Mechanistic Interpretation and Utilization of Viscoelastic Behavior of Polymer Solutions for Improved Polymer-Flood Efficiency

Abstract

There are increasing laboratory and field evidences that the viscoelastic characteristics of polymer solutions help improve polymer-flood efficiency. Extensive rheological measurements and laboratory corefloods with partially hydrolyzed polyacrylamide polymers with very high molecular weight were carried out to delineate the role of their viscoelastic behavior in improving oil recovery from polymer flood. Different polymer solution's elastic contribution is modeled in the polymer's apparent viscosity in porous media, which is implemented in UTCHEM simulator for quantification of improved reservoir sweep. As the application range of polymer flood is extended to recover more viscous oils with use of polymers at high concentrations and with very high molecular weights, a mechanistic understanding of polymer rheology in porous media and accurate numerical modeling are essential for successful field implementation of polymer flood. Oscillatory and shear viscosity measurements and polymer flow coreflood experiments were carried out for different shear rates (and flow velocities and permeabilities in core), polymer concentrations, and molecular weights. The polymer's shear-thickening characteristic was correlated with the Deborah number via its molecular relaxation time, which is in turn determined from the rheological data. An apparent viscosity model that accounts for both shear-thinning and shear-thickening behavior of polymer in porous media was developed, which fit the laboratory data well. The model was then implemented in a compositional chemical flooding simulator and successfully history-matched published coreflood oil recovery experiment. Through systematic rheological measurements and corefloods, and their use in the apparent viscosity model for simulation, the elastic contribution of different polymers in improving polymer-flood efficiency is quantified. Specifically, a polymer solution's shear-thickening behavior is characterized in terms of the molecular relaxation time determined from bulk rheology measurements. Introduction For a majority of oil reservoirs, large amounts of oil are still left unrecovered even after extensive waterflooding, due to reservoir heterogeneity. Once water channels are formed through high-permeability zones, the water subsequently injected simply bypasses the oil stranded in the low-permeability zones. To recover the bypassed oil, a low-concentration polymer solution that viscosifies water is injected to improve the reservoir sweep efficiency. An extensive research has been carried out for more than 50 years to develop and refine the "polymer flooding?? process (see e.g., Sorbie, 1991). Polymer flooding is now considered to be a technically and commercially proven improved oil recovery process, especially since its large-scale application at Daqing field in northern China where about 300,000 barrels of incremental oil per day is attributed to polymer flooding (Wang et al., 2001). Like applications elsewhere, a partially hydrolyzed polyacrylamide (HPAM) is employed, which is available at low cost worldwide.