CO2 Wettability of Minerals Relevant to Sealing Efficiency of Caprocks for CO2 Storage in Saline Aquifer

The capillary-sealing efficiency of the caprock is one of the major factors that control the safety of geological CO2 storage. Possible changes in wettability due to geochemical processes could possibly decrease the capillary entry pressure and reduce the sealing integrity of the caprock. Changes in wettability have been measured by measuring the CO2 contact angle mineral substrates mica, quartz, calcite and feldspar in the presence of brine for pressures extending from atmospheric pressure up to 400 bars and for temperatures of 36 °C and 66 °C. The change in contact angle as function of pressure was non-significant for CO2 drop on the surface of quartz, feldspar and calcite. There was a significant change in contact angle on mica in which case the wettability changed from strongly water-wet to intermediate water-wet. The minimum in contact angle near the critical pressure was observed at 36 °C for three minerals (feldspar, calcite, quartz) and was maybe masked in the case of mica by the significant decrease in contact angle with increasing pressure. A relatively strong anomaly in CO2 compressibility is observed at 36 °C, less so at 66°C, indicating that the observed minimum may be related to the near-critical behavior of the CO2-phase.