Enhancement of Oil Recovery Using a Low Concentration of Surfactant to Improve Spontaneous and Forced Imbibition in Chalk

Abstract

Abstract A study was undertaken with the objective of evaluating the use of low concentrations of surfactant to improve oil recovery. Various concentrations of commercial surfactants were screened for long term stability at high temperature in seawater. Cloud points, precipitation, and the stability of surface tension were used as screening criteria. Spontaneous imbibition tests at ambient and reservoir temperatures were conducted using reservoir chalk plugs that were moderately water-wet. A selected surfactant, when added at low concentrations (100 to 500 parts per million of active surfactant) to the imbibition water reduced the residual oil saturation over imbibition tests conducted without surfactant. Acceleration of spontaneous imbibition was observed in tests that had improved oil recovery. Forced imbibition tests for viscous displacement were conducted by flow tests. Although low concentrations of surfactant did not lower oil-water interfacial tension below single digits, a reduction in residual oil saturation was obtained in the forced imbibition tests over tests without surfactant. Measurement of surfactant adsorption indicated that low adsorption at reservoir conditions could be obtained below the critical micelle concentration (CMC) of some surfactants. The combination of improved recovery with low adsorption suggests that the addition of surfactant to injected water may improve the economics of surfactant-enhanced water flooding under appropriate conditions. Introduction Surfactants have long been considered for improving oil recovery from oil reservoirs. Although many pilot tests and a few field tests have been conducted, the economics of surfactant injection have rarely been favorable. Most surfactant studies focused on the injection of a high concentration of surfactant to create ultralow interfacial tension between oil and water via microemulsions followed by a slug of polymer designed to mobilize the surfactant bank and oil. The front end cost of such a process was only favorable if residual oil saturations could be reduced to near zero, surfactant adsorption was not excessive, the sweep efficiency was excellent and oil prices were high. In contrast, consideration herein was given to improving the economics by a process that used only a dilute amount of surfactant to minimize adsorption and to recover only a portion of the residual oil. Published field tests1,2 show that it is possible to have an economic surfactant process at low surfactant concentrations. Others have examined in the laboratory, the possibility of dilute surfactant flooding for improving oil recovery3,4,5. This paper will focus on improvements observed in the displacement of oil in the laboratory and the approach to minimize surfactant losses. Description and Application of Equipment and Processes Preparation of Materials. Kansas outcrop chalk and field reservoir chalk were used in this study. Field reservoir plugs were extracted by alternation of toluene and methanol soaks until no hydrocarbon discoloration was observed (1–2 months). All plugs were one inch in diameter and up to 3.5 inches in length. The wett ability of the Kansas outcrop plugs was altered by aging under field crude oil at initial water saturation using established methods6 and then extracted like the field reservoir plugs. Spontaneous imbibition tests were used to establish a baseline measurement and to select moderately water-wet plugs for further testing. The field stock-tank crude oil was centrifuged to remove paraffins that were solid at near-ambient temperatures. N-decane of 99+% purity was used and filtered through a silica gel column before use. The brines were equilibrated with chalk material and filtered before use. Other fluids were used as purchased. Surfactants were commercially available products. They were diluted with synthetic North Sea water to desired concentrations before use. Preparation of Materials. Kansas outcrop chalk and field reservoir chalk were used in this study. Field reservoir plugs were extracted by alternation of toluene and methanol soaks until no hydrocarbon discoloration was observed (1–2 months). All plugs were one inch in diameter and up to 3.5 inches in length. The wett ability of the Kansas outcrop plugs was altered by aging under field crude oil at initial water saturation using established methods6 and then extracted like the field reservoir plugs. Spontaneous imbibition tests were used to establish a baseline measurement and to select moderately water-wet plugs for further testing. The field stock-tank crude oil was centrifuged to remove paraffins that were solid at near-ambient temperatures. N-decane of 99+% purity was used and filtered through a silica gel column before use. The brines were equilibrated with chalk material and filtered before use. Other fluids were used as purchased. Surfactants were commercially available products. They were diluted with synthetic North Sea water to desired concentrations before use.