Aquifer disposal of acid gases: modelling of water–rock reactions for trapping of acid wastes

Abstract

The pore space of deep saline aquifers in the Alberta (sedimentary) Basin offers a significant volume for waste storage by “hydrodynamic trapping”. Furthermore, given the slow regional fluid flow in these deep saline aquifers, ample time exists for waste-water/rock chemical reactions to take place. A geochemical computer model (PATHARC) was used to compute the interaction of industrial waste streams comprising CO 2, H 2SO 4 and H 2S with the minerals in typical carbonate and sandstone aquifers from the Alberta Basin. The results support the idea that these acids can be neutralized by such reactions and that new mineral products are formed, such as calcite, siderite, anhydrite/gypsum and pyrrhotite, thereby trapping the CO 3, SO 4 and S ions that are formed when the acid gases dissolve in the formation water. Siliciclastic aquifers appear to be a better host for “mineral trapping” than carbonate aquifers, especially with regard to CO 2. Carbonate aquifers may be more prone to leakage due to high CO 2 pressures generated by reaction with H 2SO 4 and H 2S. Even though permeability decreases are expected due to this “mineral trapping”, they can be partially controlled so that plugging of the aquifer does not occur.