Dependence of Polymer Retention on Flow Rate

Abstract

The efficient use of aqueous polymer solutions as mobility control agents demands an understanding of the mechanisms of mobility reduction. Although much work has been directed toward this goal, there have been very few physical insights or conclusive explanations. This paper presents experimental results that imply a reasonably consistent physical interpretation and demonstrate that the amount of polymer retained increases with flow rate. polymer retained increases with flow rate. In Fig. 1, effluent concentration and mobility reduction profiles are plotted vs pore volumes injected for a 500-ppm polysaccharide solution (XC biopolymer, Xanco, Div. of Kelco Co.) with 2 percent NaCl in a 121-md Berea core 1 in. in diameter and 6 in. long. Effluent concentration fractions were determined by measuring efflux times for effluent and inlet samples in a constant-head capillary viscometer. At Position A, the flow was stopped for 16 hours and then resumed at the same pressure drop. This resulted in sharp increases in effluent concentration and mobility reduction relative to previous steady-state conditions. The result can be interpreted as follows. Under a positive pressure gradient, some polymer molecules become trapped and deformed polymer molecules become trapped and deformed within the porous structure. Cessation of flow eliminates hydrodynamic drag and permits the molecules to assume relaxed, random-coil configurations. This facilitates migration to larger flow channels and permits transport when flow is resumed. Further, if flow permits transport when flow is resumed. Further, if flow is stopped for a sufficiently long time, concentration gradients favor the diffusion of polymer molecules into less constricted regions of the porous matrix. When flow is resumed, the resulting increase in the concentration of flowing polymer increases viscosity and, hence, mobility reduction. Permeability may also increase, but evidently this effect is overwhelmed by the attendant viscosity increase. Subsequently, polymer trapping recurs and decreases the effluent concentration below the steady-state value. This, in turn, lowers the in-situ solution viscosity and also the mobility reduction. When all trapping sites are once again saturated, the system returns to its initial steady state. Positions B and C indicate points at which the pressure drop across the core was increased without pressure drop across the core was increased without interrupting the flood. In these cases additional polymer is immediately retained, lowering both the polymer is immediately retained, lowering both the effluent concentration and the mobility reduction. The minima and asymptotic approaches to steady state with continued injection are as described above, except that a lower equilibrium mobility reduction results for each increase in pressure drop. This can be attributed to lower polymer solution viscosities at high shear rates (pseudoplastic, non-Newtonian behavior). Here again, the possible lower permeability caused by added polymer retention, which permeability caused by added polymer retention, which opposes the effect of viscosity on mobility reduction, is dominated by the viscosity contribution to the mobility reduction. These results are consistent with data reported by Desremaux et al. Similar behavior has been observed for polyacrylamide solutions (Pusher 700, Dow Chemical U.S.A.) polyacrylamide solutions (Pusher 700, Dow Chemical U.S.A.) in Berea sandstone in a different type of experiment. A 6-in. core, which had been saturated with a filtered 2,500-ppm solution of polyacrylamide in 2 percent NaCl, was flushed with 2 percent NaCl at a constant pressure drop of 20 psi until the flow rate stabilized pressure drop of 20 psi until the flow rate stabilized and a residual permeability was determined. Without interrupting the flow, the pressure was lowered to 15 psi. A detectable quantity of polymer was found in psi. A detectable quantity of polymer was found in the effluent by monitoring capillary viscometer efflux times. Furthermore, the stabilized residual permeability reduction, which is the ratio of brine permeability reduction, which is the ratio of brine permeability before the addition of polymer to brine permeability before the addition of polymer to brine permeability after the addition of polymer, dropped 25 permeability after the addition of polymer, dropped 25 percent. These findings were repeated after another percent. These findings were repeated after another reduction in pressure drop. Further increases in permeability were caused by eliminating the pressure permeability were caused by eliminating the pressure gradient for periods as short as 10 minutes and then resuming flow. However, the presence of small quantities of polymer in the effluent ceased to be detectable. The residual permeability reduction decreased from its first stabilized value of 19.4 to 3.3 after 146 PV of brine had been injected with intervening PV of brine had been injected with intervening periods of no flow. There was no indication that the periods of no flow. There was no indication that the trend would change when experimentation was suspended at this point. It must be concluded that solutions of polysaccharide and polyacrylamide will lose more molecules polysaccharide and polyacrylamide will lose more molecules through interaction with porous rock at larger flow rates, and the interaction is somewhat reversible. P. 1307