Verification of confinement impact on flow using a black oil simulator

Abstract

This research study for a Master of Science degree has been conducted under the Unconventional Reservoir Engineering Project (UREP) at the Marathon Center of Excellence for Reservoir Studies (MCERS) in the Petroleum Engineering Department of Colorado School of Mines. The main objective of the research is to verify the impact of confined PVT behavior (pore size impact) on flow in unconventional reservoirs by considering the impact of gas-oil drainage capillary pressure on oil and gas production (depletion) profiles. Unlike conventional oil and gas reservoirs, pore sizes in unconventional resources can be prohibitively small for the flow of hydrocarbon fluids. Moreover, extreme pore confinement causes deviations from conventional (bulk) fluid phase behavior. When confinement increases, capillary pressure between the phases occupying the pore system increases. Black oil simulators that are commonly used in the industry are not formulated to handle the impact of capillary pressure on phase behavior, and they do not evaluate fluid properties at corresponding gas- and oil-phase pressures. This may be a significant problem in simulation of nano-porous unconventional reservoirs where fluid phase equilibrium calculations are affected due to capillary forces in confinement. Studies have shown that the confinement manifests itself as suppression in bubble-point pressure which extends the under-saturated portion of the formation-volume-factor curve for oil. This study uses a black-oil simulator, COZSim, which was developed earlier under UREP to include the capillary-pressure impact on phase behavior. In COZSim, individual phase pressures are derived from the bulk pressure using the capillary pressure and the excess pressure, which is the additional pressure required for the two phase to be in a thermodynamic equilibrium. A correlation that represents the bubble-point suppression as a function of the solution gas-oil ratio ($R_{s}$) and capillary pressure ($P_{c}$) is used in the simulator. Sensitivity tests have been performed to show the impact of phase behavior due to confinement on fluid flow and production from wells in unconventional reservoirs. Various simulation models have been built to study the impact of single-, dual-, triple- and quintuple-porosity formulations on calculated oil and gas production profiles at different capillary pressure values. Results of the simulations show considerable dependence of oil and gas production not only on the magnitude of the confinement but also on the formulation that is used.