Introduction to Geomechanics: Rock Failure, Testing and Wellbore Instability
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The extensive slide-pack starts with introducing physics and basics on geomechanics. A lot of stress and rock strength concepts are explored. Then it moves on to explain the importance of the discipline for drilling, injection, sanding. Apart from giving theory to understand more difficult content that follow, it throws in practical application and prepares good ground for further study of geomechanical literature.
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Introduction to Geomechanics: Rock Failure, Testing and Wellbore Instability
- 1. Introduction to Geomechanics Farida Ismayilova, Drilling Geohazards Specialist 1
- 2. Agenda 2 Basic concepts Rock failure Testing rock strength Wellbore instability Water injection Sanding Stress cage “I am dying, Sir” Well Geomechanics
- 3. Why the industry needs geomechanics ?? To.. 3 Understand stress distribution in the field/basin Prevent/remediate mud losses (stress cage, loss circulation material) Choose the optimal well trajectory (well intersection angle) Predict pore pressure and fracture gradient resulting from water injection and depletion (stress path) Understand sanding (rock matrix failure), fracture propagation in case of water injection by fracturing Advise on wellbore instability issues
- 5. Important terminology 5 − Stress: − Effective stress: ’ − Strain: − Elasticity: E, v − Tensile failure − Pore collapse − Shear failure − Failure criteria − Yield surface − Mohr-Coulomb Deformation Failure Applied geomechanics deals with the measurement and estimation of stresses within the earth, and how those stresses apply to oilfield operations.
- 6. Stress and pressure 6 Geomechanics deals with stress and pressure Pressure is that part of the boundary forces supported by the fluid phase only Effective stress is the net force acting a – Axial stress r – Radial stress po Pore pressure A Fa a = A • - stress changes • p - pressure changes • C - chemistry changes • T - temperature changes All lead to ΔV - volume changes Quantifying ΔV is fundamental to geomechanics
- 7. Stress 7 Stress is force divided by the area the force is working on. Units Pa (Pascal=N/m2) SI PSI (pounds per square inch)
- 8. Strain 8 Strain is a measure of deformation. Strain (elongation or shortening).
- 9. Effects of increased force 9 What happens if you deform an object with an increased force? Typically the object will compress in the direction of applied load and expand in the direction of no load (or less load). No Force Force applied
- 10. Relationship between load and deformation (stress and strain) 10 The slope of stress-strain curve with constant confining pressure gives the stiffness, often referred to as theYoung’s Modulus.
- 11. Hooke’s law (simple definition) 11 Hooke postulated that: The deformation (strain) is proportional to the load (stress) and inversely proportional to the stiffness. Elastic Limit Permanent strain or plastic deformation Failure
- 12. Relationship between compression and expansion 12 The negative ratio of expansion over shortening is called the Poisson’s Ratio (n). Note that in geomechaincs compression is positive!
- 13. 13 Effective stress principle Terzhagis equation σ’ - Effective stress, σ - total stress Pp - pore pressure.
- 14. Effective stress principle 14 Imagine a pore in between three grains in the reservoir without a pore fluid. The effective stress is equal to the load on the grains, i.e. the total stress. Imagine the same pore in a reservoir filled with water. The pressure of the water is unloading the grains and reducing the grain contact force, i.e. reducing the effective stress. Imagine we inject water at high pressure into the pore. The higher pressure of the water is unloading the grains further and reducing the grain contact force further, i.e. reducing the effective stress even further.
- 15. Effective stress principle 15 In the earth, the stresses are going through the solid. The pressure in the pore is carrying some of the load. How effective the pore pressure is to unload the grains depends on the grain structure/geometry and is given by the poroelastic constant. Pf
- 16. Poroelasticity, effective stress 16 • Where is the poroelastic constant and is the pore pressure. is theoretically a value between the porosity of the rock and 1 Weak rocks has usually ~1 Other effective stress coefficients exists for acoustics, resistivity and permeability, these can be larger than one. f P − = ' Poroelasticity describes the interaction between fluid flow and solids deformation within a porous medium.
- 17. Pressures (stresses) in formations 17 OBP – Overburden pressure PP – Pore Pressure FP - Fracture Pressure
- 18. 18 Generalized cube showing simplified geomechanical modeling inputs.
- 19. 19 Diagram showing the definitions of vertical and lateral strain,Young’s modulus and Poisson’s Ratio μ shear modulus – restoring force/area
- 20. Max horizontal stress Min horizontal stress Fracture opening against (perpendicular) the min horizontal stress – σh when Pwb >FG Overburden Fracture Direction and Effect of Stress Contrasts 20
- 21. 21 Schematic showing the Anderson fault classification system.The relative magnitudes of the stresses with depth dictate the type of faulting in a given region.
- 22. 22 Rock Failure
- 23. Rock failure 23 When rocks are loaded past their elastic limit (i.e. permanent irreversible deformation), cracks start to grow in the cement between the grains and grains start to crack. The cracks are initially distributed across the whole rock volume. However, as the cracking progresses, a localization of cracks takes place forming larger global fractures that can be observed with the naked eye. The way the global cracks develop depends on the external load and the rock properties. It is the effective stresses that matter to deformation
- 24. Formation Pressure Integrity Test 24
- 25. − curves, for rock deformation leading to shear failure 25 25 Stress difference 1 - 3 Axial strain - a peak strength seating, microcrack closure “elastic” part of - curve massive damage, shear plane develops damage starts Sudden drop cohesion breaking continued damage ultimate or residual strength a r r = 3 a = 1 tmax planes slip planes axial cleavage Used to define elastic parameters S= σ
- 26. Rocks fail in three primary modes 26 Tensile Failure Shear Failure Pore Collapse/Compaction • Volume of grains = Const. • Volume of pore volume reduced • Permanent deformation (plastic)
- 27. Pulling the rock apart 27 If you pull on a rock specimen it will fail along a fairly well defined global crack. The micro cracks have been concentrated in a plane with maximum tensile stress. This failure is brittle. The rock’s tensile strength is relatively low.
- 28. Compressing the rock with equal stresses 28 The loading condition with equal stresses around the specimen is referred to as hydrostatic. The same loading condition as if you wrap the specimen in waterproof rubber and drop it into the deep ocean. The rock is generally ‘strong’ under this loading condition. The micro cracks will be distributed across the sample with no visible cracks. In some cases one can observe compaction bands. As the crack distribution reaches a certain density, the grains and fragments can find a new packing arrangement by reducing the porosity (pore collapse). The rock loses some of its load bearing capacity at pore collapse yield. It gets less stiff (softer) the failure is ductile. 28 σ3 = σ1 σ1
- 29. Compressing the rock with unequal stresses 29 As the load is increasing in one direction while it is kept constant or reduced in another, the micro cracks will tend to coalesce to form global shear bands. The angle of the shear band, relative to the maximum principal stress, will be determined by a strength measure called internal friction angle. The load bearing capacity is reduced to a residual governed by the friction of the fracture surfaces. This failure mode is called shear failure and is generally brittle. 29 σ 3 σ 1
- 30. Failure mechanics 30 Typical test specimen for uniaxial or triaxial test Typical rock behavior in triaxial tests (i.e., the peak strength is increased as the confining stress (3)is increased Brittle Ductile
- 31. Shear failure of a sandstone 31 a r = 3 a = 1 High quality cylinder L = 2D Flat ends High angle shear plane Zone of dilation and crushing
- 33. Measuring rock strength 33 Direct measurements with confinement (3) Compressive strength:Triaxial testing machine Encase the core in an impermeable sleeve Confining stress is applied 3 first σa is then increased while… Pore pressure constant Record Δσa, εa, εr, ΔV More exotic tests ΔT, Δp, even Δchemistry Creep tests (constant σ, measure ε) Hollow cylinders Triaxial Test Stresses ′r = ′3 ′a = ′1 σ′n tf Shear planes
- 34. Testing Rock Strength 34 Triaxial test animations Presenting triaxial test results Using triaxial test results for wellbore stability analysis Unconfined Compression Strength test video
- 35. Triaxial Test 35 Confining Pressure (psi) Compressive Stress (psi) 1000 2000 3000 4000 5000 6000 5000 10000 15000 20000 25000 30000 Break Risk area Safe area
- 36. Triaxial CompressionTest Horizontal Cross Section ofWellbore Mudweight effects: 1. Radial (confining) stress 2. Reducing tangential (axial) stress 36
- 37. Triaxial Compression Test Results / Mudweight Effect Triaxial Compression Test Results 0 5000 10000 15000 20000 25000 30000 0 1000 2000 3000 4000 5000 6000 Confining Pressure (psi) Effective Comprassive strength (psi) Core 2 37 Horizontal Cross Section of Wellbore Effective Compressive stress (psi)
- 38. 38
- 39. Confining Pressure Compressive stress Normal Stress Shear Stress Mohr’s stress circle Coulomb’s failure line 39
- 40. Normal Stress Shear Stress Mohr’s stress circle Coulomb’s failure line 2 40
- 42. Introduction to Geomechanics Part 2 Farida Ismayilova, Drilling Geohazards Specialist, BP 42
- 43. 43 Wellbore Stability – from drilling perspective
- 44. Wellbore instability issues 44 Wellbore instability problems arise when stresses around wellbore exceed the rock strength Cost: 10% of drilling costs, $500-$1000MM/year to industry Causes: High in situ stress, low rock strength, drilling fluid/shale interaction, incorrect mud weight, surge/swab Consequences: Hole collapse, tight hole, stuck pipe, hole cleaning problems, torque/drag, washouts, lost circulation Solutions: Planning adjustments:Well path, mud system and mud weight, drill string, casing, cementing Operational adjustments: pipe movement and trip speed, wiper trips, mud weight and rheology, ROP, torque/drag monitoring
- 45. General concepts 45 Improper Drilling Practices Undesirable Formations Improper Well Trajectory Incorrect Mud Causes Near-wellbore stress exceeds formation strength Fracturing & Lost Circulation Hole Collapse, Hole Cleaning, Stuck Pipe Instability Cuttings Hole Closure Overgauge Hole Lost Circulation stuck Adverse Effects
- 46. Effect of mud weight 46 Increasing mud weight promotes stability Support by the drilling fluid pressure helps keeps the blocks in place, and so promotes stability. But fluid-loss control needs to be good to stop fluid leaking into the rock and destabilizing the blocks! Pmud
- 47. 'Predicting' onset of instability 47 We now have methods of estimating in situ stresses (e.g. Sv and σhmin from PPFG plots, an estimate of σHmax). We also have methods of measuring or estimating rock strength and deformation. We can calculate stresses around wellbore. Putting these together allows prediction of shearing initiation on the borehole wall, giving …an estimate of 'breakouts initiation' or the onset of Wellbore Instability.
- 48. Cause: • Insufficient mud weight Consequences: • Borehole breakout • Poor hole cleaning / pack- offs • Excessive trip / reaming time • Stuck pipe • Poor logs • Poor cement quality Breakout Instability mode: Shear failure of intact rock 48 Breakout Breakout
- 49. Causes: • Mud weight too high • Thermal cooling in HPHT wells • high horizontal stress anisotropy h < v < H Consequences: • Mud losses • Wellbore breathing (ballooning) • Lost circulation/ well control Breakouts breakouts Tensile fractures h H H – Maximum horizontal stress h – Minimum horizontal stress H – Maximum horizontal stress h – Minimum horizontal stress breakouts Tensile fractures h H H – Maximum horizontal stress h – Minimum horizontal stress H – Maximum horizontal stress h – Minimum horizontal stress breakouts Instability mode: Tensile failure of intact rock 49
- 50. Operational mud weight window 50 Borehole Geometry ‘STABLE WINDOW’ ‘SAFE WINDOW’ Pore Frac. High MW Low MW A A’ Sh Borehole Geometry ‘STABLE WINDOW’ ‘SAFE WINDOW’ Borehole Geometry ‘STABLE WINDOW’ ‘SAFE WINDOW’ Pore Frac. High MW Low MW A A’ Sh Borehole Geometry ‘STABLE WINDOW’ ‘SAFE WINDOW’ Pore Frac. High MW Low MW A A’ A A’ Sh
- 51. Controllable factors in the wellbore instability problem 51 Well Trajectory (azimuth & deviation) Drilling Fluid weight & chemistry; fluid-loss additives for StressCage™ implementation Drilling Practices Parameters (controllable) that affect near-wellbore stresses, and resisting formation strength, that can combat instability.
- 52. Causes of wellbore instability that have to be designed for … 52 Adverse formations Reactive shales Fractured formations Plastic formations Over-pressured formations Depleted formations Weak formations In situ conditions Abnormal horizontal stresses Abnormal temperatures Solutions mud type / chemistry chemistry, fluid-loss control, practices mud density mud density fluid-loss control, StressCage™ mud density mud density, trajectory mud type, drilling practices
- 53. What does wellbore stability mean? Stable wellbore Breakout - Acceptable only when failure is limited – focus on mud weight first Washout - Avoidable – focus on mud and practices first Severe WBS problems - Complex issue – be open-minded and address root cause(s) Unstable wellbore
- 54. Wellbore instability is the major cause of unscheduled events and associated trouble costs. Consequences: Planning: well path, mud system / mud weight rheology Operational: ECD & torque/drag monitoring, trips, ROP Solutions: high in-situ stress, low rock strength, fluid/shale interaction, incorrect mud weight / ECD, surge/swab Causes: Stress exceeds strength Stuck Pipe STUCK Hole Cleaning Hole Washout Lost Circulation Shale Instability Torque/Drag WellControl Key-seating Stuck Tight Hole Causes, consequences, costs, solutions
- 55. 55 Wellbore Stability – Intersection Angle Bedding plane vs trajectory
- 56. Why does weak bedding cause wellbore instability? In laminated shales, the cohesion and shear strength of bedding planes can often be substantially lower than that of the equivalent intact or non-bedded rock. This is clearly seen in laboratory experiments where boreholes are drilled at different angles to bedding through blocks of laminated shale (photos courtesy of Oakland and Cook) 56 Wellbore drilled normal to bedding Wellbore drilled parallel to bedding
- 57. Roof Collapse Mechanism of Instability 57
- 58. Strength Anisotropy 58 Plugged parallel to bedding Plugged at 45 deg to bedding Will they all have the same strength? If not which is the strongest and the weakest? Plugged normal to bedding Anisotropy is properties variation depending on direction.
- 59. Plugged at 45 deg to bedding Will they all have the same strength? If not which is the strongest and the weakest? Plugged parallel to bedding Strength Anisotropy 59 Photos from John Cook, Schlumberger Plugged normal to bedding
- 60. Plugged at 45 deg to bedding Will they all have the same strength? If not which is the strongest and the weakest? Strength Anisotropy 60 Plugged parallel to bedding Plugged normal to bedding Plugged at 45 deg to bedding Plugged at 45 deg to bedding Photos from John Cook, Schlumberger
- 61. Will they all have the same strength? If not which is the strongest and the weakest? Strength Anisotropy 61 Plugged parallel to bedding Plugged normal to bedding Plugged at 45 deg to bedding Strong Weak
- 62. Strength vs Well Trajectory 62 Strong StrengthTesting or Vertical well Strong Strong
- 63. Strong Weak Weak + High-angle well Strength vs Well Trajectory 63 StrengthTesting or Vertical well + Strong Strong Photos from John Cook, Schlumberger
- 64. 64 Water Injection vs Geomechanics
- 65. Water Injection vs Geomechanics 65 Pros: Slows down depletions Prevents pore collapse, compaction Keeps pore pressure, subsequently fracture gradient high Cons: Water injection creates fractures High injection rates damages rock matrix around the wellbore of the injector When the water reaches a producer, water brings solids, sand particles. Again destroys the integrity of rocks, completion (quality of gravel pack which is mostly used in ACG)
- 66. Fracture Direction and Effect of Stress Contrasts Overburden Max horizontal stress Min horizontal stress Injection fluid pressure Injection Pressure 66 Layer A Layer B Layer C Injection will not initiate any fractures and the injected fluid will not propagate. Injec. Pres. < Min/Max horiz stress oooooofor Layers A,B,C
- 67. Max horizontal stress Min horizontal stress Injection fluid pressure Overburden Injection Pressure Fracture Direction and Effect of Stress Contrasts 67 Layer A Layer B Layer C Injec. Pres. > Min horiz. stress ooooooat Layer B Injection Pressure is increased to allow injected volume propagate through creating fractures. Fractures resulting from injection will open against the minimum horizontal stress
- 68. 68 Sanding issues vs Geomechanics The production of solids together with the reservoir fluid Besides sand, solid production can include chalks, coals, limestone, etc.
- 69. World Map of Sanding Regions 69 In 2016 around 65% of production came from sand prone reservoirs at BP. That can grow due to both inherently weaker rocks and more extreme operating conditions in terms of reservoir pressure depletion and sand face drawdown. Regions facing sanding issues Courtesy of Hans Vaziri/Yuxing Xiao
- 70. Sand Production Patterns 70 • Sporadic sanding – transient sand production caused by abrupt changes in downhole flowing pressure. Most frequently occur during: ▪ Shut-in ▪ Bean-up • Continuous sanding – massive sanding generally due to: ▪ Excessive drawdown ▪ Excessive depletion
- 71. Sanding and Rock Mechanics 71 Sanding is a Rock Mechanics Issue The important rock mechanics factors are: The tensile and shear strength of the reservoir The in situ stresses and pore pressures The hydrodynamic drag forces on the matrix Alteration of rock properties (damage) Weakening of cohesion due to rock/fluid interaction Alterations of stresses and pressures with time Understanding the physics is vital Completion strategy is critical Well management is key Courtesy of Dussault Maurice
- 72. Mechanisms of Sand Production 72 Rock failures due to high effective stresses High draw down, low confining stress (low frictional strength) High depletion Drag forces of the produced fluid bring the failed materials from the perforation tunnels or formation to the wellbore. The failure of the perforation due to the onset of water production (reduction in capillary pressure). Cohesion is destroyed, weakening As “cavity” grows, high shear stresses on the walls lead to weakening and dilation
- 73. Failure of Formation Causes Sand Production 73 Shear failure caused by production drawdown Low wellbore pressure Increased effective stresses Shear failure caused by reservoir depletion Differential increase in stresses Shearing can lead to breaking of bonds and changing of material properties Tensile failure caused by high flow rate Self stabilizing because as cavity grows, fluid gradient becomes smaller and sand production tends to stop Localized failure (e.g. wormhole) may lead to additional stress concentration and further sand production
- 74. Mechanism of Sand Production 74 SAFE TENSILE FAILURE SHEAR FAILURE Pore pressure Drawdown pressure •High drawdown results in shear failure •High pore pressure causes tensile failure After Morita et al. (1987)
- 75. 75 Depletion – Stress Cage Losses – Loss Circulating Material
- 76. Depletion 76 Non-uniform Depletion and Stress Arching Uniform Depletion
- 77. Stress Cage Concept 77 Stress case - prevention LCM – remediation (Lost Circulation Material) Stress Cage short fracture(s) are induced as wellbore pressure exceeds fracture pressure LCM mud block and isolate the induced fractures from the wellbore Allows the fluid within the fracture to drain into rock matrix Further fracture growth is prevented.
- 78. Loss Circulation Material vs Fracture Width 78 100micron=0.1mm
- 79. 79 Tornado Plot representing fracture width sensitivity to different parameters.
- 80. 80 Stress Cage Envelope of experience (global dataset) Increases 100micron=0.1mm • The more fracture pressure is exceeded, the wider fracture is initiated. • There is a limit on the size of stress cage particle. Increasing the limit will result in sagging of the material in the solution.
- 81. 81