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Polymer retention occurring in porous media induces significant mobility reduction as well as poor sweep efficiency. In the heterogeneous reservoir, it is very complicated and difficult to analyze performance of polymer due to permeability-dependent retention. Among various mechanisms of retention, mechanical entrapment of polymer affects mainly the propagation of injected polymer solution by forming macromolecular polymer plugging into pore throats, especially occurring in low permeability zone. Besides, polymer adsorption contributes to mobility reduction and polymer retention. The influence of heterogeneity on polymer retention has been not investigated accurately in the reservoir scale and limited to experimental studies, even though both mechanisms are highly dependent on permeability. In this paper, extensive numerical simulations incorporating mathematical modeling on the retention mechanisms have been performed to examine polymer propagation and flow behavior at various heterogeneous reservoir models. All reservoir models have same average permeability and different Dykstra-Parsons' coefficients (V DP ) to represent wide range of reservoir heterogeneity.The results show the amount of polymer adsorption is at high level in low permeability zone. The mechanical entrapment also exhibits same trend with adsorption. Most of polymer retention is accumulated in the low permeability zone, which causes a substantial increase resistance factor in the area. After polymer flooding, total amount of polymer accumulated by retention in the reservoir with V DP =0.72 is 2.5 times more than that of homogeneous reservoir. Furthermore, 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 DP =0.72 is reduced about 33% compared to the case of V DP =0.14. This paper could be helpful for an accurate assessment of polymer retention during polymer floods in the heterogeneous reservoirs.

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Significant injectivity loss during polymer injection measured particularly in the near wellbore has been reported. This challengeable issue is identified as bridging polymer adsorption caused by the bridging of pore throats via macromolecular polymers previously stretched under elongational flow conditions occurring in the vicinity of the injection well. There has been no attempt to describe this phenomenon by numerical simulation model because the conventional Langmuir isotherm widely used in reservoir simulation is not able to contain this bridging-adsorption observed in complicated polymer flooding experiments. This study focuses on the development of numerical model for the bridging adsorption by implementing population balance theory and performs extensive simulation verifications with small core-scale reservoir rock. To reflect distinct flow condition to induce bridging adsorption, the rate of bridging adsorption is established by considering the relationship of expanded polymer under shear force and narrow pore size. To verify the feasibility of new model, simulation results are compared with experiment output reported in previous studies. The simulation results indicate that a considerable amount of bridged polymer can be generated in the low permeability cells only if the polymer solution is exposed to high shear velocity related with shear rate. This is in accordance with a number of previous experimental reports. In addition, the mechanism to induce permeability reduction is totally different from that of conventional Langmuir's isotherm which is widely incorporated in commercial simulators. With buildup of bridging-polymer, the adsorption model can enable the application of numerical simulation targeted at chemical EOR process to be wider.

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Polymer retention is one of the most important factors to govern polymer propagation through porous media, determining whether successful or not. The focus of previous studies has been limited to polymer concentration loss caused by the retention; not only change in polymer concentration, but also reduction in reservoir permeability is the main issue for theoretical transport study. Due to the lack of accuracy of Langmuir isotherm describing the polymer retention mechanisms, this study proposes a new type of matching interpretation method to correlate the permeability reduction factors from experiments to permeability. In order to solve the problem of poorly matching results between estimation and observation, use of nonadsorptive constant conditionally selected in matching process was made. Based on the threshold permeability reduction factors, approximate critical permeability can be calculated to which nonadsorptive constant would be applied. Results showed significant improvements in the estimation of permeability reduction for both low and high permeability cores. In addition, effects of permeability reduction on polymer transport in field scale were analyzed using the proposed matching model. Thus, not only does this interpretation method help to evaluate prediction for accurate flow behavior, but also unwanted risk can be evaluated.