Summary

Wettability alteration in unconventional liquid reservoirs (ULRs) can improve fracture-treatment performance and consequently oil recovery by changing capillary forces when shifting intermediate- and oil-wet reservoirs to water-wet. Wettability can be modified while fracturing the formation by adding surfactants, in proper concentrations, to completion fluids favoring the process of imbibition and increasing current ULR recovery factors of less than 10% of the original oil in place (OOIP). This study combines the effect of wettability and interfacial-tension (IFT) alteration by surfactants and the corresponding effect on spontaneous imbibition in ULRs from the Permian Basin through a correlated experimental work flow, which includes conducting contact-angle (CA) and ζ-potential experiments, IFT measurements, and spontaneous-imbibition experiments combined with computed-tomography (CT) methods to evaluate and compare the efficiency of different surfactants in altering wettability and recovering hydrocarbons from siliceous core at reservoir temperature.

Wettability-experiment results showed that all surfactants change ULR core-wetting affinity from oil- and intermediate-wet to water-wet at commonly field-used concentrations. However, the anionic surfactant showed better results in changing CAs. In addition, the anionic surfactant better reduced the IFT than nonionic and mixed surfactants, and surfactants performed better than fracturing fluid without surfactant additives. Finally, spontaneous-imbibition results showed that the anionic surfactant was better for recovering oil from shale core, which agrees qualitatively with the previous results where the anionic surfactant showed the lowest CAs and IFT. However, both anionic and nonionic surfactants were better in displacing liquid hydrocarbons and had higher penetration magnitudes obtained by CT methods compared with fracturing water without surfactant. From the results obtained, it can be concluded that the addition of proper surfactants to fracturing fluids has the potential of improving oil recovery by wettability and IFT alteration, with the anionic surfactant showing lower CAs and IFT, better imbibition, and higher oil recovery than nonionic and mixed surfactants in these siliceous ULRs from the Permian Basin.

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