Acid gas injection in the Brazeau Nisku Q carbonate reservoir: Geochemical reactions as a result of the injection of an H2S-CO2 waste stream

Abstract

Storage of greenhouse gases in geological media is regarded as a potential option for reducing release of CO2 emissions to the atmosphere. Industrial analogues are found in acid gas injection which injects mixtures of waste streams of H2S and CO2 (derived from gas processing of sour gas) into spent oil and gas reservoirs and aquifers for disposal. This is currently occurring at over forty sites in western Canada. To date no leakage has been reported. A review of these acid gas operations has been undertaken in the chapter with a view to evaluating their storage security. The reservoir temperature and pressure, before injection of an acid gas stream of 85% H2S and 15% CO2, was 110°C and 11 MPa. Geochemical mass transfer modeling using GAMSPATH.99 under these conditions predicts that the iron containing minerals in the reservoir will break down rapidly to form iron sulphide minerals. This was verified experimentally in the laboratory at much lower pressures through observation of significant reaction of siderite to iron sulphide at 54°C and 0.5 MPa in a CO2-H2S atmosphere in two weeks. Solubility trapping and ionic trapping are significantly larger, by approximately 10%, if the acid gas-charged fluids react to equilibrium with the reservoir rock, compared to no reaction with the reservoir minerals. In Brazeau, mineral trapping of H2S by iron sulphide minerals dominates due to the high H2S content of the injected gas, reaching trapping levels exceeding 20 moles of H2S per 1000 cc of pore space. In the absence of H2S, mineral trapping of CO2 by carbonate minerals would have dominated.