Cappilary alteration of shaly caprocks by carbon dioxide
Résumé
The efficiency of the CO2 geological storage process in aquifers and hydrocarbon reservoirs is controlled by several factors. In the case of reservoirs with a shaly caprock, one critical factor is the capillary-sealing potential of the caprock. This potential can be expressed in terms of a maximum storage pressure, equal to the hydrostatic pressure in the caprock plus the CO2 capillary entry pressure in the brine-saturated caprock. It is therefore controlled by the CO2-brine interfacial tension, the water-wettability of shale minerals, and the pore size distribution within the shaly rock. By means of contact angle measurements, we provide experimental evidence that the water-wettability of minerals representative of shales, such as mica and quartz, is significantly altered in the presence of CO2 under pressures typical of geological storage conditions. Those minerals, known to be strongly water-wet in the presence of oil, turn out to be intermediate-wet in the presence of dense CO2. We discuss the consequences of such wettability alteration on the maximum CO2 storage pressure, which can be converted into a maximum CO 2 height stored in the reservoir. In the case of hydrocarbon reservoirs initially close to capillary leakage, the maximum CO2 storage pressure should be only a fraction of the initial virgin pressure.