Potential of Improving Oil Recovery with Surfactant Additives to Completion Fluids for the Bakken

Fracture treatment performance in Bakken liquid-rich shale reservoirs can be improved by altering rock wettability from oil-wet to water-wet. The use of surfactant additives for altering wettability 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 nanopores associated with unconventional liquid reservoirs (ULRs) and complex as the contact angle (CA) and IFT varies simultaneously. This study carefully evaluates these interactive variables and compares the performance of anionic, nonionic, blended, and complex nanofluid (CnF) surfactants in recovering liquid hydrocarbon from siliceous and carbonate Bakken shale cores through spontaneous imbibition experiments. In addition, we also analyze the effect of wettability, surfactant adsorption, and IFT alteration on the process. CA measurement is used in determining the original wettability of Bakken cores, as well as the wettability alteration effectiveness of each surfactant. The results show Bakken ULR original wettability in the oil-wet region, but most importantly, all surfactants are capable of shifting the wettability of the core to the water-wet region at field-used concentrations. However, we observed that the extent of wettability alteration strongly depends on rock lithology and surfactant type. These results were also corroborated by zeta potential measurements. In addition, IFT measurement of Bakken crude oil shows that all surfactants cause a reduction in IFT value. Surfactant dynamic adsorption measurements also show the dependence of rock lithology on surfactant performance giving higher adsorption values on carbonate rocks to negatively charged surfactants and higher adsorption values on siliceous surfaces to more positively charged surfactants. Last, the surfactant potential of improving oil recovery in ultralow permeability Bakken cores is investigated by spontaneous imbibition experiments. Through the experiment, oil recovery is recorded and the system is scanned using computed tomography (CT) in order to analyze the movement of fluid in the core and to compare the performance between surfactants and slickwater without an additive. The results suggest that surfactants are better on recovering oil from a shale core, displacing more oil, and having higher imbibition than slickwater with no additive. However, oil recovery depends on surfactant type and rock mineral composition. These findings are consistent with CA, zeta potential, surfactant adsorption, and IFT measurements. From the results obtained, it can be concluded that altering wettability and moderately reducing IFT when surfactant additives are added to aqueous solutions can improve oil recovery in Bakken cores. These findings give an important understanding for designing completion fluid treatments and flowback schedules for these ULRs.