Pore-scale modeling of thermal-hydro-mechanical-chemical coupled rock dissolution and fracturing process

Acid fracturing can effectively improve the connectivity inside underground reservoirs, which is of significance for geothermal exploitation and geological carbon sequestration. It involves a complex thermal-hydro mechanical-chemical (THMC) coupled process. This work first develops a pore-scale THMC coupled model combining the lattice Boltzmann method (LBM) and the discrete element method (DEM) to describe the acid fracturing process, particularly covering chemical damage, rock dissolution, and solute transport. A chemical damage variable based on the cohesive bond is proposed to characterize the alteration of mechanical parameters caused by local rock dissolution. The change of fluid pathway due to rock dissolution is handled by the lattice Boltzmann solute transport model combined with the volume of pixel method. Based on the proposed model, the influence of injection pressure and Damkohler number (Da) on the THMC coupled rock dissolution and fracturing process is analyzed. Results indicate three types according to dissolution morphologies, i.e., permeation, cavity, and permeation-cavity. The first and second types are related to the reaction-controlled and diffusion-controlled processes, respectively. Particularly, the permeation-cavity type occurs at conditions with high injection pressure and high Da and is the combination of the first two types, where the dissolution around the main fracture presents the cavity type but converts into the permeation type around secondary fractures. Additionally, the evolution patterns of reactive surface area and dissolution rate with time differ significantly with rock dissolution types. In particular, for the permeation type, the aperture of fracture far away from the injection hole could be larger than that near the injection hole under the THMC coupling, which is quite different from HM and THM coupling.