Separation of Oil and Water Produced by Micellar-Solution/ Polymer Flooding

Abstract

Summary The phase behavior of produced fluids from a micellar/polymer project is dominated by produced sulfonate equivalent-weight distribution, total sulfonate production, and aqueous-phase salt concentration and type. Produced fluids at Marathon Oil Co.'s 219-R Project showed evidence of having passed through a salinity gradient created by reservoir brine at the leading edge of the displacement and fresh polymer water behind the micellar solution. During early production, when aqueousphase salt concentration was relatively high, highequivalent-weight sulfonates were permanently entrained in produced oil. Significant amounts of water also remained. As the salt content of produced water declined, high-equivalent-weight sulfonates moved to middle and aqueous phases. The middle and aqueous phases carried significant quantities of oil during these periods. All three problems-water in oil, oil in the middle phase, and oil in water-were corrected by treatment with demulsifying chemicals that rendered all sulfonates highly water- soluble. Water-soluble amines and alcohols were effective. Because of large quantities of sulfonate production and resulting low oil/water tensions, extended retention times were needed in separation vessels. In the absence of adequate retention (highest sulfonate production), a freshwater wash of the oil with an appropriate demulsifying chemical after initial oil/water separation removed the remaining sulfonate (and water) from the oil. All production from the 219-R Project was successfully treated and sold with strict quality control. Data from laboratory corefloods pertinent to the characterization of produced-fluid phase behavior are presented. Introduction Vinatieri has described emulsions that can be produced as a result of micellar-solution/polymer flooding. Characteristics of these emulsions depended on salinity of the aqueous phase when significant amounts of sulfonate were present. Nelson and Pope have presented detailed descriptions of phase behavior of fluids produced from laboratory corefloods. Given the proper salinity gradient, ". . high-salinity formation brine, moderate-salinity chemical slug, and low-salinity drive. . . "produced fluids exhibited the phase behavior noted by others when sulfonate-containing microemulsions are equilibrated with brine and oil. Phase behavior of produced fluids observed by Nelson and Pope included (1) one phase (brine), (2) two phases (oil and brine), (3) two phases (upperphase microemulsion and brine), (4) three phases (oil, middle-phase microemulsion, and brine), and (5) two phases (oil and lower-phase microemulsion). This sequence of phase behavior in produced fluids resulted from injection of a small (10%PV) chemical slug of moderate salinity followed by low-salinity polymer solution. The foregoing indicates that emulsion problems may occur in a field project. At Marathon Oil Co.'s 219-R Project, crude-oil sulfonate, p-amyl alcohol, and n-butanol were used to formulate the displacing micellar solution. Dow Chemical Pusher Tm 700 was used in the mobility buffer. Appearance of sulfonate in produced water coincided with increased oil production. As sulfonate and oil production increased, water carry-over in oil and oil carryover in water at the initial point of oil/water separation became excessive. Significant amounts of emulsion formed in chem-electric heater treaters, and the water content of oil could not be reduced to pipeline quality. These problems persisted in the presence of excessive demulsifying chemical. Upper-phase microemulsion produced at the 219-R Project was successfully upgraded (enough sulfonate was removed) to meet pipeline and refinery standards by proper application of a treatment chemical to make sulfonates more water-soluble and by introduction of a freshwater wash. The oil in lower-phase microemulsion was separated by extended retention and treatment with the same chemical. Middle-phase microemulsion was collected in a separate system. Oil was recovered from these emulsions by treatment with fresh water, caustic treating, chemical, and heat. All oil, whether in upper-, middle-, or lower-phase microemulsion, was recovered and sold. Laboratory Data The phase behavior of micellar solutions with reservoir brine and oil has been represented in pseudoternary diagrams. When surfactant, oil, and brine are used as pseudocomponents, distinct regions appear in ternary representation. The regions may include single- and multiple-phase areas. Compositions that are in the single-phase region are microemulsions that are undersaturated with respect to both brine and oil. JPT P. 1459^