Numerical Modelling on CO 2 Storage Capacity in Depleted Gas Reservoirs

Making an accurate estimate of the CO 2 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 CO 2 storage capacity in depleted gas reservoirs. First, we show a simple volumetric equation that gives the CO 2 storage capacity in a depleted gas reservoir, which considers the same volume of CH 4 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 CO 2 storage capacity as structurally trapped gas within 1% of that estimated from numerical simulations. The most significant factor influencing the CO 2 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 CO 2 trapping. All the simulation results show a fairly consistent amount of trapped CO 2 as a dissolved component in water, which is 15∼17% of the structurally trapped CO 2 . Overall, our study presents the validity and the limitation of the simple material balance equation for estimating the CO 2 storage capacity, which helps with designing a CCS project at the early stage.