Fully Coupled Anisotropic Porothermoelasticity Modeling for Inclined Borehole in Shale Formations

Existing porothermoelastic models assume that shale is isotropic and disregard material anisotropy. When material anisotropy is considered, the shale-bedding plane must be assumed to be perpendicular to the borehole axis. However, in the actual drilling of shale reservoirs, the assumptions above are idealized to correctly identify the mechanisms of wellbore instability in shale reservoirs. Therefore, based on the generalized porothermoelastic theory and the generalized plane assumption, a pseudo three-dimensional finite element model that considers complete material anisotropy is established. The effects of elastic anisotropy, solid thermal expansion coefficient anisotropy, permeability anisotropy, thermal osmosis coefficient anisotropy, and thermal conductivity anisotropy on effective stress and pore pressure are analyzed parametrically. The following conclusions are based on ΔT = − 50 K (cooling) and k^T > 0: when the permeability of the formation is less than 1e−19 m², thermal osmosis governs the distribution of pore pressure. When the formation is drilled, the pore pressure distribution under elastic anisotropy differs completely from that under elastic isotropy. As time progresses, the pore pressure increases under the effect of hydraulic pressure; however, the higher the elastic anisotropy ratio, the faster the pore pressure increases. Additionally, the greater the elastic anisotropy ratio, the greater is the effective hoop stress. As time progresses, the elastic anisotropy ratio increases and the effective hoop stress decreases. The greater the elastic anisotropy ratio, the greater are the anisotropy ratio of the solid thermal expansion coefficient, the permeability anisotropy ratio, the thermo-osmotic anisotropy coefficient ratio, and the thermal conductivity anisotropy ratio, which is conducive to wellbore stability. Based on a quantitative evaluation of the parameter anisotropy, the elastic anisotropy exerts the greatest effect on wellbore stability.