Simulation and interpretation of compaction patterns around boreholes excavated in high-porosity rocks

Boreholes excavated in porous rocks exhibit a variety of inelastic deformation patterns. This paper investigates how nucleation, propagation, and coalescence of compaction zones, including those resulting from compaction localization, affect the mechanics of excavated boreholes in high-porosity rock formations. For this purpose, a recently developed controllability framework able to differentiate among multiple modes of compaction banding is used to guide the interpretation of numerical simulations for boreholes excavated in porous rock. The goal is to explain the link between heterogeneous deformation patterns forming around the excavated zone and in situ stress conditions. To carry out the analyses, the theory is combined with a strain-hardening constitutive law calibrated against Berea sandstone laboratory evidence, thus capturing the stress-dependence of both homogeneous and heterogeneous compaction. The resulting simulations of idealized boreholes show that the inelastic compaction around the excavated zone depends dramatically on the far-field stress state, leading to transitions from isotropic-distributed plastic deformation modes under in-plane isotropic stress states, to dog-ear-, slot-, and/or butterfly-shaped inelasticity in the presence of strongly anisotropic in situ stress. It was found that each of these scenarios can be explained in terms of controllability indices computed at locations exhibiting inelastic response, thus establishing a link between the computed modes of global borehole deformation and multiple types of compaction localization processes (e.g., compactive shear bands, shear-enhanced compaction bands, and pure compaction bands).