A mineral-water-gas interaction model of pCO2 as a function of temperature in sedimentary basins

Abstract

Accurate prediction of CO2 partial pressure (pCO2) in sedimentary basins is important for reducing risk in natural gas exploration, predicting non-hydrocarbon gas in geological formations, improving reservoir quality prediction, optimizing production and reservoir management operations, and understanding geological storage of CO2. It has been proposed for some time now that pCO2 in sedimentary basins is buffered by water-mineral-gas interactions and is primarily related to the temperature. However, the pCO2 values predicted from thermodynamic calculations do not always match field observations.Here, we developed a new and general geochemical model to predict pCO2 in sedimentary basins. By treating each component of the geochemical model of CO2-water-rock reactions rigorously, our model is able to match observed pCO2 in a number of sedimentary basins around the world. The thermodynamic treatments introduced in the model include: (a) a gas mixture including CH4(g), CO2(g), H2S(g), and H2O(g) rather than a single gas in previous models; (b) the Peng-Robinson Equation of State to calculate the fugacity coefficients in the gas mixture rather than assuming ideal gas; (c) taking into account of the hydrostatic pressure of the system and make pressure corrections on equilibrium constants of gas, aqueous species, and minerals reactions. For sedimentary basins with complicated uplift and subsidence history, we introduced reaction path modeling to account for the P-T history. The geochemical model allows the input of basin-specific conditions and serves as a tool to identify the key processes that control pCO2 in sedimentary basins.