As naturally fractured reservoirs present wide ranges of geological characteristics and complex flow mechanisms between matrix and fracture, reservoir simulation is highly necessary to properly evaluate production performance. Pressure maintenance by gas and/or water injection is often required in naturally fractured reservoirs to control production GOR and to extract more oil from matrix rock. We studied by simulation particularly about effects of oil compressibility below bubble-point pressure on pressure maintenance and production performance.

We first developed and validated a 3-phase 3-dimensional dual-porosity model with the streamline method considering dissolved gas, capillary pressure and gravity. 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.

The oil and gas compressibility definitions presented by Perrine, that have been being used conventionally, have inherently 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. To correct those, new derivation based on the basic compressibility definition was introduced.

Simulating peripheral water-injection with and without crestal gas-injection, results of pressure and production performance were compared for the new and conventional compressibility formulation. With the new compressibility, higher degrees of effectiveness were demonstrated in pressure maintenance. Gas injection also showed effectiveness in pressure maintenance, though it caused higher production GOR and a faster rise of water cut.

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