CO2 plume management in saline reservoir sequestration

Abstract

Abstract A significant difference between injecting CO 2 into saline aquifers for sequestration and injecting fluids into oil reservoirs or natural gas into aquifer storage reservoirs is the availability and use of other production and injection wells surrounding the primary injection well(s). Of major concern for CO 2 sequestration using a single well is the distribution of pressure and CO 2 saturation within the injection zone. Pressure is of concern with regards to caprock integrity and potential migration of brine or CO 2 outside of the injection zone, while CO 2 saturation is of interest for storage rights and displacement efficiency. For oil reservoirs, the presence of additional wells is intended to maximize oil recovery by injecting CO 2 into the same hydraulic flow units from which the producing wells are withdrawing fluids. Completing injectors and producers in the same flow unit increases CO 2 throughput, maximizes oil displacement efficiency, and controls pressure buildup. Additional injectors may surround the CO 2 injection well and oil production wells in order to provide external pressure to these wells to prevent the injected CO 2 from migrating from the pattern between two of the producing wells. Natural gas storage practices are similar in that to reduce the amount of “cushion” gas and increase the amount of cycled or working gas, edge wells may be used for withdrawal of gas and center wells used for gas injection. This reduces loss of gas to the formation via residual trapping far from the injection well. Moreover, this maximizes the natural gas storage efficiency between the injection and production wells and reduces the areal extent of the natural gas plume. Proposed U.S. EPA regulations include monitoring pressure and suggest the “plume” may be defined by pressure in addition to the CO 2 saturated area. For pressure monitoring, it seems that this can only be accomplished by injection zone monitoring wells. For pressure, these wells would not need to be very close to the injection well, compared to monitoring wells intended to measure CO 2 saturation via fluid sampling or cased-hole well logs. If pressure monitoring wells become mandated, these wells could be used for managing the CO 2 saturation and aquifer pressure distribution. To understand the relevance and effectiveness of producing and injecting brine to improve storage efficiency, direct the plume to specific pore space, and redistribute the pressure, numerical models of CO 2 injection into aquifers are used. Simulated cases include various aquifer properties at a single well site and varying the number and location of surrounding wells for plume management. Strategies in terms of completion intervals can be developed to effectively contact more vertical pore space in relatively thicker geologic formations. Inter-site plume management (or cooperative) wells for the purpose of pressure monitoring and plume management may become the responsibility of a consortium of operators or a government entity, not individual sequestration site operators.