Extrapolation of black- and volatile-oil fluid properties with application to immiscible/miscible gas injection

Abstract

Black-oil fluid properties are determined by lab measurements or can be calculated through flash calculations of the reservoir fluid. Allowing for variable bubble-point pressures in black-or volatile-oil models requires a table of fluid properties be extended above the original bubble-point. Reservoir simulation accuracy, however, may be affected by discontinuities in the input data and poor predictions of extrapolated fluid properties. Common practice is to add surface gas to the original oil in the lab and increase the pressure to reach a new bubble-point. Another approach is to use linear extrapolation of oil and gas K-values with pressure on a log-log plot, where K-values are equal to 1.0 at a pseudo-critical or convergence pressure. The latter approach results in discontinuities in the phase behavior.We calculate continuous black-oil fluid properties above the original bubble-point by adding a fraction of the equilibrium gas at one bubble-point pressure to achieve a larger bubble-point pressure. This procedure continues until a critical point is reached at the top of the pseudocomponent pressure-composition diagram. Unlike other methods commonly used or recently proposed, the approach provides a smooth and continuous pressure-composition curve to the critical point. The model further allows for reinjection of produced gas, methane, or CO 2 to increase oil recovery for both volatile and black oils. Further, the approach allows the use of black-oil or volatile-oil properties for tight rocks where capillary pressure alters the saturation pressures by decreasing the bubble-point pressure or increasing the dew-point pressure. Bubble-point pressure in the new model is a function of both capillary pressure (effective pore radius) and gas content. The phase behavior is illustrated using ternary diagrams for up to four components (water, oil, gas, and CO 2 or CH 4 ) and three phases (aqueous, oleic, gaseous) to allow for miscible and immiscible injection of various gases. The new phase behavior could be easily incorporated in a compositionally-extended black-or volatile-oil simulator. The approach could also be used to model gas condensate reservoirs with or without gas injection and capillary pressure. Finally, 1-D slim-tube simulations are made with the extrapolated black oil fluid properties to estimate minimum miscibility pressure (MMP) to demonstrate the applicability of the approach. Shocks occur in the 1-D displacement consistent with a compositional simulator.

A Review of Gas Injection in Shale Reservoirs: Enhanced Oil/Gas Recovery Approaches and Greenhouse Gas Control

“…It should be noted that in this study and another similar study [10], the MMP was measured by using the vanishing interfacial tension (VIT) method, which has been shown to have significant limitations even for conventional reservoirs [53]. Nojabaei and Johns [54] studied the effect of large gas-oil capillary pressure on fluid properties and saturation pressures when the produced gas was injected to enhance oil recovery. They showed that as the original oil mixed with the injection gas, the effect of capillary pressure on recoveries would get smaller.…”

“…Recent studies visualized the gas sweep volume in ultra-tight shale plugs by using CT images [33,58], indicating that gas could make contact with the oil that is trapped in nanosized pores. Furthermore, the nanoconfinement effect may influence the estimations of MMP and alter the fluid properties, so the inclusion of capillary pressure effect and the shift in critical properties results in more accurate recovery prediction [52,54]. In addition, the mechanism of molecular diffusion in shale reservoirs is controversial in the literature.…”

Evaluation of the Effectiveness of Continuous Gas Displacement for EOR in Hydraulically Fractured Shale Reservoirs

“…The superiority of gas-injection-based EOR methods is attributed to the increased reservoir energy (pressure) that can enhance production. Their effectiveness can be further augmented through the use of miscible gases, the dissolution of which in the oil enhances its mobility because of a reduction in its viscosity and density, in addition to a reduction in the oil-gas capillary pressure (Nojabaei and Johns 2016;Du et al 2018).…”