Two-phase flow pipe network method for simulation of CO2 sequestration in fractured saline aquifers

A two-phase flow unified pipe network method is developed to simulate CO2 evolution in fractured saline aquifers. Fractures are explicitly represented in the proposed method. The two-phase flow in both rock matrix and fractures is considered by using different equivalent pipe flow models respectively, namely the two-phase matrix pipe flow model and the two-phase fracture pipe flow model. The equivalent flow coefficients of the pipe flow models are derived based on flow rate equivalence. The coupling of the fracture pipe flow and matrix pipe flow is treated by applying the extended capillary pressure conditions. Brooks-Corey relative permeability model and capillary model are adopted to simulate CO2 (non-wetting phase) invasion into the brine (wetting phase) saturated formation, which is a typical drainage process. Accurate Equations of State for calculating density and viscosity of CO2 are incorporated to reflect its change in hydraulic characteristics during the injection processes. The proposed method is simple yet robust and not sensitive to the mesh quality. The complex geological and topological features of fracture networks can, therefore, be well retained in the proposed method. The anisotropy and heterogeneity characteristics of the fractured rock mass caused by the fracture networks can be accurately represented. The proposed method is verified by comparing to other numerical methods. Both 2D and 3D models with complex fracture networks are presented to demonstrate the feasibility of proposed method. Numerical examples show that fractures can significantly affect the distribution and evolution of CO2 in aquifers and the differences of entry capillary pressures for fractures and matrix rock should be accurately simulated.