Linear chemo-poroelasticity for swelling shales: theory and application

Optimization of drilling fluid parameters such as mud weight, salt concentration, and temperature is essential to alleviating instability problems when drilling through shale sections. The selection of suitable mud parameters can benefit from analyses that consider significant chemo-mechanical processes involved in shale-drilling–fluid interactions. This paper describes the development of a scientifically robust and practical physicochemical theory for describing shale deformation. The theory considers chemical and poroelastic processes and couples ion transfer in the mud/shale system to formation stresses and pore pressure. The field equations are derived within the framework of a linear, Biot-like isotropic poroelastic theory. These field equations are solved analytically for the problem of a wellbore in shale to yield the solute mass fraction, pore pressure, and the stress distributions around the borehole. The solution is developed using the generalized plane strain approach and is used to study the impact of solute transfer on stress and pore pressure fields. The analyses indicate that solute transfer causes the chemical-osmosis to become time-dependent. As time increases, the transfer of ions leads to osmotic pressure dissipation and re-establishment of a pore pressure regime characteristic of hydraulic flow. Furthermore, it is observed that while the wellbore may be supported by a mud pressure of significant magnitude, the rock can experience a tensile effective radial stress due to the physicochemical interaction between the shale and the drilling mud. Hence, the contribution of the physicochemical processes can significantly impact the stress and pore pressure distributions around a borehole and should be considered when optimizing drilling mud properties.