Impact of Surfactants on Hydrocarbon Mobility in Shales

Abstract

Summary Unconventional reservoirs such as Wolfcamp and Eagle Ford formations have played an important role in boosting the oil and gas production in the United States, but unfortunately, primary recovery from these reservoirs seldom exceeds 10%. Thus, operators are exploring enhanced oil recovery (EOR) techniques such as miscible gas injection (huff ‘n’ puff) and surfactants to increase the production from shales. This study evaluates several commercial surfactants and the commonly used solvent limonene for their ability to increase hydrocarbon recovery. The results show that the various surfactants at 2 gallons per ton (gal/t) or 0.8 wt% concentration recover up to 29, 33, and 34% hydrocarbons from Lyons sandstone, Wolfcamp, and Eagle Ford rock samples, respectively. This is significantly more than the base case (no surfactants), which recovers only 16, 19, and 14%, respectively. The increased recovery by surfactants can be partially explained by the reduction in interfacial tension (IFT) between crude oil and brine (up to 90%) caused by the surfactant solutions. Another important reason governing the hydrocarbon recovery is the ability of the surfactants to prevent asphaltene precipitation. This study focused on the interaction of the surfactants with the asphaltenes and found some surfactants can cause a linear decrease in asphaltene precipitation with increasing surfactant concentration. Finally, the contact angle measurements were used to study the change in wettability of the rock surface caused by surfactant solutions that can preferentially change the oil-wet and mixed-wet pores to more water-wet pores, thereby further aiding the hydrocarbon recovery. This study shows that an integrated approach including a broad spectrum of measurements such as aqueous stability, IFT, contact angle, phase behavior, spontaneous imbibition hydrocarbon recovery, and asphaltene precipitation is required to adequately characterize any surfactant and its ability to increase the hydrocarbon recovery.