Wettability behaviour of CO2 at storage conditions

Abstract 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 physical–geochemical processes could possibly decrease the capillary entrance pressure and reduce the sealing integrity of the caprock. Changes in wettability have therefore been investigated by measuring the CO2 contact angle on some selected minerals typical for reservoir rocks in the presence of brine at reservoir conditions. In this paper, a set of CO2 contact angle data are reported for quartz, feldspar, calcite and muscovite mica, representing reservoir and seal rock, at pressure, temperature and salinity conditions representative of a CO2 storage operation. Among these minerals, quartz, feldspar and calcite are strongly water wet with non-significant change in contact angle versus pressure while the water wettability of muscovite mica changed from strongly water-wet to intermediate water-wet with increasing pressure. Also wettability alterations of CO2 as function of time for calcite and muscovite mica at constant pressure, temperature and salinity were investigated. The water wettability of the calcite mineral did not change significantly at the time scale of the laboratory experiment while there was a marked decrease in the water wettability of mica. A maximum in contact angle near the critical pressure was observed at 36 °C for feldspar, calcite, and quartz and was maybe masked in the case of muscovite mica by the significant increase in contact angle with increasing pressure. A relatively strong change in CO2 compressibility is observed at 36 °C, less so at 66 °C, indicating that the observed maximum may be related to the near-critical behaviour of the CO2 phase.