Surfactant imbibition is a new method to improve the oil recovery from tight rocks. The idea of this method is to introduce diluted surfactant solution (0.1-0.25% in active concentration) to stimulate the oil recovery from low or ultra-low permeability formations. The loss of surfactant from aqueous solutions during propagation into tight rocks with ultra-low permeability is a major concern for surfactant imbibition EOR. This loss is attributed to adsorption or retention on rock surfaces. Generally, surfactant adsorption/retention is affected by surfactant structure, mineralogy (for example, clay content), salinity, pH, Eh, microemulsion viscosity, crude oil, co-solvent and other variables. Some static adsorption and kinetic adsorption studies have been reported on sandstone, limestone, and permeable carbonate reservoirs, but few on ultra-lower permeable rocks, such as the Bakken Formation, which consists of Lower Shale Member, Middle Dolostone/ Siltstone Member, and Upper Shale Member, so far.

This paper investigates the degree of surfactant adsorption loss of candidate surfactant formulations in tight rocks with ultra-low permeability using a UV-spectrometer. Two core samples from the Bakken Formation with permeability that ranged from 0.004 to 0.008 md and seven surfactant formulations of various types (anionic, nonionic, and blended) were studied. In laboratory experiment, the core samples were smashed and mixed with a surfactant-brine system at concentration and pH value of interest. The mixture was shaken with an electronic shaker until equilibrium was reached. Then the mixture was centrifuged and the supernatant was taken out to measure the absorbance. In order to quantify the adsorption loss, a relationship (calibration curve) between concentration and absorbance was built for each surfactant solution by using a series of standard solutions.

Based on laboratory observation, we found: (1) one candidate surfactant formulation (blended) showed a linear relationship between concentration and absorbance, and an adsorption capacity in an average of 0.62 mg/g; (2) three surfactant solutions showed an adsorption capacity that ranged from 11. 91 to 33.08 mg/g, which was much higher than expected and considered to be caused by solution turbidity; (3) two surfactant solutions did not present any absorbance peaks in the range of measuring wavelength; (4) surfactant adsorption capacity was reduced when alkaline was added on anionic surfactant, and other surfactant types. This result encourages us to investigate the surfactant desorption in our future work. (5) Even though the lowest adsorption capacity of the surfactant adsorption (0.62 mg/g) was still higher than 0.1 mg/g compared with the conventional permeable rocks, for an ultra-low permeable reservoir, these surfactants are still effective in improving oil recovery, so it is positive to use them to the field application.

You can access this article if you purchase or spend a download.