Testing an Improved Method to Determine Inaccessible Pore Volume

Inaccessible pore volume (IAPV) is intended to characterize the fraction of aqueous pore space in a porous medium that is not accessible to flowing polymer. Previous IAPV literature is contradictory in that no correlation is evident between measured IAPV values and rock permeability or porosity or polymer molecular weight or size in solution. Prior work by Gilman and MacMillan (1987) and Wang et al. (2021) demonstrated that much of the previous contradictory reports may result from the inadequacies of methods to measure IAPV. In particular, the “double-polymer/tracer bank” method incorporates a water flush between two polymer/tracer banks. The unfavorable mobility ratio as water displaces the second polymer bank causes viscous fingering and overestimation of IAPV if insufficient water is flushed. Dean et al. (2022) proposed a potentially improved method to determine IAPV, where a low concentration polymer bank is displaced by a more-concentrated, more-viscous polymer bank—so the mobility ratio is always favorable during the displacement. This paper tests this method for determining IAPV.
The tests used sandstone cores and bead packs with permeabilities ranging from 113 to 18600 mD, porosities ranging from 0.188 to 0.402, and core lengths ranging from 30.48 to 122-cm. Many tests involved 500-ppm HPAM (with no potassium iodide tracer) displacing 250-ppm HPAM (with a KI tracer). However, three sets of tests made five replicate determinations of IAPV using successive polymer banks with HPAM concentrations starting at 62.5 ppm and doubling in stages to 2000 ppm. This procedure tested the reproducibility of the method and whether the IAPV measurement depended on HPAM concentration.
With a given data set, multiple methods can be used to assign an IAPV value, including (1) the area between polymer and tracer breakout curves, (2) the difference in pore volume (PV) throughput (between polymer and tracer) upon attaining an effluent concentration of 50% of the injected concentration, and (3) the difference in PV during first breakout of polymer ahead of tracer. Significant differences in calculated IAPV values were noted for these different methods.
In general, the method of Dean et al. (2022) provides a substantial improvement over previous methods when measuring IAPV. However, caution must be exercised when interpreting the results of these tests with respect to projecting polymer-flood performance. Important uncertainties arise from assessments of IAPV—notably, in rock with permeability above 200 mD. These uncertainties arise partly because polymer retention can vary with polymer concentration—violating a key assumption of the method of Dean et al. From this work, it is arguable whether a significant IAPV exists in Berea sandstone or bead packs with permeability greater than 200-mD (when using 18-20-million g/mol HPAM).