Transport of HPAM Solutions in low Permeability Porous Media: Impacts of Salinity and Clay Content

Abstract

Chemical 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 in these cases. 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. The intrinsic viscosity and hydrodynamic size of partially hydrolyzed polyacrylamide (HPAM) solubilized in brines of variable ionic strengths and hardnesses were firstly investigated. Polymer injection corefloods were then conducted using granular packs (sand and clays mixtures) with similar petrophysical characteristics (permeability 60-80 mD) but having several controlled mineralogical compositions. The granular packs were especially characterized in terms of structure (SEM) and specific surface area (BET) before and after polymer injections. The main observables from the coreflood tests were the resistance and residual resistance factors generated by the polymer, the polymer inaccessible pore volume and its irreversible retention. Homogenous and reproducible granular packs were successfully prepared thanks to a dedicated methodology and using different ratios of quartz and clays (kaolinite and illite separately). Results from the viscometric analysis showed that the intrinsic viscosity of the HPAM solutions decreased with increasing total salinity, as expected from charge screening, and that it decreased sharply in presence of divalent cations, even at low ionic strength, which was less expected. Coreflood experiments showed that polymer retentions, resistance factors and irreversible resistance factors increased significantly:–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;–in presence of kaolinite and illite, even at low ionic strength and hardness. The polymer inaccessible pore volume was significantly impacted by the presence of clays, but not by the ionic strength and hardness. Analysis of the results indicated that these effects could not be attributed only to polymer adsorption linked to the increase of specific surface area, but that more complex polymer adsorption/retention mechanisms occur depending on the clay type (layer charge and expandability). This systematic study allows dissociating the impacts of salinity, hardness and clay contents/types on the transport of polymer solutions in low permeability porous media. The results obtained 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.