H2S Risk Assessment: Hydrogeochemical 3D Reactive Mass Transport Modeling and Mass Balancing of Acid Gas Generation and Distribution in Anhydrite-Sealed Oil and Gas Reservoirs

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 ca. 100°C. The interactions lead to the formation of toxic and corrosive hydrogen sulfide (free H2S gas and dissolved H2S). Such interactions are caused by the instability of hydrocarbons in the presence of water and a reactive reservoir rock matrix containing water-soluble anhydrite. The mass conversions of the inorganic water-rock-gas interactions which are triggered by the redox degradation of hydrocarbons establish a certain, thermodynamically defined state of chemical equilibrium. Any approach to geochemically model “acid gas generation” and “H2S-risk distribution” in petroleum systems should be based on a conceptual model that adequately reproduces the intimately interconnected and interdependent nature of all isochronous hydrogeochemical reactions, whether they are kinetically controlled or establish equilibrium species distributions. Such approaches rely (1) on the thermodynamical calculation of chemical equilibrium species distribution, (2) on the coupling of kinetically controlled oil degradation and sulfate reduction by oil-derived reductants to the equilibrium calculations, and (3) on the calculation of diffusive mass transport through the free pore water network and the irreducible water film. The key to model TSR, “acid gas generation, and “H2S-risk distribution” is not to consider and model any single, isolated reaction like the kinetically controlled sulfate reduction which depends on the thermal history. The actual key to model TSR, the fate and behavior of sulfidic sulfur, and a realistic “H2S-risk distribution” in petroleum reservoirs is an overall reproduction of the hydrogeochemical reactive transport processes which temporally and spatially evolve in a complex network of oil/petroleum-water-rock-gas interactions under reservoir conditions. 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) and take the following boundary conditions into account: gas reservoir; carbonate (dolomite plus calcite) reservoir rocks; anhydrite seal; 140°C; 600 bar total pressure; kinetic rate constant for sulfate reduction by CH4 = 1.08 x 10-16 mol s-1 l-1; mass transport is restricted to diffusion; modeled time span is 10 Ma