Direct Thickeners for Mobility Control of CO2 Floods

Heller, J.P.; SPE, New Mexico Petroleum Recovery Research Center Dandge, D.K.; New Mexico Petroleum Recovery Research Center Card, R.J.; American Cyanamid Corp. Donaruma, L.G.; Polytechnical Inst. of New York Polytechnical Inst. of New York October 1985 Abstract This paper describes efforts in an experimental search for polymers that are sufficiently soluble in dense CO2 that polymers that are sufficiently soluble in dense CO2 that they could serve as mobility control agents. The operation of the apparatus designed and built for the measurement of solubility in condensed gases is described. A modified version of this apparatus has been used to measure viscosity by timing the fall of a cylinder in a tube. More than a dozen polymers have been found that are soluble at least in the parts-per-thousand (ppt) range in liquid and in dense supercritical CO2. As pressures and temperatures are varied. the solubilities of these polymers generally are found to increase with increasing CO2 density. Certain generalizations have been made concerning the influence of various polymer properties on their solubility in dense CO2. These properties include structure, stereochemistry, and molecular weight. Although the viscosity enhancements of the solutions measured thus far are insufficient for purposes of mobility control, they provide clues that point toward those features of polymer provide clues that point toward those features of polymer molecules that yield greater thickening properties. Also discussed are considerations involved in the application of direct thickeners in the mobility control of CO2 floods and the advantages in the use of such CO2-soluble polymers in place of methods that involve the injection of water. Introduction There is a considerable degree of optimism about the usefulness of CO2 as a displacing agent in EOR operations. At the moderately high pressures and reasonable temperatures common in many oil reservoirs, CO2 is capable of extracting enough of the light ends, in the region of its contact with the crude oil. that a highly efficient displacement in porous media is possible in the laboratory, especially in thin-tube tests. This multiple-contact miscibility, which has been observed in many laboratories, occurs above a threshold minimal miscibility pressure. The higher density attained by CO2 in this pressure. The higher density attained by CO2 in this range has been specifically referred to by Holm and Josendal and by Orr et al. The effectiveness of CO2 in displacing oil from reservoirs is marred, however, by its extremely low viscosity. The viscosity of dense CO2 remains low (in the range from 0.03 to 0.08 cp [0.03 to 0.08 mPas]) despite its relatively high density (above 0.45 g/cm 3 ) under reservoir conditions. Thus, the viscosity of CO2 is lower by more than an order of magnitude than that of either crude oil or the brine occupying the remainder of the pore space of the reservoir rock. The resulting high mobility ratio leads to severe instability of the frontal region and significantly degrades the macroscopic efficiency of the displacement process. Some method of mobility control is required for efficient use of CO2 to increase greatly the quantity of producible oil. A promising approach to the goal of decreasing the mobility of CO2, well-explored in the laboratory and close to field trial, has been the use of foam. Such a composite fluid in which CO2 is used in conjunction with an aqueous surfactant solution is considerably less mobile in a porous medium than CO2 alone. The work was reviewed and some results were presented. 10 This work, however, pursues a different means to decrease the mobility of the CO2 displacement fluid. A direct thickener, a soluble polymer that sufficiently increases the viscosity of dense CO2, would be superior to a foam-like dispersion in two important respects that arise from the fact that a directly thickened CO2 could be injected without water. Therefore, there would be less trapped oil caused by increased water saturation, and a higher displacement efficiency could be attained. Furthermore, without injected water, problems resulting from corrosion would not be as severe as they have been in other CO2 projects. Although polymers are used in waterfloods to control mobility ratio, no data are available on similar use of such agents in CO2 floods. In fact, except for a preliminary report of this study, no search for viscosity-increasing polymers soluble in liquid or supercritical CO2 has been polymers soluble in liquid or supercritical CO2 has been reported in the literature. Francis has written two classic, general papers on solubility of simple organic and inorganic compounds in dense CO2. These give mutual solubilities of CO2 with more than 250 substances in various two- and three-component mixtures. Recently, Stahl et al. have described a method of microanalytical evaluation of the dissolving power of supercritical gases. Lundberg and Ali have been working on gas/polymer solutions at high temperatures and pressures. These researchers, however, seek low-viscosity solutions of polymers in dense gases like CO2, butane, propane, and ethane. SPEJ P. 679