This paper describes the incorporation of asphaltene deposition and geochemistry in the simulation of CO2 floods. The asphaltene deposition model includes reversible and irreversible asphaltene precipitation followed by surface deposition and pore-throat plugging. Simulation runs are performed to examine the effect of various deposition model parameters. A preliminary investigation of the inclusion of geochemistry in a CO2 EOR process is conducted. Simulation runs show that the injection of CO2 initially induces Calcite dissolution. However, if CO2 is left in the reservoir after the flood, Calcite precipitation resulting from the mineralization of the injected CO2 may occur. This aspect is important for the long-term sequestration of CO2
The injection of CO2 for EOR and sequestration processes has received much interest recently. This paper discusses the modelling of two important aspects in the simulation of CO2 EOR processes that have not been addressed in conventional EOS compositional simulators, i.e. (1) asphaltene precipitation and deposition and (2) reactions with the minerals in the reservoir.
Asphaltene precipitation and deposition during CO2 EOR has been reported in the literature1–4. Precipitation can occur anywhere in the reservoir, although it manifests itself frequently in the production wells at solvent breakthrough. Asphaltene deposition could be detrimental to the CO2 operation as it could cause plugging of the formation and/or wellbores.
CO2 is highly soluble in the aqueous phase. When CO2 dissolves in the aqueous phase, it dissociates into H+ and HCO3 ions. The resulting ions react with the minerals in place and induce mineral dissolution and precipitation. This aspect is important from a sequestration point of view as the mineralization of CO2 provides a very safe venue for long-term storage of CO2.
Kohse and Nghiem5 and Nghiem et al.6 describe an approach for incorporating asphaltene precipitation, flocculation and deposition in an equation of state (EOS) compositional reservoir simulator. They have also validated the model against experimental data5. The following sections briefly describe the method.
The precipitated asphaltene is modelled as a pure solid. The pure solid is described within the framework of the EOS model by splitting the heaviest pseudo component in the oil characterization into a non-precipitating component and a precipitating component. These two components have identical critical properties and acentric factors, but different interaction parameters with the light components in the system. The precipitating component may be considered to include asphaltene and resin molecules. Let s1 represent the solid phase in equilibrium with the fluid hydrocarbon phases. The equations for thermodynamic equilibrium between oil, gas and s1 are:
Equation(1) (Available in full paper)
Equation (2) (Available in full paper)
Equation (1a) expresses the equality of fugacity of component i in the oil and in the gas phase. Equation (1b) expresses the equality of fugacity of the precipitating component (with index nc since it is the last component) in the oil phase and in the precipitated solid s1. The component fugacities in the oil and gas phases are calculated from the Peng-Robinson (PR) EOS7. The solid fugacity, fs1, is given by: