Are rocks still water-wet in the presence of dense CO2 or H2S?

The various modes of acid gas storage in aquifers, namely structural, residual, and local capillary trapping, are effective only if the rock remains water-wet. This paper reports an evaluation, by means of the captive-bubble method, of the water-wet character in presence of dense acid gases (CO2, H2S) of typical rock-forming minerals such as mica, quartz, calcite, and of a carbonate-rich rock sampled from the caprock of a CO2 storage reservoir in the South-West of France. The method, which is improved from that previously implemented with similar systems by Chiquet et al. (Geofluids 2007; 7: 112), allows the advancing and receding contact angles, as well as the adhesion behavior of the acid gas on the mineral substrate, to be evaluated over a large range of temperatures (up to 140 degrees C), pressures (up to 150 bar), and brine salinities (up to NaCl saturation) representative of various geological storage conditions. The water-receding (or gas-advancing) angle that controls structural and local capillary trapping is observed to be not significantly altered in the presence of dense CO2 or H2S. In contrast, some alteration of the water-advancing (or gas-receding) angle involved in residual trapping is observed, along with acid gas adhesion, particularly on mica. A spectacular wettability reversal is even observed with mica and liquid H2S. These results complement other recent observations on similar systems and present analogies with the wetting behavior of crude oil/brine/mineral systems, which has been thoroughly studied over the past decades. An insight is given into the interfacial forces that govern wettability in acid gas-bearing aquifers, and the consequences for acid gas geological storage are discussed along with open questions for future work.