The Effect of Chemical Environment on Crack Propagation in Pressure-Sensitive Rocks

Abstract Fracture propagation due to fluid pressurization is frequently encountered in the oil and gas industry, enhanced geothermal systems (EGS) as well as carbon capture, utilization and storage (CCUS). In the operation of hydraulic fracturing, acidizing treatment is often incorporated for low-permeability, tight, unconventional reservoirs, to soften the rock and promote crack connectivity, which has been proven effective for carbonate-rich reservoirs. However, the complex interplay between the evolution of the stress field, deformation, hydraulic properties and chemical processes (e.g., mineral dissolution) during the stimulation and maintenance phases demands a sophisticated understanding. How a fluid-driven crack propagates in a stressed frictional rock undergoing mineral mass removal as surrounded by a chemical environment, remains elusive. Here we investigate the acid-assisted fracking problem for pressure-sensitive rocks by adopting a coupled reactive-chemo-mechanical model, considering a combined effect of micro-cracking enhancement on the chemically driven shrinking of the yield surface, a dissolution induced ductile transition post-yield, as well as a chemically affected elastic modulus. Our numerical results show that the subcritical propagation of a single blunt-tip crack can be chemically driven and the yielding concentrates at the crack tip which expands penetrating into the material in front of the tip point. A typical three-region development of Mode I crack propagation is identified by the plotting of crack propagation velocity versus stress intensity factor. By adopting the Drucker–Prager yield criterion, more pronounced material yielding and chemical mass removal arise in the near-tip region compared to the frictionless reference case. Microstructural heterogeneity in the form of inhomogeneous distribution of the initial porosity leads to a substantial acceleration of the crack propagation, which is attributed to the interaction between the microstructure and the chemo-mechanical process during evolution, as well as a distinct self-organized pattern of micro-bands formed in front of the crack tip.