Wettability, Oil and Rock Characterization of the Most Important Unconventional Liquid Reservoirs in the United States and the Impact on Oil Recovery

摘要

Rock wettability dominates fluid flow and influences rock-fluid interaction impacting oil recovery. Siliceous and carbonate cores from Bakken, Eagle Ford, Wolfcamp and Barnett are used investigate original wettability of these unconventional liquid reservoirs (ULR). In addition, we carefully analyze the relation of rock mineralogy, oil type and total organic content (TOC) to wetting affinity. The objective of this experimental study is to characterize wettability states from four of the most important ULR in the United States and its impact in oil recovery. Original wettability from ULR cores is quantified initially by contact angle (CA). The results show that all liquid rich shale plays evaluated exhibit intermediate-wet behavior. However, TOC and oil type influence wetting affinity whereas rock mineralogy does not exhibit an effect on original wettability. Moreover, zeta potential experiments are utilized to assess the stability of thin water films on the shale rock surface and its correlation to wettability. The measurements indicate low zeta potential values that confirm an unstable water film, which can be interpreted as intermediate wetting behavior consistent with CA results. In addition, petrophysical properties such as permeability, porosity and pore size distribution using mercury injection capillary pressure (MICP), XRD, total organic carbon-analyses and scanning electron microscopy (SEM) images, as well as oil properties like IFT, API gravity, and oil total acid and basic number are measured to further understand wettability states from these ULR. Finally, spontaneous imbibition experiments are performed to investigate wetting affinity and fluid penetration in ULR cores. Using modified Amott cells, preserved and aged cores are submerged in water or oil to gauge wetting affinity by measuring fluid imbibition. Moreover, time-lapse, computed tomography (CT) determined penetration magnitude. Lastly, the potential of water imbibition as a technique for improving oil recovery during hydraulic fracturing ULR is investigated by submerging aged cores in water to represent soaking during shut-in of the well after stimulation. The results show that both water and oil are capable of imbibing into ULR matrix pore space thus demonstrating their intermediate-wetting state, which is consistent with CA and zeta potential experiments. In addition, spontaneous imbibition experiments performed in aged shale cores indicate the efficacy of recovering oil by water imbibition when hydraulically fracturing the formation, which opens discussions indicating the scope for improved and/or enhanced oil recovery in ULR.