Gel Dehydration by Spontaneous Imbibition of Brine From Aged Polymer Gel

Abstract

Summary This work investigates dehydration of polymer gel by capillary imbibition of water bound in gel into a strongly water-wet matrix. Polymer gel is a crosslinked-polymer solution of high water content, where water can leave the gel and propagate through porous media, whereas the large 3D polymer-gel structures cannot. In fractured reservoirs, polymer gel can be used for conformance control by reducing fracture conductivity. Dehydration of polymer gel by spontaneous imbibition (SI) contributes to shrinkage of the gel, which may open parts of the initially gel-filled fracture to flow and significantly reduce the pressure resistance of the gel treatment. SI of water bound in aged Cr(III)-acetate-hydrolized-polyacrcylamide (HPAM) gel was observed and quantified. Oil-saturated chalk-core plugs were submerged in gel, and the rate of SI was measured. Two boundary conditions were tested: all faces open (AFO) and two-end-open oil-water (TEO-OW), where one end was in contact with the imbibing fluid (gel or brine) and the other was in contact with oil. The rate of SI was significantly slower in gel compared with brine, and was highly sensitive to the ratio of matrix volume to surface open to flow, decreasing with increasing ratios. The presence of a dehydrated gel layer on the core surface lowered the rate of imbibition; continuous loss of water to the core increased the gel layer concentration and thus the barrier to flow between the core and fresh gel. Severe gel dehydration and shrinkage up to 99% were observed in the experiments, suggesting that gel treatments may lose efficiency over time in field applications where a potential for SI exists. The implications of gel dehydration by SI, and its relevance in field applications, are discussed for both gel and gelant field treatments.