Thermodynamic Analysis of Organic/Inorganic Reactions Involving Sulfur: Implications for the Sequestration of H2s in Carbonate Reservoirs

A thermodynamic analysis has been made of water-gas-rock-hydrocarbon interactions involving sulfur in carbonate reservoirs. Mineral-solution equilibria calculations indicate that formation waters produced from carbonate reservoirs at temperatures between 40°C and 160°C in the Paris Basin, Alberta Basin, and the North Sea are saturated with respect to calcite and dolomite, but undersaturated with respect to anhydrite. The calculations also suggest that increasing the activity of dissolved H 2 S (aH 2 S (aq) ) in the reservoirs favors the formation of elemental sulfur, which constitutes a potential sequestration reaction for H 2 S. It is also shown that high partial pressures of H 2 S generated by thermochemical sulfate reduction in carbonate reservoirs may be controlled by metastable equilibrium states between hydrocarbons, organic sulfur compounds, elemental sulfur, and H 2 S at partial pressures which are 4 to 6 orders of magnitude higher than in clastic reservoirs where the H 2 S partial pressures are controlled by pyrite and iron carbonates. Depending on the H/C ratios of the hydrocarbons and organic sulfur compounds, H 2 S may be consumed or produced as a result of petroleum sulfurization reactions in carbonate reservoirs.