Carbon dioxide (CO2) gas is a major component of flue gases emitted from fossil fuel-burning power plants, but these flue gases often also contain other gases such as hydrogen sulfide (H2S) or sulfur dioxide (SO2). Because separation of these gases may be an expensive process, innovative ideas such as using the mineralogy of deep saline systems to co-sequester mixed flue gas streams could significantly lower retrofitting costs and is an alternative being considered at some storage sites. Advanced oxy-combustion processes lead to relatively pure waste streams, but flue gases still contain non-negligible quantities of other gases such as oxygen (O2), argon (Ar), and nitrogen (N2). Because of limitations of previous simulators in handling the injection of multi-component and multi-phase mixtures, previous modeling studies of co-injection of mixed gas waste streams have either simplified hydrogeological systems (batch conditions), or considered only the co-injection of gases already dissolved in an aqueous phase rather than as constituents of the non-aqueous phase.
A better approach is to simulate the injection of supercritical/gas mixtures where each component is in equilibrium between multiple fluid phases. This requires the incorporation of customized “equations of state” into the simulator that describe the properties of mixed fluids (such as density and viscosity) as functions of controlling system variables (such as pressure and temperature). A mode of STOMP-CO2 has been developed for simulation of mixed gases, and equations of state have been developed and tested against standards for 10 different gases (CO2, O2, N2, Ar, methane [CH4], ethane [C2H6], propane [C3H8], butane [C4H10], H2S and SO2.). In addition, the various components of a mixed gas stream have a significant effect on the reactivity of the injected gas with reservoir rocks and fluids. Reaction models for selected multi-gas systems have been incorporated into the multicomponent reaction chemistry module of STOMP and are being tested against laboratory and field experimental data (see figure below).