Alteration of bentonite when contacted with supercritical CO2

Abstract:

Deep saline formations overlaid by impermeable caprocks with a high sealing capacity are attractive CO₂ storage reservoirs. Shales, which consist of mainly clay minerals, are potential caprocks for the CO₂ storage reservoirs. The properties of clay minerals in shales may affect the sealing capacity of shales. In this study, changes in clay minerals’ properties when contacted with supercritical (SC) CO₂ at various conditions were investigated. Bentonite, whichis composed of primarily montmorillonite, was used as the clay material in this study. Batch reactor tests on wet bentonite samples in the presence of SC CO₂ with or without aqueous phases were conducted at high pressure (12 MPa) and moderate temperature (50 ^oC) conditions for a week. Results show that the bentonite samples obtained from the tests with SC CO₂ had less change in porosity than those obtained from the tests without SC CO₂ (vacuum-drying) at a given reaction time, indicating that the bentonite samples dried in the presence of SC CO₂ maintained their structure. These results suggest that CO₂ molecules can diffuse into interlayer of montmorillonite, which is a primary mineral of bentonite, and form a single CO₂ molecule layer or double CO₂ molecule layers. The CO₂ molecules can displace water molecules in the interlayer, resulting in maintaining the interlayer spacing when dehydration occurs. Noticeable changes in reacted bentonite samples obtained from the tests with an aqueous phase (NaCl, CaCl₂, or sea water) are decreases in the fraction of plagioclase and pyrite and formation of carbonate minerals (i.e., calcite and dolomite) and halite. In addition, no significant exchanges of Na or Ca on the exchangeable complex of the montmorillonite in the presence of SC CO₂ occurred, resulting in no significant changes in the swelling capacity of bentonite samples after reacting with SC CO₂ in the presence of aqueous phases. These results might be attributed by the CO₂ molecule layer, which prevents diffusion of ions into the interlayer.