Numerical and experimental investigation of polymer-induced resistance to flow in reservoirs undergoing a chemical flood

Polymers are commonly used in chemical EOR flooding to provide mobility control to the injected fluid slugs. Water-soluble polymer injection in porous media usually results in polymer entrapment in the subsurface formation from adsorption, mechanical trapping, and hydrodynamic retention. These different entrapment mechanisms lead to permeability reduction (R_k). The term resistance factor (R_F) is used to measure the impact of permeability reduction and viscosity enhancement from the polymer. The residual resistance factor (R_RF) represents the residual permeability reduction to chase fluids (usually water) following a polymer flood. These phenomena are more prevalent in high molecular weight polymers. The effects of R_RF are also sometimes evident when polymer injection is suspended (e.g., operational issues) and other fluids are injected in the formation instead. Post-polymer chase water injection is considered a viable option to improve economics of a polymer flood. Current models validate this optionality because they overestimate the efficiency of the chase water flood. Lab and field observations indicate that chase water after chemical flood can cause rapid breakthrough, and therefore it is generally not recommended. The shortfall lies in the assumption that permeability reduction during polymer flood is an irreversible mechanism and therefore the existing simulation models assume that the permeability reduction during and post-polymer flood is the same. In this work, we show that the permeability reduction mechanism can be somewhat reversible, and therefore the current models underestimate chase water injectivity, and more seriously can overestimate the efficiency of chase water displacement. For optimizing the slug size, over predicting chase water displacement efficiency has a critical impact on the project economics and it may wrongly indicate premature switching time from chemical to water injection. We provide an alternative permeability reduction model that does not assume irreversibility. The new model decouples permeability reduction during polymer flood from R_RF during chase water flood. We validate the model using experimental data. Several test cases are provided to compare the two models and we finally demonstrate the applicability of the proposed model in an onshore polymer flood pilot. The new model also highlights the seriousness of the inefficiency of the post-polymer chase water injection.