Effects of oil compressibility on production performance of fractured reservoirs evaluated by streamline dual-porosity simulation

As naturally fractured reservoirs (NFR) present wide ranges of geological characteristics and complex flow mechanisms between matrix and fracture, reservoir simulation is highly necessary to properly evaluate production performance. The production performance depends on fracture distributions and matrix/fracture properties as well as on fluid properties. We studied by simulation particularly about effects of oil compressibility below bubble-point pressure on production performance. Also evaluated was how the compressibility effects vary with different fracture spacing that is generally uncertain even in a production stage and should be characterized by simulation. We first developed and validated a 3-phase 3-dimensional dual-porosity model with the streamline method. The fluid compressibility is a primary parameter that directly affects the reservoir performance. We accounted for compressibility effects with the total compressibility in the 3-D pressure equations, and with the effective density in the 1-D flow equations along streamlines. A flash-calculation algorithm was incorporated to treat the gas and oil phases correctly. We also considered dissolved gas, capillary pressure and gravity The oil and gas compressibility definitions presented by Perrine, that have been being used conventionally, show a physical inconsistency such that oil compressibility below the bubble-point pressure increases with the increase of density, and that the mass of gas phase remains constant with changing pressures below the bubble-point pressure. To correct those, new derivation based on the basic compressibility definition was introduced. With constant water-injection and liquid production rates in a 5-spot well pattern, simulation results using the new and conventional compressibility formulation were compared for different fracture and fluid properties such as shape factor and solution gas-oil ratio. With the new compressibility, we obtained slower declines of the reservoir pressure, higher oil-production rates, lower water-production rates, and slower increases of production GOR. Differences between the results with the new and conventional compressibility are augmented as the fracture spacing decreases.