Improving Oil Recovery in the Wolfcamp Reservoir by Soaking/Flowback Production Schedule With Surfactant Additives

Abstract

Summary Improving oil recovery from unconventional liquid reservoirs (ULRs) is a major challenge. We have demonstrated in previous laboratory studies the effect 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 ULRs by experimentally simulating the fracture treatment to represent surfactant imbibition in a ULR core fracture during soaking and flowback. In addition, we analyze the effects of wettability and IFT alteration as well as surfactant adsorption on the process. A coreflooding system was combined with the computed-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 a 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 (CA) and pendant-drop methods. Coreflooding results showed that surfactant solutions had higher imbibition and recovered more oil from liquid-rich core compared with 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. In addition, surfactants showed adsorption capacity following a Langmuir-type adsorption profile. 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 provide an important understanding for designing completion-fluid treatments and flowback schedules for ULRs.