Effect of Salinity and Hardness on Hydrolyzed Polyacrylamide Rheology in Sandstone

In this paper, we clarify the impact of salinity and hardness on the rheology of partially hydrolyzed polyacrylamide (HPAM) in sandstones with permeability greater than 200 md. These findings are particularly relevant for modelers and simulators of polymer flooding, as they provide critical insights into HPAM injectivity, the conditions leading to fracture initiation, and the potential significance of viscoelasticity for enhancing oil recovery—particularly in mobilizing capillary-trapped residual oil, whether or not fractures are present.

To contextualize the study, the literature review first summarizes how various parameters—including polymer concentration, molecular weight (Mw), rock permeability, and oil saturation—affect HPAM rheology in sandstone reservoirs. This foundation sets the stage for our experimental investigation, which focuses on high-Mw HPAM (18–20 million g/mol with 30% hydrolysis) across a range of reservoir-relevant conditions. Specifically, we assess polymer behavior in sandstones with permeabilities ranging from 252 to 838 md, salinity levels from 0.1% to 10.5% total dissolved solids (TDS), and hardness levels from 0% to 0.1% calcium chloride (CaCl₂).

Our results confirm established trends: resistance factors increased with higher HPAM concentration but declined as salinity rose. Notably, in the shear-thickening regime, the maximum resistance factor correlated strongly with the expression C[μ]/(k/ϕ)^0.5, linking rheology to polymer concentration, viscosity, permeability, and porosity. Despite the variation in brine composition, the velocity dependence of HPAM rheology in sandstone remained largely consistent across salinities between 0.1% and 5% TDS. Furthermore, even at a constant 1% TDS, varying the CaCl₂ concentration from 0% to 0.1% caused only minor changes in the velocity dependence of polymer behavior.

To deepen the mechanistic understanding, we explore the relationship between the onset of shear thickening and the inverse of the polymer solution’s relaxation time, as determined from bulk rheological measurements. Interestingly, the extent of mechanical degradation remained relatively stable across a wide concentration range—from 25 ppm to 2,000 ppm—when tested in brine containing 1% NaCl and 0.05% CaCl₂.

Overall, these results provide a robust framework for simplifying and improving polymer flooding models. By capturing how key variables interact under realistic conditions, the study supports more accurate performance projections and optimized design of EOR operations involving HPAM.