Estimating the seepage effect of SC-CO2 and water fracturing with a steady-state flow model considering capillary and viscous forces at the pore scale.

Supercritical carbon dioxide (SC-CO 2) fracturing is a promising technology for unconventional energy development and carbon capture and storage. Experimental studies have shown that SC-CO 2 fracturing can form complex fracture networks and reduce crack initiation pressure, which are different results from those when fracturing with aqueous fluids. The complex fracture networks that form from SC-CO 2 fracturing may be the result of strong seepage effects (i.e., low capillary and viscous forces). To understand the different injection behaviors induced by SC-CO 2 and aqueous fluids in low-permeability rocks, this study develops a new two-phase steady-state model based on the pore-scale network method. Although other models consider the viscous force, our model implements the capillary and viscous forces to reproduce the seepage effect. Because of the capillary force, the flow model is nonlinear and solved by iteratively solving matrix equations until a conservation of volumetric flux is satisfied. Simulation results show that the capillary force in a two-phase flow system in small pore spaces has consequential effects on its pressure distribution. Such factors lead to discontinuous pressure drops. This study shows that the seepage effect of SC-CO 2 is stronger than that of aqueous fluids and can largely avoid the capillary blockage that arises in oil-water systems. • The seepage effect estimation model for supercritical CO 2 and water fracturing in unconventional gas reservoir is proposed. • The simulation results show that capillary force has strong effect on seepage effect. • The strong seepage effect of SC-CO2 is a key factor to form complex fractures. [ABSTRACT FROM AUTHOR]