Capillary alteration of caprocks by acid gases
Résumé
The safety of acid gas geological storage is to a large extent controlled by the capillary properties of the caprock. This low-permeable (e.g., clayey) porous media usually saturated with water acts as a capillary barrier to the underlying stored acid gas, provided its water-wettability is preserved and water/acid gas interfacial tension (IFT) is high enough. The displacement or capillary breakthrough pressure, above which the stored acid gas intrudes into the caprock, is directly related to those two interfacial properties. Water/acid gas IFTs have recently been thoroughly characterized. However, little is known on the effect of acid gases (CO2, H2S and their mixtures) on the water-wettability of caprocks. We present an experimental setup and procedure for measuring contact angles on mineral substrates in the conditions of geological storage. Measurements have been carried out in a range of pressures extending up to 150 bar, both with CO2 and H2S, and with mineral substrates representative of caprock minerals such as quartz and mica, as well as with a substrate sampled from the caprock of a depleted gas reservoir. We observed that the wettability alteration of mica is moderate in the presence of dense CO2, but pronounced in the presence of dense H2S. In contrast, the wettability of quartz and of the 'real' caprock substrate is not altered by dense CO2 or H2S. In addition to those substrate- and acid gas-dependent wettability effects, the much lower water/acid gas IFTs as compared to water/hydrocarbon gas IFTs are responsible for a loss in capillary-sealing potential of a given caprock when a hydrocarbon gas is replaced with acid gas, especially when the acid gas is rich in H 2S. This potential, as evaluated by the displacement or capillary breakthrough pressure, should be determined very carefully when planning an acid gas geological storage operation. Copyright 2008, Society of Petroleum Engineers.