Comparison Of Oleic And Aqueous Micellar Processes For Tertiary Oil Recovery

Abstract

Abstract A comparison is made of the oil recovery efficiency obtained by injecting surface active agents in an oil phase vehicle vs. injecting these agents in an aqueous phase vehicle. Mobilities and mobility ratio of the flowing phases, sensitivity of the micellar slug and phases, sensitivity of the micellar slug and polymer solution to reservoir brine, mechanism polymer solution to reservoir brine, mechanism of oil bank formation, slug sizes used for each process, and adsorption of surfactant from the process, and adsorption of surfactant from the micellar slugs are considered in this analysis. Also discussed are 1) the importance of phase relationships between the crude oil, brine and injected surfactant-solvent on process performance, and 2) the effect on oil recovery efficiency of preflushing the rock prior to a micellar-polymer preflushing the rock prior to a micellar-polymer flood. Introduction The Miscellar-polymer processes used to recover oil from watered-out reservoirs can be classified under two primary categories. In one, the surface active agents are injected primarily in an oil phase vehicle, and in the other, an aqueous phase vehicle is used. The compositions of the slugs used in the two processes are shown on the generalized phase diagram (FIGURE 1). A given oil, water, and surfactant system can produce aqueous, water-external, oil-external, or bicontinuous phase, micellar solutions. However, there are distinct differences in the oil displacement performance of the solutions; these difference are performance of the solutions; these difference are described below. MICELLAR SLUG Mobility FIGURE 1 shows the viscosities of typical oil-in-water and water-in-oil microemulsions, and aqueous micellar solutions, as a function of oil and water content. The soluble oils, or oil-external systems, are characterized by the presence of enough oil to maintain a high viscosity over a rang of water content from 0 to 70%. Consequently, the micellar slug used in these processes has a viscosity sufficiently high to immediately create, and to maintain, a favorable mobility ratio between slug and the low mobility, oil-water bank which it mobilizes in the reservoir. This favorable mobility ratio minimizes dispersion of the slug. By contrast, the aqueous micellar solutions, as injected, contain less than 5% oil and have low viscosities. In properly designed aqueous slugs, the eventual formation of microemulsions, which occurs in-situ with the residual oil in the reservoir, tends to increase the viscosity of the micellar slug. However, the mobility ratio that prevails during at least the initial stages of the flood is unfavorable, and a portion of the micellar slug tends to bypass the oil bank which may be formed by the slug. Even in linear laboratory systems the lack of initial mobility control appears to be reflected in lower oil recovery efficiency as shown by comparison of the first two floods listed on TABLE 1. Flood No. 2 was identical to Flood No. 1 except that the oil was left out of the micellar slug causing the aqueous slug to have a viscosity of only 5 cp. A larger decrease in oil recovery efficiency is likely to occur in the reservoir due to lack of mobility control because of unbounded radial flow and permeability variations. Aqueous micellar systems often rely upon the addition of polymer to the slug to attain the proper initial slug viscosity. However, the presence of polymer in a slug has the negative effect of making the phase relationships with oil and water less favorable (larger two-phase area) and thereby decreasing oil recovery efficiency. FIGURE 2 shows the ternary phase relationships for oil-surfactant-brine systems with and without polymer present in the brine. As shown in this figure, the addition of polymer causes a large increase in the two-phase area where appreciable oil is present. TABLE 1 presents data (Floods 2–5) which show the adverse effects on oil recovery of adding polyacrylamide polymer (Cyanatrol 9SOS) to two different aqueous micellar slugs. The economics of using polymer rather than oil in the slug are not a simple trade-off.