An Excess-Bubble-Point-Suppression Correlation for Black Oil Simulation of Nano-Porous Unconventional Oil Reservoirs
The average pore size in unconventional, liquids-rich reservoirs is estimated to be less than 100 nm. At this nano-pore scale, capillary forces play an important role on phase behavior that is not considered in conventional PVT studies. Confinement on phase behavior of black-oil fluids manifests itself as bubble point pressure suppression, extension of the undersaturated portion of the formation volume factor curve, and alteration of the equilibrium gas composition. Studies show that the magnitude of the bubble point suppression is more than the capillary pressure and may amount to hundreds of psi. These phenomena can be modeled through compositional solution of the phase behavior at differing gas- and oil-phase pressure values that are due to capillary pressure. However, black-oil simulators cannot perform the compositional phase behavior calculations to estimate the total bubble point suppression due to confinement. In this study a correlation that expresses the bubble point pressure suppression as a function of the capillary pressure and the solution gas oil ratio (Rs calculated through conventional PVT which is the input in black oil simulator) was developed, such that it can be used as a simulation model input. The correlation data was based on three unconventional oil samples evaluated at different saturation pressures and compositions. To use the correlation, a modified black oil simulator that can handle the PVT data at different oil- and gas-phase pressure values is required. The source code of the black oil simulator used in this study was modified to include the total bubble point suppression into the PVT calculations. The impact of the confined phase behavior on flow was quantified through simulation runs. The results showed that the grid blocks with different capillary pressure values reach the bubble point at different times. During depletion, the grid blocks with higher capillary pressure values remained in undersaturated conditions longer, impacting the gas production and pressure profiles.