Numerical analysis of different fracturing mechanisms between supercritical CO₂ and water-based fracturing fluids

Hydraulic fracturing using water-based fluid consumes a large amount of freshwater resources and pollutes a reservoir by substantially decreasing its matrix permeability. To address the problems caused by water-based fracturing fluids, the possibility of using SC-CO₂ as a fracturing fluid was studied for its special properties (such as low viscosity, high density, and miscibility with hydrocarbons). Previous experimental studies have indicated that SC-CO₂ is superior to water-based fracturing fluid in inducing complex fractures at the laboratory scale, while the mechanism of complex fractures induced by SC-CO₂ remains unclear. This study develops a new numerical model to simulate the different performances of SC-CO₂ fracturing and water-based fluid fracturing to determine the mechanisms of complex fractures. The numerical model couples an unsteady flow model based on the pore-scale network method and a solid model using the finite element method with cohesive zone elements. The unsteady flow model reproduces a two-phase flow considering viscous and capillary forces at the pore scale. Our simulation results show that both viscous and capillary forces contribute to the different fracturing performances between SC-CO₂ and water-based fluid. The capillary force should be considered in the flow model when simulating fracturing in low matrix permeability rock.