Are rocks still water-wet in the presence of dense CO 2 or H 2S?
Résumé
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 (CO 2, H 2S) of typical rock-forming minerals such as mica, quartz, calcite, and of a carbonate-rich rock sampled from the caprock of a CO 2 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°C), pressures (up to 150bar), 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 CO 2 or H 2S. 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 H 2S. 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.