Capillary imbibition is the dominating recovery mechanism in naturally fractured reservoirs if matrix is water wet and continuous supply of water through fractures exists. The mechanics of the process still needs clarification for complex conditions. Matrix boundary, i.e., the contact areas of fracture fluids with matrix blocks, is one of the causes of this complexity. Capillary imbibition rate and ultimate recovery are significantly affected by the partial contact of water with the matrix due to phase segregation in fracture by gravity or impervious layer covering some part of the matrix.

This study aims at analyzing the recovery behavior experimentally for different boundary conditions and proposes remedies to increase the ultimate recovery. Berea Sandstone samples saturated with two types of oil are exposed to counter-and co-current capillary imbibition at ambient temperature and pressure conditions. Matrix boundary conditions, i.e., the matrix-water contact areas, are changed by covering the sandstone sample surface using epoxy. The oil recovery in non-contacted area of the matrix is due to gravity segregation and also re-imbibition of water that is already imbibed by the matrix blocks. To remedy low oil recovery from the parts of the matrix where no contact with water exists, the same experiments are repeated using low IFT solutions. It is observed that the low IFT solution yields in higher oil recovery from the non-contacted part of the core sample. But, the recovery rate decreases when the low IFT solution is used instead of brine especially for the unfavorable boundary conditions.

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