (Right photo)

HTHP Drilling FluidTo meet the requirement of HTHP conditions where conventional water base fluid fails to work

Advantage Stable up to 236C. Can be weighed up to 2.20 specific gravity. Can be prepared in fresh as well as salt water

Fluid for Sub hydrostatic pressureIt is also meant to prevent pay zone damage where mud weight requirement is

Low Toxicity Oil Base MudWater based drilling fluid sometimes fail to deliver. Oil base system comes to rescue. It gives excellent well bore stability, flat rheology profile and superior lubricityAdvantageIncreased ROPBorehole stabilityImprovement in lubricityGood control over HTHP fluid lossComplication free drilling

FLUIDS FOR PAY ZONE DRILLINGNon Damaging Drilling Fluid

NDDF Based on MCCIt is meant to prevent pay zone damage. Non damaging and degradable constituents are used for its formulation. Acid degradable Micronised Calcium Carbonate is used as bridging material. The fluid is water based. AdvantagePrevention of pay zone damageEnhancement of ProductivityApplicationRecommended for drilling pay zone of all development wells

NDDF Based on Na formate/ K formateSodium/ Potassium formate are high specific gravity solids free fluids(SG~ 1.33 with Sod formate and upto 1.55 with Pot. Formate) which do not effect rheology adversely. The salts are inhibitive in nature and sustain temperature ~2250C.AdvantagePrevention of pay zone damageEnhancement of Productivity

* Microbubble Fluid

It can be used for drilling depleted reservoirs and simultaneously controlling partial to total mud loss encountered without any LCM pill.

Advantage

It creates a microenvironment of bubbles and high low shear viscosity which instantly stops loss of drilling fluid.

Microbubble resists coalescence and aggregation and remains as discreet bubble when entering into producing zones leading to less invasion of reservoir and easy clean up during flow back.

Being solid free no mud cake is formed and chances of differential sticking are minimized.

Total well cost can be reduced considerably.

Comparative Drilling Fluid Performance

Case Study: Geomechanics Modeling

*Postdrill stability analysis of Well A, Mehsana Asset from Sonic trends

Caliper10.8 14Mud Weight maintained13.33 14.41 ppg

ONGC a Wealth CreatorTrajectory Analysis

ONGC a Wealth Creator

Octahedral stresses we call the normal and shear stresses that are acting on some specific planes inside the stressed body, the octahedral planes. If we consider the principal directions as the coordinate axes (see also the article: Principal stresses and stress invariants), then the plane whose normal vector forms equal angles with the coordinate system is called octahedral plane. There are eight such planes forming an octahedron as it is illustrated in Fig. 1.

*************10********Terzaghis law ststes that the Total Overburden - Pore Pressure = Effective vertical Stress.*If a rock fractures then it fails due to the Minimum Horizontal Stress (assuming that the Vertical Stress is the largest stress).**If a rock fractures then it fails due to the Minimum Horizontal Stress (assuming that the Vertical Stress is the largest stress).*Radial Stress is dependant upon MW only.Tangential Stress is dependant upon MW and Far Field Stresses.Axial Stress is dependant upon Far Field Stresses only.Wellbore Stresses are important because it is these Stresses that lead to failure of the rock.If a rock fractures then it fails due to the Minimum Horizontal Stress (assuming that the Vertical Stress is the largest stress).

The maximum Tangential Stress occurs at a position that is 90 degrees from the maximum stress . In other words in the direction of the minimum stress.

*****************************Modified Lade is one of 5 available failure model. Requires an estimate of intermediate principal stress using a value halfway between FG and OBG is for demo purposes only.

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geomechanics

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