TOUGH–RDCA modeling of multiple fracture interactions in caprock during CO2 injection into a deep brine aquifer

Abstract

The interaction between multiple fractures (e.g., hydraulic fractures and pre-existing natural fractures) is important in the analysis of a number of geoengineering applications, such as in the evaluation of the stability and integrity of caprock during underground CO2 sequestration. Here, we present new developments and applications of a model for analyzing coupled multiphase fluid flow and geomechanical processes during fracturing involving multiple fractures and their interactions. Based on a numerical code, i.e., rock discontinuous cellular automaton (RDCA), we introduce a discontinuous displacement function for representing multiple discontinuities, and develop an algorithm to deal with a propagating fracture that interacts with other discontinuities. By applying multiphase fluid pressure to fracture surfaces, the RDCA has the ability to simulate multiphase fluid flow-driven fracturing. The RDCA technique incorporates the discontinuity of the crack independently of the mesh, such that the cracks can be arbitrarily located within an element. This method does not require any remeshing for multiple cracks growth, an aspect that greatly reduces the complexity and improves efficiency in modeling multiple-fracture propagation. The RDCA is integrated with the TOUGH2 multiphase flow and heat-transport simulator by a sequential coupling algorithm, using mixed FORTRAN and C++ programming. The coupled TOUGH2 and RDCA code is applied to simulate the multiple fracture interaction in caprock induced by CO2 injection into a deep brine aquifer. The simulation results show hydraulic fracture trajectory, fracture aperture, and pressures as a function of injection time. Fluid flow (driven by CO2 injection) into natural fractures and their transition to hydraulic fractures is simulated. The injection pressure profile shows the complexity of the fracturing and its impact on CO2 migration and caprock integrity.