Polyacrylamide Adsorption and Readsorption in Sandstone Porous Media

Abstract

Summary The term polymer retention describes all mechanisms that remove the polymer from the flowing solution, with adsorption being its primary cause. This phenomenon can lead to detrimental effects during polymer enhanced oil recovery (EOR). In this paper, we present an investigation of dynamic polymer adsorption in sandstone-outcrop cores using polymer solutions. We study the effects of permeability and polymer concentration on the adsorption under two conditions: on virgin cores (adsorption) and a previously polymer-flooded core (readsorption). According to the results, two concentration plateaus and two regions of concentration-dependent adsorption characterize the polymer adsorption in a virgin porous medium, following a proposed Type IV isotherm. The transition between the first plateau and the second adsorption region occurs near to the overlapping concentration from dilute to semidilute regimes (cp*). Polymer readsorption increases slightly with the successive injection of banks with a higher polymer concentration, following a Type I (Langmuir) isotherm. For that case, we propose a readsorption mechanism on the basis of the desorption of a polymer molecule section and the adsorption of a new free polymer molecule. The adsorption and readsorption isotherms are similar until cp*, while the adsorption is much higher than readsorption for concentrations higher than cp*. Therefore, if the polymer concentration of the mobility control bank is greater than cp*, the total polymer loss during field applications can be reduced by preinjecting a polymer bank of lower concentration.

Evaluation of Polymeric Materials for Chemical Enhanced Oil Recovery

“…General agreement is that polymer adsorption is the dominant mechanism for polymer retention [126,127,129], even though all mechanisms for polymer retention could be at play during polymer flooding in oil reservoir rocks due to the intrinsic heterogeneity of the geological formations [126,127]. Polymer retention in porous media depends on polymer type, polymer molecular weight (MW), polymer concentration, flow rate, temperature, polymer slug size, brine salinity (i.e., adsorption increases as brine salinity increases), absolute rock permeability, oil saturation, rock wettability, and minerology [113,124,126,128,130,131].…”

“…If polymer retention is significant, it causes a substantial loss of polymer concentration from the flowing polymer solution, which results in a reduced viscosity of the injected polymer solution and in reservoir rock damage (i.e., plugging) that affects the integrity of the polymer solution and its propagation through the oil reservoir, thus resulting in a delay of oil displacement and oil recovery [126,127]. Polymer retention in porous media is caused by polymer adsorption onto the rock surface, mechanical entrapment of polymer molecules at small pore throat constrictions relative to the size of the polymer molecules, and hydrodynamic retention [126,128] that "is caused by hydrodynamic drag forces [that] trap polymer molecules in flow-stagnant zones", as explained by Ferreira and Moreno [129]. The mechanical entrapment of the polymer significantly increases as the rock permeability decreases [126].…”

“…Polymer adsorption onto rock surfaces is considered instantaneous and irreversible [127], although polymer adsorption takes place through physical adsorption that, in principle, should be reversible. The issue, as Ferreira and Moreno [129] explained, is "because of the large size of the polymer molecule, the polymer adsorbs in multiple sites on the rock surface, making the desorption of the entire molecule difficult, even though individual segments of the macromolecule can desorb". Furthermore, the large sizes of polymer molecules influences the way that the polymer adsorbs onto the rock surface.…”

“…Furthermore, the large sizes of polymer molecules influences the way that the polymer adsorbs onto the rock surface. As Figure 6 shows, polymer molecules adsorbed onto rock surfaces display different hypothetical conformations, such as trains, loops, and tails, with different levels of interactions between the polymer and the solid surface [129]. of the entire molecule difficult, even though individual segments of the macromolecule can desorb".…”

“…Furthermore, the large sizes of polymer molecules influences the way that the polymer adsorbs onto the rock surface. As Figure 6 shows, polymer molecules adsorbed onto rock surfaces display different hypothetical conformations, such as trains, loops, and tails, with different levels of interactions between the polymer and the solid surface [129]. Recently, Ferreira and Moreno [129] conducted an experimental study of dynamic polymer adsorption using a polyacrylamide-based terpolymer to establish the adsorption isotherms on a porous medium that has not been contacted by the polymer previously (i.e., initial adsorption) and the adsorption of the polymer on a porous medium previously contacted by the same polymer (i.e., readsorption).…”