Shale Stability: Drilling Fluid Interaction and Shale Strength

This paper presents main results of a shale stability study, related to the understanding of shale/ fluid interaction mechanisms, and discusses shale strength correlation. The major shale/ fluid interaction mechanisms: Capillary, osmosis, hydraulic, swelling and pressure diffusion, and recent experimental results are discussed. Factors affecting the shale strength are discussed, and a sonic compressional velocity-log based correlation for strength is proposed. Recommendations for modeling and improving shale stability are described, based on the current understanding of shale stability. Introduction Shales make up over 75% of the drilled formations, and over 70% of the borehole problems are related to shale instability. The oil and gas industry still continues to fight borehole problems. The problems include hole collapse, tight hole, stuck pipe, poor hole cleaning, hole enlargement, plastic flow, fracturing, lost circulation, well control. Most of the drilling problems that drive up the drilling costs are related to wellbore stability. These problems are mainly caused by the imbalance created between the rock stress and strength when a hole is drilled. The stress-strength imbalance comes about as rock is removed from the hole, replaced with drilling fluid, and the drilled formations are exposed to drilling fluids. While drilling, shale becomes unstable when the effective state of the stress near the drilled hole exceeds the strength of the hole. A complicating factor that distinguishes shale from other rocks is its sensitivity to certain drilling constituents, particularly water. Shale stability is affected by properties of both shale (e.g. mineralogy, porosity) and of the drilling fluid contacting it (e.g. wettability, density, salinity and ionic concentration). The existence and creation of fissures, fractures and weak bedding planes can also destabilize shale as drilling fluid penetrates them. Drilling fluids can cause shale instability by altering pore pressure or effective stress-state and the shale strength through shale/fluid interaction. Shale stability is also a time-dependent problem in that changes in the stress-state and strength usually take place over a period of time. This requires better understanding of the mechanisms causing shale instability to select proper drilling fluid and prevent shale instability. The basic shale stability problem can be stated as follows: Shale with certain properties (including strength) normally lies buried at depth. It is subjected to in situ stresses and pore pressure, with equilibrium established between the stress and strength. When drilled, native shale is exposed suddenly to the altered stress environment and foreign drilling fluid. The balance between the stress and shale strength is disturbed due to the following reasons: • Stresses are altered at and near the bore-hole walls as shale is replaced by the drilling fluid (of certain density) in the hole. • Interaction of drilling fluid with shale alters its strength as well as pore pressure adjacent to the borehole wall. Shale strength normally decreases and pore pressure increases as fluid enters the shale. When the altered stresses exceed the strength, shale becomes unstable, causing various stability related problems. To prevent shale instability, one needs to restore the balance between the new stress and strength environment. Factors that influence the effective stress are wellbore pressure, shale pore pressure, far away in situ stresses, trajectory and hole angle, etc. The effective stress at any point on or near the borehole is generally described in terms of three principal components. A radial stress component that acts along the radius of the wellbore, hoop stress acting around the circumference of the wellbore (tangential), axial stress acting parallel to the well path, and additional shear stress components. To prevent shear failure, the shear stress -state, obtained from the difference between the stress components (hoop usually largest and radial stress smallest), should not go above the shear strength failure envelope. To prevent tensile failure causing fracturing, hoop stress should not decrease to SPE 54356 Shale Stability: Drilling Fluid Interaction and Shale Strength Manohar Lal, SPE, BP Amoco