Influence of coupled chemo-poro-thermoelastic processes on pore pressure and stress distributions around a wellbore in swelling shale

Abstract

Drilling efficiency can be improved by optimizing the drilling fluid parameters such as density, salt concentration, and temperature. Selection of an appropriate mud can benefit from considering the impact of mud chemistry and temperature on stress/pore pressure variation around the wellbore. A pore pressure and stress analysis is presented in this work that is based on a coupled thermoelastic model of a chemically-active rock saturated by a binary electrolyte fluid consisting of a solute and diluent. The analysis is carried out within the framework of a theory that considers conventional poro-thermal expansibility as well as the variation of chemical potential with temperature, and takes into account direct flow of diluent, solute, and heat by diffusion. In addition, flow of diluent by chemical osmosis and thermal osmosis, and flow of solute by thermal filtration are included. The field equations are rooted in constitutive equations that are based on the principles of irreversible thermodynamics. Their solution yields the redistribution of pore pressure, temperature, solute mass fraction, and stress that result from drilling. An example is used to show the influence of the coupling between hydraulic, thermal, and chemical processes on stress and pore pressure distributions. The results indicate an interesting interaction between the temperature, chemistry, and stress. When the solute concentration in the mud is larger than in the formation both the effective radial stress and tangential stress increase, becoming more compressive, and vice versa. When the higher salinity mud is cooler than the rock the stress deviator decreases at the borehole wall suggesting enhanced wellbore stability with respect to shear failure. This is consistent with porothermoelastic analysis and field experience. Also, results show that cooling a high salinity mud increases the potential for tensile failure.