USE OF POLYMERS TO RECOVER VISCOUS OIL FROM UNCONVENTIONAL RESERVOIRS

Abstract

This final technical progress report summarizes work performed the project, 'Use of Polymers to Recover Viscous Oil from Unconventional Reservoirs.' The objective of this three-year research project was to develop methods using water soluble polymers to recover viscous oil from unconventional reservoirs (i.e., on Alaska's North Slope). The project had three technical tasks. First, limits were re-examined and redefined for where polymer flooding technology can be applied with respect to unfavorable displacements. Second, we tested existing and new polymers for effective polymer flooding of viscous oil, and we tested newly proposed mechanisms for oil displacement by polymer solutions. Third, we examined novel methods of using polymer gels to improve sweep efficiency during recovery of unconventional viscous oil. This report details work performed during the project. First, using fractional flow calculations, we examined the potential of polymer flooding for recovering viscous oils when the polymer is able to reduce the residual oil saturation to a value less than that of a waterflood. Second, we extensively investigated the rheology in porous media for a new hydrophobic associative polymer. Third, using simulation and analytical studies, we compared oil recovery efficiency for polymer flooding versus in-depth profile modification (i.e., 'Bright Water') as a function of (1) permeability contrast, (2) relative zone thickness, (3) oil viscosity, (4) polymer solution viscosity, (5) polymer or blocking-agent bank size, and (6) relative costs for polymer versus blocking agent. Fourth, we experimentally established how much polymer flooding can reduce the residual oil saturation in an oil-wet core that is saturated with viscous North Slope crude. Finally, an experimental study compared mechanical degradation of an associative polymer with that of a partially hydrolyzed polyacrylamide. Detailed results from the first two years of the project may be found in our first and second annual reports. Our latest research results, along with detailed documentation of our past work, can be found on our web site at http://baervan.nmt.edu/randy/. As an overall summary of important findings for the project, polymer flooding has tremendous potential for enhanced recovery of viscous oil. Fear of substantial injectivity reduction was a primary hurdle that limited application of polymer flooding. However, that concern is largely mitigated by (1) use of horizontal wells and (2) judicious injection above the formation parting pressure. Field cases now exist where 200-300-cp polymer solutions are injected without significant reductions in injectivity. Concern about costs associated with injection of viscous polymer solutions was a second major hurdle. However, that concern is reduced substantially by realization that polymer viscosity increases approximately with the square of polymer concentration. Viscosity can be doubled with only a 40% increase in polymer concentration. Up to a readily definable point, increases in viscosity of the injected polymer solution are directly related to increases in sweep efficiency and oil recovery. Previously published simulation results - suggesting that shear-thinning polymer solutions were detrimental to sweep efficiency - were shown to be unfounded (both theoretically and experimentally).