Microvisual investigation of polymer retention on the homogeneous pore network of a micromodel

• A new image-based technique for direct visualization of polymer retention.
• Investigation of hydrodynamic retention, microgel, and mechanical entrapment.
• Polymer retention under water-wet and oil-wet conditions.
• Visualization of microgel transition from mobile to immobile conditions.
• Impact of mechanical degradation of polymer on polymer retention.

A new experimental technique is reported for visualization of polymer retention on the solid surfaces of porous media. Etched silicon micromodels with well-characterized pore networks were used during single-phase flow to examine the retention of 0.2 wt% partially hydrolyzed polyacrylamide (HPAM) solution. Image analysis included an image subtraction and an RGB-based global thresholding technique for quantification of polymer retention/adsorption. Results are reported as the percentage of porosity occupied by immobile polymer.Three factors were investigated including the salinity of displacing water, the change of wettability of the micromodel surface, and mechanical degradation of polymer. With respect to salinity in displacing water, the experiment confirmed that 5 wt% NaCl results in less polymer retention (6.3±0.3%) than without NaCl (7.5±0.3%). The increase in Na+ concentration was sufficient to induce contraction of the size of the flexible HPAM molecules and, therefore, decrease the thickness of polymer adsorption on the grain. Two methods were used to alter the initially strongly water-wet surface of the micromodel. Wettability was changed by the deposition of crude oil and CTAB (cetyl trimethylammonium bromide) in aqueous solution. The micromodels treated using crude oil and CTAB showed polymer retention of 15.0±0.3%% and 5.0±0.3%%, respectively. Oil-wet micromodels aged by crude oil showed larger polymer retention than the polymer retention on water-wet micromodels (7.5±0.3%). Otherwise, the polymer retention on the CTAB treated micromodel was lower than the polymer retention on the water-wet micromodel. Finally, polymer solution flowed through a 7 µm filter was tested. Average polymer retention was 4.3±0.3% and this is 3.2±0.3% point lower than the value of unfiltered polymer solution. Further investigation of microgels within pore networks provided a chance to demonstrate that a size and structural flexibility of microgel leading to a transition from mobile to immobile conditions.