Effects of reservoir size and boundary conditions on pore-pressure buildup and fault reactivation during CO 2 injection in deep geological reservoirs

The excessive burning of the fossil fuels has caused severe global climatic changes such as increasing the global temperature, causing initiation of the wild fire, rising the sea level, increasing the floods, storms, amount of rain and snow. One of the effective global mitigation strategies is sequestration of huge quantity of CO2 deep below the ground level for a long period of time. An important issue is to ensure the permanency and safety of the sequestration process due to the associated pore-pressure buildup. It is necessary to have correct estimates of the pore-pressure buildup, ground uplift and re-activation of any existing fault during the process of CO2 injection and long-term storage. In this investigation, the effects of reservoir size and boundary conditions are investigated by means of geomechanical modeling of the deep Biyadh sandstone reservoir in Saudi Arabia. Currently, carbon dioxide is not injected into the actual Biyadh reservoir. In this investigative modeling, CO2 is injected for an injection period of ten years using a single injection well at the center of the reservoir. The developed modeling scheme for a single injection well has been extended further to include multiple injection wells. For multiple injection wells, the reservoir size and locations of injection wells are varied to evaluate their effect on the pore-pressure buildup and ground uplift. The reservoir stability analysis has been performed using Mohr–Coulomb failure criterion for both small and large reservoir models, with the same injection parameters. The simulation results demonstrated that pressure buildup and ground uplift are relatively higher for reservoirs with small sizes and closed boundaries; while in the case of large sizes and open boundaries, the pore-pressure buildup and ground uplift are relatively lower. Moreover, the effect of the reservoir size and boundary conditions on the reactivation of faults during CO2 injection has been evaluated. The stability analysis performed in this study shows that injecting CO2 into larger size reservoir is safer as compared to smaller size reservoir. Injecting CO2 with multiple injection wells will cause pore-pressure buildup of huge magnitudes. The modeling results show that suggesting a representative volume for the reservoir during CO2 injection can under-estimate the pore-pressure buildup and fault re-activation that can cause the reservoir failure and leakage of the stored CO2.