Improving oil recovery in the Wolfcamp unconventional liquid reservoir using surfactants in completion fluids
Abstract Well performance after stimulation in unconventional liquid reservoirs (ULR) can be enhanced by altering rock wettability, and by moderately reducing interfacial tension (IFT) using surfactants in completion fluids. However, ULR lithology plays a major role in surfactant selection due to rock-fluid interactions having direct impact on oil recovery. This experimental study evaluates and compares the efficiency of different groups and blends of surfactants on recovering liquid hydrocarbons from siliceous and carbonate Wolfcamp shale cores by analyzing the effects of lithology, oil and surfactant type on wettability and IFT alteration, and their impact on imbibition and oil recovery. ULR wettability was determined by contact angle (CA) experiments, and its alteration was evidenced when adding surfactants to completion fluids. The results showed that at field-used concentrations, all tested surfactants altered wettability from oil and intermediate-wet to water-wet. These findings were consistent with zeta potential results when assessing surfactant solution film stability on siliceous and carbonate shale rock surfaces as an indication of water wetness. In addition, IFT between crude oil and completion fluids was measured to gauge IFT reduction by surfactants, showing higher reductions by anionic surfactants compared to nonionic and blended surfactants. Finally, imbibition potential for completion fluids with and without surfactants was tested in spontaneous imbibition experiments. Oil recovery from Wolfcamp shale cores was tracked in real time, and CT scan technology was simultaneously used to monitor frac fluid penetration in ULR cores. The results showed that cores submerged in completion fluids with surfactant additives have higher hydrocarbon recovery and better fluid imbibition than those in frac fluids without surfactants. In addition, siliceous shale cores had higher oil recovery and fluid penetration when anionic surfactants were used. Conversely, carbonate shale cores showed better hydrocarbon recovery and penetration when submerged in nonionic-cationic surfactants. Hence, it can be concluded that the addition of surfactants to completion fluids improved oil recovery by wettability alteration and IFT reduction, and core lithology, oil and surfactant type impacted fluid imbibition and oil recovery and should play a major role on surfactant selection. These findings give important understanding for designing completion fluid treatments and flowback schedules for these ULR.