Recently Mittermeir (2014) introduced a new material balance (MB) method applicable to naturally fractured dual porosity reservoirs which correctly considers the matrix-fracture fluid transfer too. The MB model is built by two columns, representing the matrix and the fracture continua. The overall MB is calculated as usual, determining the relation between pressure, OOIP, production and water influx. On top of that the fluid distribution in the fracture and matrix will be considered also. In the fracture full phase separation is assumed and the actual phase contacts define three sections. In the bottom section the fracture injects water by capillary imbibition, in the top section the gravitation forces the gas to invade the matrix and in the middle section expansion drive is active. The fracture-to-matrix injection rates are determined from the relation recovery factor versus injected pore volume (called recovery curves). They are derived from laboratory and field data or estimated on the basis of sound engineering judgment. The paper presents the calculation scheme and a successful application to the Sabah field (Libya) that had more than 500 MMstb original oil in place and a 35-year history of production. The new MB method matches both the reservoir pressure and the positions of the phase contacts. It also provides aquifer and matrix-fracture fluid transfer models. For the first time ever it becomes possible to realistically – this means by fully considering the governing recovery mechanisms and thus the matrix-fracture transfer – calculate material balance for naturally fractured dual porosity reservoirs.

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