Quantifying the reimbibition effect on the performance of gas-oil gravity drainage in fractured reservoirs: Mathematical modelling

As a major oil production mechanism, gravity drainage in fractured oil reservoirs is heavily affected by the reimbibition phenomenon. In this work, a stack of oil-saturated two matrix blocks surrounded by oil-saturated fractures are considered as a synthetic fractured reservoir. A mathematical computer program is developed to numerically simulate the fluids' flow through the matrixes and fractures employing the finely gridded single porosity concept. Simulation results are validated by checking the continuity of the gas volume through the reservoir. The percentage of oil recovered by gravity drainage is determined employing the new material balance equation approach. A new procedure is employed for deactivating the reimbibition phenomenon, introducing the zero transmissibility for the oil phase in the case of oil transfer from the fracture to matrix region. According to the simulation results and considering the proposed model, under the constraints of constant gas injection and oil production rates, fully or partially deactivating the reimbibition phenomenon causes a faster gas movement through vertical fractures. The reason for this is that some of the oil leaving the upper matrix block goes to vertical fractures and causes a faster gas breakthrough in them due to reduced available pore volume to gas. In this case the oil recovered by gravity drainage from the system is decreased. Inactivating this phenomenon in the simulated case study causes a 40 % reduction in oil production by gravity drainage from the matrix blocks until the breakthrough time.