CCS in Depleted Gas Fields: Impact of Imposed Reservoir Pressure Limits on Storage Capacity and Efficiency

Storage capacity of CCS in depleted gas fields is mainly controlled by the produced gas volume and the pressure to which a field will be repressurized. While imposing a pressure limit can mitigate CO2 containment risk, it may reduce storage capacity and hence the storage efficiency of a field. The strong pressure dependency of storage efficiency indicates the need to strike the right balance between containment risk and storage efficiency, and the need to investigate legacy well integrity. This will allow a balanced evaluation by policy- and decision makers of the added value of conservative limits and storage efficiency. A fast analytical method is derived to assess the impact of imposed pressure constraints on storage efficiency, to determine either lost or gained capacity. It provides decision makers with a tool to optimally balance storage capacity and containment. The data for this method is based on the portfolio of CCS projects and proprietary models that the Geological Survey of the Netherlands currently holds. As an extension of the method, a simplified function is proposed to estimate the density of CO2 for realistic pressure and temperature ranges for depleted fields. With this method a series of pressure limits were evaluated. Restricting bottom hole pressures to hydrostatic pressures, instead of planned final average reservoir pressures, could reduce storage capacity with 10 - 12% and storage efficiency with 8 - 10%.