A Surfactant System for the Oil-Wet Sandstone Of the North Burbank Unit

Abstract

Oil-displacement tests in water- and oil-wet sandstone cores with similar permeability, porosity, and pore structure showed that oil recovery was reduced and sulfonate retention was increased for the oil-wet case. A promising aqueous surfactant system was selected for a pilot test in the North Burbank Unit. Introduction A study of the feasibility of surfactant flooding in the North Burbank Unit reservoir in Osage County, Okla., required a knowledge of the effects of wettability on recovery since the North Burbank reservoir is strongly oil-wet. Previous laboratory studies, of surfactant systems containing petroleum sulfonates have been restricted to tests in water-wet rock. A pilot test of one process has been carried out in the oil-wet Bradford process has been carried out in the oil-wet Bradford sandstone, but no supporting laboratory work has been published. This study compares the performance in both published. This study compares the performance in both water- and oil-wet sandstone of a surfactant system recommended for a field trial in the North Burbank Unit. Surfactant systems have been investigated for recovering the crude oil that remains after waterflooding. The usual procedure in a surfactant flood is sequential injection of a brine preflush to condition the reservoir, a slug of the surfactant system to displace residual oil, and a graded mobility buffer to drive the surfactant slug. Surfactant-Polymer System The surfactant system recommended for the North Burbank Unit contained 5-weight-percent Witco TRS 10-410 petroleum sulfonate, 3-percent isobutyl alcohol, and petroleum sulfonate, 3-percent isobutyl alcohol, and 92-percent brine. The sulfonate product was about 60-percent active sulfonate with an average equivalent weight of about 420 and a relatively narrow distribution of equivalent weights. This surfactant system was chosen after studying the bulk phase behavior and oil displacement characteristics of many surfactant formulations. The screening process for selecting an effective aqueous surfactant system was based on phase and interfacial tension behavior, and on oil recovery from cores. The more efficient surfactant systems formed three bulk phases when equilibrated with reservoir crude oil. The phases when equilibrated with reservoir crude oil. The sulfonate was concentrated in the phase with intermediate density. The phase-volume diagram in Fig. 1 displays the relative volumes of the phases formed when two parts of a surfactant system containing 5-percent TRS 10-410, 3-percent isobutyl alcohol, and varying concentrations of sodium chloride were equilibrated against one part of Burbank crude oil. The minimum interfacial tensions (Fig. 1) occurred between 1.0- and 1.7-percent sodium chloride concentrations where the sulfonate was concentrated in the intermediate phase. The concentration of sodium chloride was 1.5 percent in the surfactant slugs considered in the rest of this paper. After injection into a porous medium, the aqueous surfactant system porous medium, the aqueous surfactant system equilibrated in situ with the reservoir crude oil to form a sulfonate- and oil-rich microemulsion that displaced most of the residual oil. The polymer solutions used to displace the surfactant slugs all contained the same partially hydrolyzed polyacrylamide (Betz Laboratories, Inc., Hi-Vis), which is polyacrylamide (Betz Laboratories, Inc., Hi-Vis), which is an anionic, high-molecular-weight, organic copolymer. It had a molecular weight ranging from 10 to 16 million and a degree of hydrolysis ranging from 21 to 32 percent. Burbank Reservoir Considerations The North Burbank Unit (Fig. 2) covers about 36.5 sq miles in Osage County west of Pawhuska, Okla. JPT P. 501