Abstract

The large permeability contrast between fractures and matrix in highly fractured carbonate reservoirs has been a hindrance to the efficient recovery of the oil from the matrix. Gas oil gravity drainage (GOGD) has been the most appealing option to date. However it is a slow process. Cyclic Pressure Pumping (CPP) is proposed which has the potential of increasing the rate of recovery over GOGD under certain conditions. CPP utilises the fact that due to the much lower permeability in the matrix, the matrix pressure lags behind the fracture pressure during a rapid pressure change. A sudden drop in pressure in the fractures results in expansion of fluids in the adjoining matrix which are discharged into the fracture system. The fluids in the fracture system can then be produced. The reservoir can be re-pressurised by gas injection and depressurised by producing the fluids in an alternate manner.

This process originally called batch gas cycling was suggested in the 50s for improving recovery in non-homogeneous reservoirs over conventional gas flooding. It was tried successfully in a small field in the late 50s. Laboratory experiments in the 70s on low-permeability matrix in contact with high-permeability fractures indicated the effectiveness of the process.

The process was re-discovered during reservoir production optimisation simulation studies in a heavy oil steam injection project in Oman and will be applied to minimise oil loss during steam plant shut downs. Early field observations support the process. The paper presents a study of the process using both dual permeability and single porosity simulation modeling. It also highlights the conditions which are favorable to the process as well as certain practical problems that could be encountered in implementing it.

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