TMGAS: A new TOUGH2 EOS module for the numerical simulation of gas mixtures injection in geological structures

Injection of gas mixtures in geological structures is performed for a variety of different purposes including natural gas storage operations, CO2 flooding and Water Alternate Gas injection for Enhanced Oil Recovery, and more recently for the sequestration of GreenHouse Gases and the disposal of acid gases produced by sour hydrocarbon reservoirs. The injection of multi-component mixtures into saline aquifers and hydrocarbon reservoirs requires the understanding of the processes that govern the performances of injection wells, the fluid migration within the reservoirs, and the possible recovery of additional hydrocarbon reserves. Conventional compositional simulators employed by the oil industry, while implementing complex phase equilibrium algorithms for the hydrocarbon phases, often give only an approximate description of the aqueous phase behaviour and properties. In particular, the injection of mixtures mainly composed by acid gases such as CO2 and H2S drives pronounced modifications of aqueous phase chemical equilibria promoting water–rock reactions with possible alteration of reservoir and cap rock, as well as of wellbore cement sheaths. The gas mixtures usually injected in undersaturated conditions with respect to water, progressively vaporise the aqueous phase, concentrating the dissolved solids until they precipitate as new mineral phases, eventually reducing the initial rock porosity and permeability. Vaporised water is transported further from the injection wellbore to possibly condense again in the presence of proper thermodynamical conditions. TMGAS, a new Equation of State module for the TOUGH2 reservoir simulator, has been developed to model the injection of mixtures of inorganic gases and hydrocarbons in geological structures at conditions conventionally found in deep sedimentary basins.