Coupled thermo-hydro-mechanical simulation of CO2 enhanced gas recovery with an extended equation of state module for TOUGH2MP-FLAC3D

As one of the most important ways to reduce the greenhouse gas emission, carbon dioxide (CO2) enhanced gas recovery (CO2-EGR) is attractive since the gas recovery can be enhanced simultaneously with CO2 sequestration. Based on the existing equation of state (EOS) module of TOUGH2MP, extEOS7C is developed to calculate the phase partition of H2O-CO2-CH4-NaCl mixtures accurately with consideration of dissolved NaCl and brine properties at high pressure and temperature conditions. Verifications show that it can be applied up to the pressure of 100 MPa and temperature of 150 °C. The module was implemented in the linked simulator TOUGH2MP-FLAC3D for the coupled hydro-mechanical simulations. A simplified three-dimensional (3D) 1/4 model (2.2 km × 1 km × 1 km) which consists of the whole reservoir, caprock and baserock was generated based on the geological conditions of a gas field in the North German Basin. The simulation results show that, under an injection rate of 200,000 t/yr and production rate of 200,000 sm3/d, CO2 breakthrough occurred in the case with the initial reservoir pressure of 5 MPa but did not occur in the case of 42 MPa. Under low pressure conditions, the pressure driven horizontal transport is the dominant process; while under high pressure conditions, the density driven vertical flow is dominant. Under the considered conditions, the CO2-EGR caused only small pressure changes. The largest pore pressure increase (2 MPa) and uplift (7 mm) occurred at the caprock bottom induced by only CO2 injection. The caprock had still the primary stress state and its integrity was not affected. The formation water salinity and temperature variations of ±20 °C had small influences on the CO2-EGR process. In order to slow down the breakthrough, it is suggested that CO2-EGR should be carried out before the reservoir pressure drops below the critical pressure of CO2.