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.

Wettability alteration in shale formations can be an important factor in improving the performance of hydraulic fracturing treatments. The use of surfactants in the frac fluid, at proper concentrations, has shown to change wettability in Unconventional Liquid Reservoirs (ULR) favoring the process of imbibition. This study evaluates and compares the efficiency of anionic and nonionic surfactants in recovering hydrocarbons in carbonate and siliceous preserved side-wall core. The techniques developed also open the door for investigation of low concentration surfactants for enhanced oil recovery (EOR) in ULR. Contact angle (CA) experiments were performed, using the captive bubble method, to measure the magnitude of wettability alteration on intermediate to oil-wet ULR core at reservoir temperature (165 °F). Different types of anionic and nonionic surfactants at field concentrations were used. The results showed that all surfactants lower the CA at the concentration tested. However, anionic surfactants showed better results as observed by lower contact angles. IFT measurements were also performed, using the pendant drop and spinning drop methods, at reservoir temperature using reservoir crude oil and anionic and nonionic surfactants at the same concentrations. The IFT reduction was similar for each type of surfactant compared to regular frac fluid without any surfactant, but anionic surfactant showed slightly better capability of reducing IFT than nonionic surfactants. Computed tomography (CT) scan methods were used to gauge the performance of these surfactants in improving oil recovery. The magnitude of penetration or imbibition into artificially-fractured ULR cores was studied for both anionic and nonionic surfactants. Frac fluids containing surfactants were mixed with a dopant salt to trace the movement of these fluids and measure the penetration numerically. Both, anionic and nonionic, surfactants have higher penetration magnitudes compared to slick water without surfactant. However, anionic surfactants displaced a higher observable amount of liquid hydrocarbon from the shale cores. This observation agrees qualitatively with the results observed in the CA experiments where anionic surfactants showed the lowest contact angles. From the results obtained, it can be concluded that anionic surfactants alter wettability in these ULR core, giving lower CA, better spontaneous imbibition and higher oil recovery than nonionic surfactants. These observed wettability changes induced by surfactants mixed in the frac fluids can improve matrix penetration with spontaneous imbibition which opens further discussions for EOR potential in shale formations.

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Fracture treatment performance in Bakken shale reservoirs can be improved by altering rock wettability, as measured with contact angle (CA), from oil-wet to water-wet. Altering the wettability with the use of chemical additives also results in alteration of the interfacial tension (IFT). The Young-Laplace equation relates the capillary pressure to IFT and contact angle. Thus, it follows that capillarity is significant in nano-pores associated with unconventional liquid reservoirs (ULR) and complex as the CA and IFT varies simultaneously. We carefully evaluate these interactive variables to improve oil recovery by alteration of capillary pressure by understanding the wetting state of siliceous and carbonate Bakken cores with and without chemical additives. We have observed that wettability can be altered from the ULR natural state of oil-wet to systems favoring frac fluid imbibition. Surfactants can be added to completion fluids, in proper concentrations, to alter wettability while hydraulic fracturing the formation. This experimental study evaluates and compares the efficiency of anionic and nonionic surfactants, and complex nanofluids (CNF) on recovering liquid hydrocarbons from Bakken shale cores by analyzing the effect of wettability and IFT alteration and their impact on spontaneous imbibition.The original wettability of Bakken cores is determined by CA measurements. Then, two surfactant types, anionic and nonionic, and CNF are evaluated to gauge their effectiveness in altering wettability. The results show that all surfactants and CNF are able to shift core wettability from oil-wet to water-wet. However, chemical additives efficacy strongly depends on rock lithology, surfactant, and CNF type. Moreover, to evaluate further wettability alteration, stability of surfactant and CNF solution films on the shale rock surface is determined by zeta potential measurements. Surfactants and CNF show higher zeta potential magnitudes than water without additives, as an indication of better stability and water-wetness, which agrees with CA results. In addition, the effect of IFT alteration is studied in solutions with surfactants and CNF, and Bakken crude oil. Higher IFT reduction is achieved by anionic surfactants, but all surfactants and CNF perform better than water alone.Surfactants and CNF potential for improving oil recovery in ultralow permeability Bakken cores is investigated by spontaneous imbibition experiments using modified Amott cells in an environmental chamber. Using computed tomography (CT) scan methods, water imbibition as penetration magnitude is measured in real time. In addition, oil recovery is recorded with time to compare the performance of surfactants, CNF, and completion fluid alone. The results suggest that surfactants and CNF are better on recovering oil from shale core displacing more oil and having higher penetration magnitudes than water without additives. In addition, oil recovery depends on surfactant and CNF type and rock mineral composition. These findings are consistent with CA, zeta poten...