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H2S Risk Assessment at the Basin Scales


High concentrations (>10vol% in reservoirs) of hydrogen sulfide (H2S), a toxic and economically-damaging gas, result from thermochemical sulfate reduction (TSR) of petroleum. TSR occurs in high temperature carbonate reservoirs, and is very detrimental to the quality and the volume of hydrocarbon resources. Therefore, accurate TSR modeling is important to lower the risks during petroleum E&P. TSR involves complex redox reactions, that lead to petroleum oxidation and reduction of sulfates. The main products of reaction are either volatile (carbon dioxide (CO2), H2S) or solid (carbonate cements, pyrobitumen). The volatile products may migrate up from deeper reservoirs in both the gas and solution (either in water or in oil) phase. We developed a post-calculator approach integrated to the TemisFlow software, in order to assess maximum H2S amounts resulting from TSR. The model is based on a mass balance approach according to the overall chemical reaction which has been first proposed by Uteyev (2011): 8 CnHm + (4n+m) CaSO4 = (4n+m) CaCO3 + (4n+m) H2S + (4n-m) CO2 + (3m - 4n) H2O The stoichiometry of the reaction is clearly related to H/C ratio of the hydrocarbon. Water may either be consumed or produced by the reaction. Temperature, pressure, porosity, HC amount, and salinity are inherited from the TemisFlow calculator. A necessary additional input is the map of sulfate minerals in the basin. The model outputs are H2S and CO2 quantities, as well as their distribution in a gas phase or dissolved in the basinal brine. The volume of dissolved sulfate and the volume of precipitated calcite are also computed, and the porosity evolution related to the TSR reaction is assessed. An identification of areas where H2S is expected to be present is then workable. Devonian carbonate reservoirs of the Nisku and Leduc formations (Alberta) include HC fields that experienced TSR and may contain up to 30% of H2S. A 3D model, taking into account subsidence history and subsequent erosional profiles, of the Alberta foreland basin has been built. Results provide the timing of the Devonian petroleum system accounting for the HC charges of Devonian-Mississippian carbonate reservoirs. This study moreover investigates the impact of critical parameters (oil composition, salinity, H/C) on the production of H2S by TSR in the Devonian Formations. Our numerical results are compared to wells data and clearly show that the H2S production occurred mainly before the last erosion.