Novel Insights on the Transport of HPAM Solutions in Low Permeability Porous Media: Impacts of Brine and Reservoir Properties

International audienceChemical EOR is now considered as an attractive option for low permeability reservoirs, in particular where lack of gas supply does not allow gas injection processes. However, its application can be challenging for permeabilities below 100 mD as poor injectivity and high chemical retention are frequently observed. This work aimed at investigating the impact of both chemical and mineralogical parameters on the transport of polymer solutions in well-controlled low permeability porous media. Selected polyacrylamide (HPAM) solubilized in brines of variable strengths and hardnesses were injected in granular sand and clays packs having similar petrophysical characteristics (permeability around 60-80 mD) but variable and well controlled mineralogical compositions. The granular packs were characterized in terms of structure (SEM) and specific surface area (BET) before and after polymer injections. The main observables of the coreflood tests were the resistance and residual resistance factors generated by the polymer, the polymer inaccessible pore volume and its irreversible retention. Viscometric analysis showed that the HPAM solutions intrinsic viscosity decreased with increasing total salinity, as expected from charge screening, with a sharp decrease in presence of divalent cations, even at low ionic strength, which was less expected. Coreflood experiments revealed that polymer retention, resistance factor and irreversible resistance factor increased significantly: (a) with increasing ionic strength and hardness for porous media of a given mineralogical composition (this appeared consistent with the outcomes of the viscometric study and confirmed the major impact of hardness); (b) in presence of clays, even at low ionic strength and hardness. The polymer inaccessible pore volume was significantly impacted by the presence of clays, but not by the brine composition. Assuming that polymer retention originated in polymer adsorption, irreversible resistance factors were translated into adsorbed layer thicknesses according to a simple capillary bundle model. This allowed discussing the results in terms of adsorbed layer density, which was showed to increase if brine hardness was increased and to be lower in presence of illite than kaolinite and pure quartz. These findings indicate that complex polymer adsorption/retention mechanisms occur depending on the clay type (layer charge and expandability). This systematic study allowed dissociating the impacts of salinity, hardness and clay contents/types on the transport properties of polymer solutions in low permeability porous media. Its results should be of interest to the chemical EOR industry as they provide guides to help tuning the injection brine composition and polymer concentration to the reservoir properties