Common Features of Carbon Dioxide and Underground Gas Storage (1)

Underground gas storage and CO, storage in depleted gas reservoirs have many common technical, reservoir mechanic and operative features, but the media involved, HC gases and CO2, are physically and chemically different. CO2 dissolved in water forms a corrosive acid, which can react with mineral compounds oldie storage and cap rock and may change the hydraulic properties in different ways. Depending on the original rock composition, carbonate, feldspar and chlorite cements may be corroded and dolomite can be precipitated. Clay or shale reservoir intercalations may become more permeable for gas. However these changes will likely not happen near the injection wells but far away because of residence time and water saturation needed for the reactions. Favorable physical properties of CO, (density, viscosity, compressibility,) will contribute in enhancing the rest gas recovery (EGR) in depleted reservoirs. The EGR capacity is limited by the termination of HC production after CO, breakthrough. Some 3 to 10% of the IGIP may be recovered additionally to the natural depletion yield. Anticline trap types with steep flanks are favourable compared to flat stratigraphic traps. High production rates accelerate the EGR. Primary injection until the reservoir pressure is raised to a medium level followed by the EGR phase is unfavorable compared with simultaneous injection/production from the beginning.