Numerical Modelling on CO2 Storage Capacity in Depleted Gas Reservoirs

Making an accurate estimate of the CO2 storage capacity before the commencement of a carbon capture and storage (CCS) project is crucial to the project design and feasibility investigation. We present herein a numerical modelling study on the CO2 storage capacity in depleted gas reservoirs. First, we show a simple volumetric equation that gives the CO2 storage capacity in a depleted gas reservoir, which considers the same volume of CH4 at reservoir pressure and temperature conditions produced from the reservoir. Next, the validity and the limitations of this equation are investigated using a numerical reservoir simulation with the various reservoir characteristics of reservoir heterogeneity, aquifer water encroachment, and rock compaction and its reversibility. Regardless of the reservoir heterogeneity, if a reservoir is subjected to a weak or moderate aquifer support, the volumetric equation provides an estimate of the CO2 storage capacity as structurally trapped gas within 1% of that estimated from numerical simulations. The most significant factor influencing the CO2 storage capacity is the reversibility of rock compaction, rather than the degree of rock compaction. If reservoir rocks have a strong hysteresis in their compaction and expansion behaviour, the material balance equation will overestimate the amount of structural CO2 trapping. All the simulation results show a fairly consistent amount of trapped CO2 as a dissolved component in water, which is 15 similar to 17% of the structurally trapped CO2. Overall, our study presents the validity and the limitation of the simple material balance equation for estimating the CO2 storage capacity, which helps with designing a CCS project at the early stage.