Wettability Alteration Using Surfactants to Improve Oil Recovery from Unconventional Liquid Reservoirs

Abstract

Improving oil recovery from unconventional liquid reservoirs (ULR) is a major challenge, and knowledge of recovery mechanisms and interaction of completion fluid additives with the rock is fundamental in tackling the problem. Fracture treatment performance and consequently oil recovery could be improved by adding surfactants to stimulation fluids to promote imbibition by wettability alteration and interfacial tension (IFT) reduction. The Young-Laplace equation relates the capillary pressure to IFT and contact angle. Thus, it follows that capillarity is significant in nanopores associated with ULR and complex as the contact angle (CA) and IFT varies simultaneously. This study analyzes the potential of improving oil recovery by imbibition using different groups of surfactants as additives to completion fluids by characterizing their interaction with oil and heterogeneous siliceous and carbonate ULR samples from the Wolfcamp, Eagle Ford, Bakken and Barnet formations as well as the effect of wettability modification and IFT reduction in maximizing well performance after stimulation.

A correlated set of experiments were proposed beginning by characterizing ULR rocks and fluids and evaluating original wettability by measuring CA and zeta-potential. Then, different types of surfactants were evaluated to gauge their effectiveness in altering wettability and IFT. In addition, adsorption measurements were performed to calculate the amount of surfactant adsorbed into the rock. Moreover, spontaneous imbibition experiments were carried out in conjunction with CT scan technology to measure oil recovery, fluid penetration (imbibition) and change of fluid saturation in the rock samples with time. Then, a core-flooding system was designed to be combined with the CT scanner to experimentally simulate the fracture-treatment and to represent surfactant imbibition in an ULR core fracture during a soaking and flowback production scheme. The results showed that surfactant solutions are capable of altering ULR wettability to water-wet with moderate reduction of IFT. However, the extent of wettability alteration strongly depends on rock lithology, surfactant and oil type. Surfactant adsorption measurements also showed the dependence of rock lithology on surfactant performance. Moreover, spontaneous imbibition and core-flooding experiments suggested that wettability alteration and IFT reduction are beneficial to oil recovery as evidenced by the improved oil recovery when surfactants were used. These findings were consistent with CA, zeta potential, surfactant adsorption and IFT measurements.

Next, to scale our laboratory results, imbibition rates and dimensionless time scaling curves were generated corroborating that fracture density and rock-fluid interactions are key parameters for oil recovery. From the results obtained, it can be concluded that moderate IFT reduction in addition to significant wettability alteration has optimum effect on improving oil recovery from these ULR. These findings provide insight in designing completion fluids and flowback schedules for these unconventional liquid resources.