Interpretation and Application of Acoustic and Transient Pressure Response to Enhance Shale (In)Stability Predictions

Abstract

Abstract The main objective of this work is to discuss the initial results of an ongoing experimental study. The study has investigated the role of chemical potential of drilling fluids on transient pore pressure, acoustic, static properties, and strength of shale formations subjected to compressive confining stress. The motivation behind this study is to develop real time wellbore (in)stability logging capability. Wellbore (in)stability in shales is a major problem costing the petroleum industry, according to conservative estimates, $700 million annually. Understanding and modeling mechanisms of shale (in)stability is an ongoing industry effort. Drilling a hole into a formation in equilibrium induces stress concentration near the borehole. A chemical interaction will occur if parameters such as chemical potential, ionic concentration, etc., of the drilling fluid and the shale formation fluid are not in equilibrium. Any differences in these parameters will alter the near wellbore pore pressure which in turn will influence the borehole equivalent stress-state and the shale mechanical strength thus affecting wellbore (in)stability. Tests conducted at compressive uniaxial and triaxial stress conditions in the past have indicated the strong influence of these chemical parameters on shale strength. In this study, transient pressure, compressional and shear wave velocities, deformation and strength of shale cores were measured as a function of time when exposed to a drilling fluid. The experiments were conducted under conditions simulating a borehole portion under geostatic stress and the wellbore surface exposed to a circulating drilling fluid. The uniqueness of this study comes from the fact that the mechanical property, acoustic velocity and transient pressure for each sample are measured simultaneously as a function of time during the experiment. This eliminates the anisotropy-associated differences obtained when different samples are used to determine the relationship between these characteristic parameters of shale formations. This comprehensive testing program will enhance our understanding of the relationship between acoustical, mechanical and chemical properties of shale as a function of time when exposed to a drilling fluid. Interpretation of static and dynamic rock properties utilizing acoustic velocity data as a function of the transient pressure of shales under identical stress state will facilitate real time wellbore (in)stability predictions using MWD/LWD information. Thus, this application provides better real-time management of (in)stability problems in the field.