Analytical approach for estimating CO2 storage capacity of produced gas reservoirs with or without a water drive

An analytical CO2 injection equation for estimating effective CO2 storage capacity is derived and verified for a gas reservoir producing gas, followed by injecting CO2 into the reservoir with a water drive. The equation can be expressed in the forms of an injection–production-ratio (IPR) equation and the p/z equation. An effective capacity coefficient can be obtained either from the ratio of effective CO2 storage capacity and theoretical CO2 storage capacity, or from the IPR and z-factors. In a gas reservoir with a water drive, the CO2 injected to increase the pressure from the abandonment pressure to the initial pressure is less than the gas produced. The single effective capacity coefficient is less than one depending on the strength of the water drive. In a gas reservoir with no water drive, the CO2 injected to increase the pressure from the abandonment pressure to the initial pressure is more than the gas produced. The single effective capacity coefficient is 1. The discussion includes examples of using the CO2 injection equation and IPR type curves to estimate effective CO2 storage capacity, and pressure behaviors in p/z plots [p/z versus (Gphc − Ginj)] during gas production and CO2 injection.

► Equations are derived for estimating CO2 storage capacity in a gas reservoir.
► Effective capacity coefficient can be calculated from IPR equation derived.
► Pressure behavior (p/z plot) of gas producing followed by CO2 injecting is studied.
► CO2 injected is less than gas produced in a gas reservoir with a water drive.
► CO2 injected is more than gas produced in a gas reservoir with no water drive.