This study investigates optimum matrix-oil-recovery strategies in naturally fractured reservoirs (NFRs) for different wettabilities and rock types. We compare the recovery efficiencies of two cases: (a) primary countercurrent spontaneous imbibition followed by the diffusion of a miscible phase (secondary recovery) and (b) primary diffusion of miscible fluid without preflush of matrix by spontaneous imbibition. For these recovery strategies, the effects of the matrix shape factor, matrix wettability, and type of miscible displacing phase on the rate of recovery and development of residual-oil saturation were clarified experimentally.

Cylindrical Berea-sandstone and Indiana-limestone samples with different shape factors were obtained by cutting the plugs 1, 2.5, and 5 cm in diameter and 2.5, 5, and 10 cm in length. The external surface except one end was coated with epoxy. Static imbibition experiments were conducted on vertically situated samples in which the fractures were at the bottom and matrix/fracture interaction took place in an upward direction. Mineral oil and crude oil were used as oleic phases. Brine was selected as aqueous phase for the primary spontaneous-imbibition recovery. For primary- and secondary-miscible-displacement experiments, n-heptane was used as solvent. Wettability of water-wet Berea-sandstone samples was altered by aging to observe its effects on the dynamics of spontaneous countercurrent imbibition and diffusion.

Parametric analyses were performed for the appraisal of the secondary-and tertiary-recovery potential of NFRs by immiscible-and miscible-fluid injections. The optimal recovery strategies (recovery rate, recovery time, and ultimate recovery) for different rock properties were identified and classified. In water-wet cases, starting the recovery with capillary imbibition followed by diffusion was found to be the optimal way (i.e., both effective and efficient). For limestone or aged-sandstone samples, starting the recovery by diffusion yielded a faster recovery rate and higher ultimate recovery.

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