Summary

Experimental methods are presented to evaluate and to measure the major contributions to surfactant and polymer losses in Berea sandstone for various oil saturations (no oil to postwaterflood residual oil). The results of linear displacement experiments are shown, and chemical losses evaluated for various sequences of injected fluids. Surfactant and polymer losses are evaluated independently, then for simultaneous injection into the core. When oil is present. chemical losses are related to phase behavior properties with salinity requirement diagrams. Both a synthetic white oil and a reservoir stock-tank crude oil are used in the experiments.

Introduction

Chemical loss in reservoir rocks is a major factor that limits the effectiveness of micellar/polymer systems for oil displacement. Excessive surfactant retention will result in adverse phase behavior properties in the porous medium, resulting in high interfacial tensions (IFT's) and retrapment of mobilized oil. Polymer loss decreases micellar and polymer slug viscosities, which may result in inadequate mobility control. For economic reasons, chemical systems that achieve good oil recovery with minimal amounts of surfactant and polymer injected must be developed. This implies low chemical retention. To design efficient micellar/polymer systems, a phenomenological understanding of mechanisms contributing to phenomenological understanding of mechanisms contributing to chemical losses is necessary. Several physics-chemical processes contribute to the overall loss of chemicals-such as (1) surfactant and polymer adsorption at the solid/liquid interface, (2) surfactant polymer adsorption at the solid/liquid interface, (2) surfactant precipitation by divalent ions, (3) surfactant trapping in precipitation by divalent ions, (3) surfactant trapping in the immobile hydrocarbon phase, (4) adverse surfactant/polymer interactions (SPI's) resulting in trapping of chemicals, and (5) mechanical retention of polymer molecules in the rock. Previous publications reported studies of some of the mechanisms responsible for surfactant or polymer retention in porous media. Few investigators have measured surfactant and polymer losses when both are injected concurrently into the rock. In this work, a comprehensive experimental study of surfactant and polymer retention in Berea sandstone is reported. Linear displacement tests were conducted in both oil-free and tertiary-oil-saturated cores. The most significant factors that contribute to chemical losses are identified for various brine salinities and oil saturations. With oil present, chemical losses are related to micellar system phase behavior with salinity requirement diagrams.

Strategy

Fig. 1 shows the experimental strategy. Chemical retention for a chosen surfactant/cosurfactant/polymer system (to be described later) was measured with fluid displacement experiments in Berea sandstone. Large chemical slugs (1 to 2 PV's) were injected, and isosalinity and salinity gradient systems were studied. In the isosalinity case, connote and chemical slug brines have the same total salinity and hardness. With the salinity gradient, connate brine salinity is higher than chemical slug salinity; this provides favorable in-situ phase behavior and results in greater oil recoveries than the no-gradient case. For a chosen salinity system (isosalinity or salinity gradient), six types of displacement tests were run. First, note that Path A in Fig. 1 shows that no oil was present in the rock. Surfactant retention (no polymer) was measured with experimental techniques to be described later (see Path al, Fig. 1). Polymer retention (no surfactant) was Path al, Fig. 1). Polymer retention (no surfactant) was determined from analysis of polymer concentration in the produced fluid cuts (see Path a2, Fig. 1). Then, produced fluid cuts (see Path a2, Fig. 1). Then, surfactant and polymer retention were evaluated when both were present in the injected chemical slug. present in the injected chemical slug. The same sequence of experiments was conducted with oil present in the core. For the surfactant-only and the surfactant/polymer cases, chemical retention was related to phase behavior characteristics of the fluid systems with salinity requirement diagrams. Produced fluid cuts (0.1 PV) were analyzed for surfactant, polymer, hardness, and chloride for all cases (Paths A and B). Alcohol cosurfactant concentrations were also measured in some cases.

Experimental Procedure

Materials. Formulation 20 is the surfactant/cosurfactant system used in all the experiments. It is composed of Petrostep S465/Petrostep S420/isopropyl alcohol (IPA)/ Petrostep S465/Petrostep S420/isopropyl alcohol (IPA)/ tertiary butyl alcohol (TBA) (46.62/19.98/16.70/16.70 wt%) in brine. The polymer was Cyanatrol 930(S), a polyacrylamide polymer (manufactured by American Cyanamid). It is a polymer (manufactured by American Cyanamid). It is a liquid emulsion polymer (30% polymer content) with an estimated molecular weight of 5 million. The brines were dilutions of a synthetic field brine (SFB) with deionized water. Table 1 describes the full-strength SFB composition.

SPERE

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