Coupled chemo-mechanical processes in reservoir geomechanics
Reservoir geomechanics investigates the implications of rock deformation, strain localization, and failure for completion and production of subsurface energy reservoirs. For example, effective hydraulic fracture placement and reservoir pressure management are among the most important applications for maximizing hydrocarbon production. The correct use of these applications requires understanding the interaction of fluid flow and rock deformations. In the past a considerable amount of effort has been made to describe the role of poroelastic and thermal effects in geomechanics. However, a number of chemical processes that commonly occur in reservoir engineering have been disregarded in reservoir geomechanics despite their significant effect on the mechanical behavior of rocks and, therefore, fluid flow. This dissertation focuses on the mechanical effects of two particular chemical processes: gas-desorption from organic-rich rocks and mineral dissolution in carbonate-rich formations. The methods employ a combination of laboratory studies, field data analysis, and numerical simulations at various length scales. The following conclusions are the results of this work: (1) the introduced numerical model for fluid flow with effects of gas sorption and shear-failure-impaired permeability captures the complex permeability evolution during gas production in coal reservoirs; the simulation results also indicate the presence non-negligible sorption stresses in shale reservoirs, (2) mineral dissolution of mineralized fractures, similar to pore pressure depletion or thermal cooling/heating can increase stress anisotropy, which can reactivate critically-oriented natural fractures; in-situ stress chemical manipulation can be used advantageously to enlarge the stimulated reservoir volume, (3) semicircular bending experiments on acidized rock samples show that non-planar fractures follow high porosity regions and large pores, and that fracture toughness correlates well with local porosity. Numerical modeling based on the Phase-Field approach shows that a direct relationship between fracture toughness and porosity permits replicating fracture stress intensity at initiation and non-planar fracture propagation patterns observed in experiments, and (4) numerical simulations based on a novel reactive fluid flow model coupled with geomechanics show that mineral dissolution (i) lower fracture breakdown pressure, (ii) can bridge a transition from a toughness-dominated regime to uncontrolled fracture propagation at constant injection pressures, and (iii) can increase fracture complexity by facilitating propagation of stalled fracture branches. The understanding of these chemo-mechanical coupled processes is critical for safe and effective injection of CO2 and reactive fluids in the subsurface, such as in hydraulic fracturing, deep geothermal energy, and carbon geological sequestration applications.