Thermochemical Sulfate Reduction in Anhydrite-Sealed Carbonate Gas Reservoirs: A 3-D Reactive Mass Transport Modeling Approach

Abstract

Acid gas generation by thermochemical sulfate reduction (TSR) evolves within a complex web of petroleum-water-rock-gas interactions in reservoirs under high temperature conditions of more than 100°C. These interactions lead to formation of toxic and corrosive hydrogen sulfide (H2S gas and dissolved H2S). Such interactions are caused by the instability of hydrocarbons in the presence of water and by a reactive reservoir rock matrix containing water-soluble anhydrite. The mass conversions of inorganic water-rock-gas interactions, which are triggered by the kinetically controlled sulfate reduction with aqueous hydrocarbons, establish a certain, thermodynamically defined state of chemical equilibrium. Any approach to geochemically and quantitatively model TSR-induced “H2S-risks” in petroleum systems should be based on a conceptual model that adequately reproduces the interdependent nature of all simultaneous hydrogeochemical processes contributing to TSR (more than 50 reactions). Such approaches should rely (1) on the thermodynamical calculation of chemical equilibrium species distribution, (2) on the coupling of kinetically controlled sulfate reduction with petroleum-derived reductants to the equilibrium calculations, and (3) on the calculation of diffusive mass transport of solutes through the free pore water network and the irreducible water film. TSR modeling is complex, and therefore, its failure often results from conceptual models which focus on only single reactions like the kinetically controlled sulfate reduction dependent on thermal history or other selected reactions of interest which are isolated from the web of interactions. The key to model TSR, the fate and behavior of sulfidic sulfur, and a realistic “H2S-risk” in petroleum reservoirs is a comprehensive reproduction of the hydrogeochemical reactive transport processes within the whole system. Consequently, we perform 3D hydrogeochemical, multi-component and multi-species reactive mass transport modeling for a semi-generic case study by using the PHAST computer code (provided by the U.S. Geological Survey). The aim is (1) to predict the temporal and spatial evolution of complex TSR interactions under reservoir conditions and (2) to test the effects of various parameters on the concentration of H2S in the gas, on the total amount of sulfidic sulfur present in the reservoir (H2S(g), H2S(aq), HS-(aq), S-2(aq) indicating reservoir souring; FeS2), and on the amount of newly formed elemental sulfur.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015