Utilization of CO2 as cushion gas for porous media compressed air energy storage

Porous media compressed air energy storage (PM-CAES) and geologic carbon sequestration (GCS) can potentially be combined when CO2 is used as the cushion gas. The large increase in density of CO2 around its critical pressure at near-critical temperature means that a PM-CAES reservoir operated around the CO2 critical pressure could potentially store more air (energy) for a given pressure rise in the reservoir. One-dimensional (1D) radial TOUGH2 simulations of PM-CAES with CO2 as the cushion gas have been carried out to investigate pressurization and gas-gas mixing effects. We find that pervasive pressure gradients in PM-CAES make it desirable to position the air-CO2 interface close to the well, but cushion gas at such locations is subject to strong and undesirable air-CO2 mixing and subsequent production of CO2 up the well. To avoid this negative effect, CO2 cushion gas should be located at the far outer margins of storage reservoirs where mixing will be very slow. In such a configuration, the super-compressibility of CO2 will not be exploited, but CO2 can be stored in the GCS context potentially earning significant value for the PM-CAES project depending on the price of carbon. (c) 2013 Society of Chemical Industry and John Wiley & Sons, Ltd