This study presents a process to investigate mass transfer between fracture and matrix in naturally fractured reservoirs. The study has three parts: (1) the effect of imbibition by capillary pressure Pc in matrix, (2) mass transfer between fracture and matrix through k*σ, and gravity drainage, and (3) combination of capillary pressure, gravity, and viscous forces.

The important results from this study are listed below.

Part-1: The counter current flow theory predicts the capillary pressure between two adjacent cells will have same value at equilibrium condition for both gas-oil and oil-water systems. In addition, the saturation in each cell has a tendency to approach a value where Pc=0 regardless the wettability of the Pc curve.

Part-2: Simulation shows injected fluid preferentially goes through high permeability fracture channel and mass transfer between fracture and matrix is mainly due to k*σ. Gravity drainage effect within a thick matrix block would be reduced due to low vertical permeability or k*σz. If k*σ=0, only viscous force is active.

Part-3: Initial estimate of critical water injection rate on oil recovery in field simulation can be calculated from dimensionless fracture capillary number derived from laboratory test. Then reservoir simulation is used to study the sensitivity of water injection rate to obtain optimal injection rate. In general lower injection rate would be favored due to lower viscous force and longer contact time with the matrix.

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