The Porochemothermoelastic Coupled Solutions of Stress and Pressure With Applications to Wellbore Stability in Chemically Active Shale

Abstract

Abstract Overcoming wellbore drilling instability in shale formations will always present one of the major challenges to drilling engineers. It is well established that the pore pressure differential between the drilling mud and the drilled formation is governed by the diffusion process which is a time-dependent phenomenon. On the other hand, shales are highly chemically active and they swell when brought in contact with aqueous drilling fluids. Chemical effects arise from imbalance in chemical potentials between the species in the formation pore fluid and wellbore drilling fluid. The transport phenomenon and the diffusive nature of the solvent and solutes in the solution affect the chemical potential, which in turn will modify the fluid pressure, e.g., via chemical osmosis, and eventually may compromise borehole stability. In addition, a temperature gradient between the drilling fluid and the formation will lead to not only induced thermal stresses but also transient thermo-induced pore pressure responses. Thus, the downhole chemical activity and temperature (mud/shale) is also time-dependent variables leading to time delayed failures in collapse or fracture gradient. This work addresses the coupled transient thermal and chemical effects on shale stability through analytical poroelastic inclined wellbore solution. The shale is modeled as an imperfect semi-permeable membrane which can allows partial transport of the solutes. In addition to chemical osmosis, both solute transport and thermal effects are taken into account to realistically model field condition. The solution is applicable to studying deep shale drilling using water based mud where thermal effect and chemical interaction are significant. The solution is helpful in the prediction and visualization of time-dependent alteration of near wellbore effective stress concentration as well as mud chemistry, density and temperature effects on the mudweight window. Specifically, borehole stability is investigated under various scenarios such as heating, cooling, high and low mud activity for simulated downhole condition as well as for a field case. Analyses reveal that cooling the wellbore and high mud activity can destabilize the near wellbore region and result in a narrower mudweight window. Ignoring the coupled solute transport and thermal effect may mislead the optimization of mud density, mud salinity or mud temperature to stabilize the wellbore over the course of time.