Recovery Of High-Viscosity Crudes By Polymer Flooding

Abstract

Abstract High-molecular-weight, water-soluble polyacrylamides were evaluated for use in the recovery of low-gravity crude oil by polymer flooding. Performance comparisons were made between a high-molecular-weight commercial polymer and two developmental materials higher in molecular weight than any commercial product. Sandpack floods using viscous (220 cp and 1140 cp) oils indicated that the highest-molecular-weight material was superior for oil recovery by polymer flooding. Based on these data improved recovery performance should be possible with the introduction of higher-molecular-weight products into the marketplace. In comparable flooding experiments, mobility -ratios using the highest-molecular-weight polymer were much lower than those of the other two polymers. Complete polymer retention or denudation of the leading edge of the injected polymer bank was observed with this material; this effect was not observed with other polymers. Denudation provides insight into the mobility reduction mechanism and, depending on flood design, it may either improve or reduce oil recovery. Forward loading of the polymer slug is discussed as a means to reduce denudation. Denudation is useful in explaining differences in polymer characteristics. The highest-molecular-weight material apparently exhibits a -narrower molecular-weight distribution than the other polymers. All molecules from this product were effective in the reduction of sandpack permeability, whereas only a small fraction of the molecules from the other polymers were useful in this regard. Introduction SUBSTANTIAL RESERVES of low-gravity crudes with relatively high viscosity are known to exist throughout the world. Of particular interest are North American reserves because of proximity to refineries, strong demand and a desire to improve self-sufficiency in petroleum. The U.S. Bureau of Mines places domestic low-gravity, in-place crude reserves at over 100 billion stock tank barrels(2,3). This estimate represents oil with gravity less than 25 °API which is mobile at reservoir conditions as evidenced by some commercial production. Less than 5% of this oil is classified as recoverable using current technology. Extensive low-gravity crude reserves are found in Canada, particularly in Alberta and Saskatchewan. Distribution of such reservoirs in the United States is fairly broad, but many fields are concentrated in Texas, California, Kansas and Wyoming. Most efforts directed toward recovery of low-gravity crude involve production stimulation via thermal processes. However, polymer flooding may have economic potential in this regard. Husky Oil, Ltd. has included flooding with a polymer among several programs for heavy crude recovery from reservoirs in the Lloydminster area of Saskatchewan.(3,4) Standard Oil Company of California has reported that polymer flooding has proved effective in substantially increasing oil recovery (13.2 °API crud) in the Huntington Beach field near Los Angeles.(3) High-molecular-weight, water-soluble polymers manufactured by various chemical companies have been evaluated for many years at Marathon Oil Company's Denver Research Center. The goal of polymer evaluation has been to find better and more economical chemicals for use in enhanced recovery processes. High-molecular-weight polyacrylamides appear more attractive than other polymers for recovery of heavy oil by polymer flooding. Reduction of water mobility with polyacrylainides is attained primarily by permeability reduction.