Summary
Improving oil recovery from unconventional liquid reservoirs (ULRs) is a major challenge, and knowledge of recovery mechanisms and the interaction of completion-fluid additives with the rock is fundamental in tackling the problem. Fracture-treatment performance and consequent oil recovery can be improved by adding surfactants to stimulation fluids to promote imbibition by wettability alteration and moderate interfacial-tension (IFT) reduction. Also, the extent of surfactant adsorption on the ULR surface during the imbibition of completion fluids is a key factor to consider when designing fracture jobs. The experimental and modeling work presented in this paper focuses on the effectiveness of surfactant additives for improving oil recovery in Wolfcamp and Eagle Ford reservoirs, as well as the extent of surfactant loss by adsorption during the imbibition of surfactant-laden completion fluid. Original rock wettability is determined by contact angle (CA) and zeta potential. Then, distinct types of surfactants—anionic, anionic/nonionic, and cationic—are evaluated to gauge their effectiveness in altering wettability and IFT. Moreover, surfactant-adsorption measurements are performed using ultraviolet/visible (UV/Vis) spectroscopy. Next, the potential for improving oil recovery using surfactant additives in ultralow-permeability Wolfcamp and Eagle Ford shale cores is investigated by spontaneous-imbibition experiments, and computed-tomography (CT) methods are used to determine fluid imbibition in real time. Finally, laboratory data are used in numerical simulations to model laboratory results and to upscale these findings to field scale. The results showed that aqueous solutions with surfactants altered rock wettability from oil-wet and intermediate-wet to water-wet and reduced IFT to moderately low values. In addition, cationic surfactant presented the highest adsorption capacity following a Langmuir-type adsorption profile. Spontaneous-imbibition results showed that aqueous solutions with surfactants had higher imbibition, and were better at recovering oil from shale core compared with water without surfactants, which agrees qualitatively with wettability and IFT alteration. However, rock lithology and surfactant type played a key role in adsorption capacity and oil recovery. Our upscaling result showed that, compared with a well that is not treated with surfactant, a 24% increase in the initial peak oil rate and an 8% increase in the 3-year cumulative oil production were observed. For the results obtained, we can conclude that the addition of surfactants to completion fluids can improve oil recovery by wettability alteration and IFT reduction, maximizing well performance after stimulation from Wolfcamp and Eagle Ford unconventional reservoirs.
Study on mechanism of spontaneous imbibition and pressurized imbibition in shale oil reservoirs
