In compositional simulation of gas-injection processes, it is often observed that gridblock oil saturations decrease far beyond the user-defined residual oil saturation, even under immiscible conditions. This numerical phenomenon occurs because oil components are allowed to vaporize into the gas phase as much as the phase equilibrium obtained with an equation of state (EOS) permits. Especially in the vicinity of gas injectors, an oil saturation of zero is sometimes predicted. On the other hand, such significant low oil saturation is rarely seen in laboratory data such as coreflood experiments and slimtube tests.
The reason for the discrepancy between the simulation results and the laboratory results described above is that bypassed oil located in dead-end pores or caused by subgrid-scale heterogeneities is not considered in the current compositional-simulation practice. To overcome this, we developed an innovative method of incorporating laboratory-based residual oil saturations. The proposed method can restrict the excessive vaporization and maintain the prescribed residual oil by accommodating a novel application of the transport coefficient (Barker and Fayers 1994).
Incorporation of the "true" residual oil saturation into the gasflood compositional simulation has been a critical problem in the industry. On the other hand, most compositional simulators allow oil saturations to be as low as an EOS predicts, but no rigorous method has been proposed to honor laboratory observations usually indicating nonzero residual oil saturations. This is why immiscible gas injection is predicted to achieve a good recovery factor despite the fact that even miscible coreflood experiment results rarely show 100% recovery.
Not only laboratory experiments but also field observations indicate that bypassed oil occurs even after the miscible injectant passes through above the minimum miscibility pressure (MMP), as stated by Stalkup (1983) and McGuire et al. (1995). Dead-end pore volume and precipitation can cause such bypassed oil under gas injection with no prior waterflood history. Although the mass transfer in microscopic scale like molecular diffusion can partially recover such bypassed oil, as described by Burger et al. (1996), there still remains the oil behind the miscible front.
For this reason, even under the miscible-flooding condition, the residual oil saturation will not reach 0% in the real heterogeneous reservoir. This residual oil is referred to as miscible flood residual oil saturation (Spence and Watkins 1980). In the conventional compositional simulation, there is no facility to actively define "true" residual oil (nonvaporizing oil) and, hence, the excessive vaporization of oil components into the gas phase is predicted. Consequently, the miscible flood residual oil saturation could not be represented in the simulation model. This frequently has caused optimistic results in which all the oil in the gridblock can be stripped by the injected gas.
The first attempt to restrict the excessive vaporization described above was made by Fayers et al. (1992), who proposed the concept of dual-zone mixing. In this concept, the phase equilibrium is established between the hydrocarbon trapped in dead-end space (uncontacted oil) and that in the permeable portion of a simulation block. Analogous to a dual-porosity model, uncontacted oil is allowed to disperse only when it is vaporized into the contacted oil or gas. The concept of the transport coefficient (Barker and Fayers 1994) was adopted to incorporate the component dependence in mass transfer. However, the work of Fayers et al. (1992) has not been used widely, primarily because of the complexity of the theory and, hence, special coding was required to accommodate the dual-zone mixing.