WELLBORE STABILITY APPLICATION

Components of Wellbore Instability

1. Fluid Shale Interaction

2. Unbalanced Stress

FLUID-SHALE INTERACTION

Shale: a fine-grained, sedimentary rock primarily composed of clay minerals and silt-sized particles.

• low permeability

• low tensile and compressive strength

1. Due to hydration and swelling

   • Clay Minerals: Shales often contain clay minerals (smectite) that swell when exposed to water-based drilling fluids. This leads to reduced mechanical strength and increased instability

   • Cation Exchange: Water entering the shale matrix may displace stabilizing cations, causing further weakening

2. Diagenesis

   • Over time, burial of sediments causes compaction, reducing porosity and permeability. As smectite transforms into illite, it releases water.

   • Drilling a hole and giving the sediments access to water can reverse the process (rocks turned into sediments).

3. Hydratable (Swelling) and Dispersable Shales

4. Implications of Poor Shale Inhibitions (Ability to slow down the hydration, swelling and disintegration of clays and shales)

   • Wellbore Instability: Hydration swelling, Disintegration, Collapse

   • Operation Challenge: Stuck pipe, Loss circulation

5. Mechanism of Various Types of Shale Inhibitors

   • Potassium System

   • PHPA System

   • Amine System

   • Glycol System

6. Design workflow towards optimum shale inhibition

   • CEC (higher CEC - swelling, lower CEC - dispersive)

   • XRD Analysis

   • Capillary Suction Time

   • Shale Discovery Test/Dispersion Test

   • Accretion Test

   • Line Swelling Test

UNBALANCED STRESS

1. Earth Stresses (also known as in situ stresses or far-field stresses)

2. Stress Regimes

3. The Basics

   • In a vertical wellbore, the mud column must provide a stabilizing force to compensate for the loss of the forces from the rock

   • But the mud doesn’t have to provide the full amount of the missing restraining force since rock has its inherent strength

   • In a horizontal wellbore, the mud column have to provide the missing restraining force from below

4. Tunnel Failure (Classic Instability → Requires More Mud Weight)

   • Wellbore fails due to the stress on the wall. If mud weight is not quite adequate, wellbore fails at the side.

   • Failure is called Breakout and the severity of the failure called Angle of Breakout

     

   • Principal Stresses at the Wellbore

     • Near-wellbore stress state (Axial, Radial, and Tangential Stress)

       • Tangential/Hoop stress = f(in situ stress, well parameter) - Pw/MW - Pp

       • If MW increases, Pw increases, Hoop stress decreases

       • Radial stress = Pw - Pp

       • If MW increases, Pw increases, Radial stress increases

       • Mohr Coulomb to obtain shear strength line. Any stress that overpasses the shear strength line will encounter shear failure

     • Far Field stress state

   • Wellbore Deformation

     • Shear failure (90 degree from maximum stress or in direction of minimum stress) favored by:

       1. Low Pw (Wellbore pressure)

       2. Big difference in stress

       3. Low rock strength

     • Tensile failure (90 degree of minimum stress or in direction of maximum stress) favored by:

       1. High Pw

   • Managing Tunnel Failure

     • Depending on the stress regime, the mud weight requirement may depend on well inclination and azimuth (Refer to stereonet and sensitivity plot)

     • Stereonet refers to collapse pressure where MW must be higher than PP and collapse pressure to avoid hole collapse and maintain in safe mud window.

     • Normal and Reverse Regime has different mud weight strategy for borehole stability.

5. Bedding Plane Failure

   • Occurs when well path’s/trajectory ‘angle of attack’ is near parallel to the bedding plane.

     

   • Usually occur at connections

   • The rocks have no ability to resist being split apart by the mud pressure. The rock is then ‘supercharged’ and will fail when ECD is relieved

6. Hole Failure Diagnosis (Differences between Classic and Bedding Plane Failure)

7. Pre-Existing Fractures

   • Due to natural fracture such as earthquake or faulting

   • Rock is mechanically weakened and exhibits high fluid flow path with low permeability reservoir matrix

   • In overbalance drilling - drilling mud enters fractures resulting in loss of circulation and damaging formation. Also, it weakens mechanical strength of formation.

8. Type of cavings

Splintered Cavings		Due to tensile failure Occur in overpressured zones drilled underbalance	Raise mud weight Avoid/minimize swab load
Angular Cavings		Due to shear failure	Raise the MW Recognize the need for more patience Improve mud inhibitions
Platy/Tubular Cavings		Result of natural fractures and bedding plane failures If mud P > Sh,min, mud will invade the fracture network	Minimize changes to wellbore P Recognize the need for more patience Minimize shocks and vibrations Avoid backreaming

9. Time-Dependent Shale Instability

   • When fluid invades, the pore pressure in shale increased. But the P cant be dissipated due to low permeability characteristics of shale.

   • Hence the pore pressure near to the wellbore can be supercharged higher.

   • With time, shale started to show sign of collapsing

   remedial:

   • MW is increased until the differential P (used to balance and stabilize the shape of wellbore) is increased to the supposed differential P

   • BUT, MW cant be further increased to avoid surpassing Fracture Gradient!