Abstract

Improving oil recovery from unconventional liquid reservoirs (ULR) is a major challenge. We have demonstrated in previous laboratory studies the impact of surfactants on spontaneous imbibition and oil recovery by means of wettability alteration and interfacial tension (IFT) reduction. Thereby, fracture treatment performance and consequently oil recovery could be improved by adding surfactants to stimulation fluids when a soaking-flowback production schedule is applied. This study evaluates the ability of different groups of surfactants to improve oil recovery in ULR by experimentally simulating the fracture-treatment to represent surfactant imbibition in an ULR core-fracture during a soaking-flowback. Also, we analyze the effect of wettability and IFT alteration on the process. A core-flooding system was combined with the computer tomography (CT) scanner to dynamically visualize the fluid movement as it penetrates the ULR sample in real-time as well as compare oil recovery between surfactants and water without additive. Wolfcamp sidewall cores were longitudinally fractured and loaded into an aluminum-carbon composite core-holder. Two different types of surfactants, anionic and nonionic-cationic, as well as water without surfactants, were injected through the fractures, at reservoir conditions, to evaluate their effectiveness in penetrating into the fractures and recovering oil from ULR core. Then, a soaking-flowback production scheme was used to simulate fracture-treatment and flowback. Changes in core wettability and IFT were determined by contact-angle and pendant-drop methods. Core-flooding results showed that surfactant solutions had higher imbibition and recovered more oil from liquid-rich core compared to water alone. The soaking-flowback production schedule aided by surfactants was able to recover up to 14% of the original oil in place (OOIP) whereas water alone recovered up to 2% of the OOIP. These observations qualitatively agree with wettability and IFT alteration measurements. Core wettability shifted from an original oil-wet to a final water-wet state and surfactants reduced IFT to moderately low values. The results showed that the addition of surfactants to completion fluids and the use of a soaking-flowback production scheme could improve oil recovery by wettability alteration and IFT reduction, maximizing well performance after stimulation. These findings give important understanding for designing completion fluid treatments and flowback schedules for ULR.

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