Primary and Secondary Oil Recovery From Different Wettability Rocks byCountercurrent Diffusion and Spontaneous Imbibition

摘要

This study investigates optimum matrix oil recovery strategiesin naturally fractured reservoirs (NFR) for differentwettabilities and rock types. We compare the recoveryefficiencies of two cases: (a) the primary counter-currentspontaneous imbibition followed by the diffusion of a misciblephase (secondary recovery) and (b) primary diffusion ofmiscible fluid without pre-flush of matrix by the spontaneousimbibition. For these recovery strategies, the effects of thematrix shape factor, matrix wettability, and type of miscibledisplacing phase on the rate of recovery and development ofresidual oil saturation were clarified experimentally.Cylindrical Berea sandstone and Indiana limestonesamples with different shape factors were obtained by cuttingthe plugs 1, 2.5, and 5 cm in diameter and 2.5, 5, and 10 cm inlength. All sides were coated with epoxy except one end.Static imbibition experiments were conducted on verticallysituated samples where the matrix-fracture interaction tookplace upward direction. Mineral oil and crude oil were usedas oleic phases. Brine was selected as aqueous phases for theprimary spontaneous imbibition recovery. For primary andsecondary miscible displacement experiments n-heptane wasused as solvent. Wettability of water-wet Berea sandstonesamples was altered to weakly water-wet to observe its effectson the dynamics of spontaneous counter-current imbibitionand diffusion.Parametric analyses were performed for the appraisal ofsecondary and tertiary recovery potential of naturally fracturedreservoirs by immiscible and miscible fluid injections. Theoptimal recovery strategies (recovery rate, recovery time andultimate recovery) for different rock properties were identifiedand classified. In water-wet cases, starting the recovery withcapillary imbibition followed by diffusion was found theoptimal way, I.e. both effective and efficient. For limestone or aged sandstone samples, starting the recovery by diffusionyielded a faster recovery rate and higher ultimate recovery.