Sweep Impairment Due to Polymers Shear Thinning

Abstract

Abstract Most polymers used in EOR exhibit shear thinning behavior. At least theoretically, shear thickening will improve sweep while shear thinning (pseudoplasticity) will impair it through exacerbating the velocity contrast and/or inducing instability. Despite this, the effect of pseudoplasticity on sweep has not been studied in detail. An in-house streamline simulator has been extended to handle polymer flooding with Newtonian and non-Newtonian behavior. Three main modifications were implemented. First, the polymer mass balance was solved along the streamlines. Second, a polymer multiplier was defined to account for polymers' viscosifying and thinning effects. Finally, an iterative approach was implemented to solve the pressure field. This is needed since the pressure depends on the aqueous phase viscosity, which for non-Newtonian fluids depends on shear stress, and hence the pressure itself. The simulator was then used to investigate pseudoplasticity effects on sweep and recovery in various reservoir models. Two cases were run. The first had a stable connate bank; hence, thinning does not induce instability and the only effect is velocity exacerbation. The second had an unstable connate bank; hence, thinning can induce instability. The results of this work prove the importance of taking polymers' non-Newtonian behavior into account for the successful design and evaluation of polymer flooding projects. This is because pseudoplasticity will impair sweep, which can deteriorate the whole economic picture of a polymer flood. Even if instability is not induced, more pore volumes will be needed, more water will be produced, and in light of a limiting water cut, less oil will be recovered; in other words, higher operations costs, higher processing costs, but less profit. Simulations carried in a 2D heterogeneous model suggest that for unconditionally stable flooding, the injection requirement will double from 2 to 4 pore volumes. In addition, in light of an 80% limiting water cut, a thinning fluid would decrease recovery by 2% of OOIP compared to a Newtonian fluid. Furthermore, if thinning does induce instability, shear thinning will reduce ultimate recovery. Simulations carried in a 2D heterogeneous model suggest that a pseudoplastic fluid would decrease recovery by 5% of OOIP compared to a Newtonian fluid. Introduction Polymer flooding is one of the most mature enhanced oil recovery (EOR) techniques. Its maturity partially stems from its ease of implementation especially where water flooding is undertaken. In polymer flooding, polymers are added to viscosify the injected water, yielding a more favorable mobility, hence a more stable displacement and a sharper front (Lake, 1989; Littmann, 1988; Sorbie, 1991). The main polymers used for this purpose: polysaccharide and polyacrylamide, are shear thinning (Lake, 1989). Although such thinning behavior is desirable from an injectivity standpoint, it is undesirable in terms of sweep and recovery—especially in heterogeneous media. Despite thinning sweep impairment and despite the maturity of polymer flooding, shear thinning effects on sweep and recovery have been almost ignored. Therefore, a real sense of the magnitude and significance of shear thinning effect on recovery has not been thoroughly investigated nor appreciated. Actually, some workers suggest just the opposite! For instance, Martel et al. (1998) state "The use of polymer solutions showing shear thinning behavior can be greatly beneficial for sweep efficiency and mobility control." In this work, a simulation study is carried to investigate the effect of non-Newtonian rheology on sweep and recovery. For this purpose, a streamline simulator has been modified to model polymer flooding with Newtonian and non-Newtonian rheology. After validating the code against analytical solutions and solutions published in the literature, the simulator was used to investigate non-Newtonian flooding performance.