Interaction of super-critical CO2 with mudrocks: Impact on composition and mechanical properties

•Geomechanical, mineralogical and geochemical analysis of mudrocks exposed to sCO2 at high temperature and pressure.•No changes in mineralogy or major element geochemistry noted.•Mudrocks noted to both strengthen and stiffen after exposure to sCO2 for six months.•Loss of water results in a complete change in geomechanical behaviour from ductile to brittle. Geological storage of carbon dioxide (CO2) is a long-discussed strategy for avoiding CO2 discharge into the atmosphere and a few industrial scale projects are underway in this regard. In order to successfully perform such a strategy, it is important that site evaluation takes into account both reservoir and caprock properties in order to trap the CO2. Many caprocks for such sites are clay-rich mudrocks, hence evaluating their seal capacity and seal integrity are critical for storage sites. Analytical studies on the impact of potential CO2-water-rock interaction are an important part of site evaluation since potential geochemical reactions may degrade the seal quality and effectiveness. This paper investigates the impact of static exposure of mudrock seals to super-critical CO2 (sCO2) at high temperature and pressure (150˚C, 29 MPa) on the mineralogy (illite-smectite, kaolinite, illite and quartz), major element geochemistry and geomechanical properties of mudrock seals over a six-month period. Mineralogy and geochemistry were determined stepwise from their preserved initial state and after 1, 4 and 6 months exposure to sCO2 in a batch reactor, with no detectable changes in any of the minerals or elements observed. This is likely due to low reactivity in silicate systems and low volume of pore fluid available to facilitate chemical reactions. Samples for geomechanical testing were exposed to sCO2 for 6 months only. Geomechanical properties changed significantly between preserved and sCO2 exposed samples tested under equivalent effective stress conditions, with sCO2 exposed samples being much stronger and stiffer than their preserved counterparts. Given that no mineralogical and geochemical changes were noted during exposure to sCO2, it is most likely that the loss of pore water during sCO2 exposure resulted in the strengthening and stiffening of these mudrocks.