Application of material balance methods to CO₂ storage capacity estimation within selected depleted gas reservoirs

Depleted gas reservoirs are potential sites for CO₂ storage; therefore, it is important to evaluate their storage capacity. Historically, there have been difficulties in identifying the reservoir drive mechanism of gas reservoirs using traditional P/z plots, having direct impacts for the estimation of the original gas in place (OGIP) and dependent parameters for both theoretical and effective CO₂ storage capacity estimation. Cole plots have previously provided an alternative method of characterization, being derived from the gas material balance equation. We use production data to evaluate the reservoir drive mechanism in four depleted gas reservoirs (Hewett Lower Bunter, Hewett Upper Bunter, and North and South Morecambe) on the UK Continental Shelf. Cole plots suggest that the North Morecambe and Hewett Upper Bunter reservoirs experience moderatewater drive. Accounting for cumulativewater influx into these reservoirs, the OGIP decreases by up to 20% compared with estimates from P/z plots. The revised OGIP values increase recovery factors within these reservoirs; hence, geometrically based theoretical storage capacity estimates for the North Morecambe and Hewett Upper Bunter reservoirs increase by 4 and 30%, respectively. Material balance approaches yield more conservative estimates. Effective storage capacity estimates are between 64 and 86% of theoretical estimates within the depletion drive reservoirs, and are 53-79% within the water drive reservoirs.