Polymer Flooding for Extra-Heavy Oil: New Insights on the Key Polymer Transport Properties in Porous Media

Primary cold production in extra-heavy oil reservoirs (7-12° API) is currently a low percent of the OOIP, as for some oil fields in La Faja Petrolífera del Orinoco (FPO), Venezuela. EOR studies are being conducted in order to increase recovery factors primarily in those thin bedded reservoirs which host up to 35% of the OOIP. Thermal EOR is usually the first option implemented to increase recovery. However, thermal methods are not suitable for thin pay zones and involve high water consumption which can create environmental issues. For these reasons, chemical EOR is becoming a feasible tool for mobility control and mobilization of residual oil. Polymer flooding applied to extra-heavy oil may require rather high concentration polymer solutions. The objective of this study is to contribute to a better understanding of the specific mechanisms involved in this context. Transport properties of polymer in porous media are investigated through a set of corefloods tests performed on model sandpacks, the high permeability (4 Darcy) and the temperature (50°C) being representative of the conditions of the FPO. Polymer transport is analyzed versus injection concentration in terms of the key petrophysical parameters controlling the performance of a polymer flooding operation, namely: retention and inaccessible pore volume, in-depth propagation and injectivity. Results show that all parameters strongly depend on polymer concentration. Specifically, the concentration dependence of polymer retention, inaccessible pore volume and in-depth propagation behavior cannot be interpreted by conventional models. Injectivity is also strongly affected by concentration: both rheo-thinning and rheo-thickening effects become more marked when concentration increases. This set of experimental data are the key for modeling the transport of high concentration polymer solutions in porous media. They also provide direct information needed, through reservoir simulation, to sanction the technical and economical feasibility of polymer flooding for extra-heavy oil.