Effects of ion transfer on stress and pore pressure distributions around a borehole in shale

摘要

The development of a scientifically robust and practical physicochemical theory for describing shale deformation while considering chemical and poroelastic processes is described herein. Ion transfer in the mud/shale system is coupled to formation stresses and pore pressure. The field equations are derived within the framework of a Biot-like isotropic poroelastic theory and allow determination of the mechanical response of shales to drilling-induced chemical, stress, and pore pressure disequilibria. These field equations are solved analytically to yield the solute mass fraction, pore pressure, and the stress distributions around a borehole using a generalized plane strain approach. The impact of ion transfer on pore pressure/stress fields around a borehole in shale has been studied by considering a vertical well. It has been determined that ion 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 has been found that, while the wellbore may be supported by a mud pressure of significant magnitude, the rock can experiences a tensile effective radial stress due to the invasion of solute into the formation. The distribution of tangential effective stress is also affected by ion transfer; it is reduced at the borehole wall, however, this reduction is initially overcome by the larger increase resulting from the reduction of pore pressure near the wellbore. The transfer of ions in the mud/shale system has a significant impact on stress and pore pressure distributions around a borehole. Hence, its contribution to borehole failure is significant and should be considered in the process of optimizing the mud properties.