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

The pore space of deep saline aquifers in the Alberta (sedimentary) Basin o ffers a significant volume for waste storage by "hydrodynamic trapping". Fu rthermore, 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 CO2, H2SO4 and H2S with the minerals in typical carbonate and sandstone aquifers from the Alberta Basin. The re sults support the idea that these acids can be neutralized by such reaction s and that new mineral products are formed, such as calcite, siderite, anhy drite/gypsum and pyrrhotite, thereby trapping the CO3, SO4 and S ions that are formed when the acid gases dissolve in the formation water. Siliciclast ic aquifers appear to be a better host for "mineral trapping" than carbonat e aquifers, especially with regard to CO2. Carbonate aquifers may be more p rone to leakage due to high CO2 pressures generated by reaction with H2SO4 and H2S. Even though permeability decreases are expected due to this "miner al trapping", they can be partially controlled so that plugging of the aqui fer does not occur. (C) 2000 Elsevier Science Ltd. All rights reserved.