The Influence of Molecular Diffusion Mechanism in the Production of Fractured Oil Reservoirs

Molecular diffusion of solution/non-equilibrium gas between oil within the matrix blocks and fracture system in the gassing zone is an important production mechanism of naturally fractured reservoirs. In well-to- moderately fractured reservoirs, the process of molecular diffusion, due to convective currents in the fissures will be active and large volumes of gas would be transported to the gas cap as reservoir pressure declines and consequently results in bubble-point pressure depletion of the oil zone. Conversely, in gas injection scenarios, molecular diffusion can transfer the gas of the gas cap to the matrix blocks and increases the bubble-point pressure of the oil. In order to evaluate the contribution of molecular diffusion in oil production of fractured reservoirs, production history of Haft-kel field was analyzed. Gas-in-place calculations indicated that in case of diffusion, extra oil of about 16.5 percent of the total recoverable reserve is produced during primary natural depletion period )20 years) of the reservoir. In other words, in the case of non-inclusion of molecular diffusion, material balance calculations overestimates oil displacement efficiency by water than that by gas to a value equal to amount of transported gas to the gas-invaded zone of the reservoir by diffusion process.