Modeling of Permeability-Dependent Retention of Low Concentrated HPAM Floods in Heterogeneous Reservoirs

Abstract

The polymer retention has been regarded as one of the major concerns in the operating process. The total polymer retention is attributed to both adsorption onto rock surface and entrapment within small pores. Traditionally, the retention has been modeled as only the adsorption by the Langmuir’s adsorption model, although macromolecules are retained in different locations within the pore space. Therefore, the model of polymer retention needs to be modified with respect to the pore size which is strongly related to the permeability. Based on the pore-level considerations, a theoretical polymer retention model is proposed that accounts for both the adsorption of polymer molecules on rock surfaces and the mechanical entrapment in pore matrices. The numerical simulations of the 1-D core were carried out to verify the independence of two mechanisms. By using the developed model, the trend and effects of polymer retention were compared to those of conventional model. The resistance factor in the two models showed the different trends in the two cases; the resistance factor in the low permeability case (31 md) calculated by the developed model was closer to that of experimental result. On the other hand, the results in the high permeability case (871 md) showed almost same in the both models. Compared with the conventional Langmuir’s adsorption model, the proposed model is more sensitive to the reservoir heterogeneity. Besides, extensive numerical simulations using the developed retention model have been performed to examine polymer propagation and flow behavior in the heterogeneous reservoir having same average permeability with different variances from 0.5 to 30. Most of polymer retention is accumulated in the low permeability zone, which causes a substantial increase of resistance factor in the area. After polymer flooding, total amount of polymer accumulated by retention in the reservoir with V=30 is 2.5 times more than that of homogeneous reservoir. Flow path of polymer solution is restricted to only high permeability zone in the highly heterogeneous reservoir. Thus, the polymer propagates through small pores at a lower rate, giving very poor sweep and irregular propagation path. Sweep efficiency of polymer flooding in the case of V=30 is reduced about 33% compared to the case of V=0.5. This paper could be helpful for an accurate assessment of polymer retention during polymer floods in the heterogeneous reservoirs. By using the developed model, the diagnosis of transport of polymer solution through the porous media could become more accurate and the uncertainty for the polymer injection design could be re-estimated.