This paper provides an introduction to the topic of water shutoff and conformance improvement. After indicating the volumes of water produced during oilfield operations, a strategy is provided for attacking excess water production problems. Problem types are categorized, typical methods of problem diagnosis are mentioned, and the range of solutions is introduced for each problem type. In the third section of the paper, the concept of disproportionate permeability reduction is introduced—where polymers and gels may reduce permeability to water more than to oil or gas. When and where this property is of value is discussed. The fourth section describes the properties of formed gels as they extrude through fractures and how those properties can be of value when treating conformance problems caused by fractures. Section 5 covers the efficiency with which gels block fractures after gel placement—especially, the impact of fluids injected subsequent to the gel treatment.

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Summary This work investigated the blockage performance of a Cr(III)-acetate-hydrolyzed polyacrylamide (HPAM) gel after placement in open fractures, with emphasis on the effect of gel maturity during placement. Polymer gel is formed through a chemical reaction between a polymer and a crosslinking agent (in a gelant solution) that occurs during the gelation time. In field applications, gelant is generally pumped from the surface, but gelation may occur during injection because of high-temperature conditions and longer pumping times; hence, partially or fully mature gel may exit the wellbore during polymer-gel injection in a fractured reservoir. Gelation alters the solution properties significantly; hence, immature gelant and fully formed (mature) polymer gel show different behavior during placement in a fractured system, and the gels deposit differently in the fracture volume. Injection of gel at different maturities in a fracture may therefore influence the ability of the gel treatment to block fractures, and hence its performance during conformance-control operations. Placement of immature and mature gels and their ability to block fractures during subsequent waterfloods were investigated in this work. Gel was placed in fractures (and in the surrounding core matrix for some application regimes) in its immature (gelant) or mature state. The gel-blockage performance was assessed by recording gel-rupture pressures and subsequent residual resistance factors during chase waterfloods. Placement of mature gel in open fractures yielded consistent rupture pressures during subsequent water injections, following linear trends for given gel-placement rates and throughput volumes. The rupture pressures were predictable and stable in all the core materials studied. Rupture pressures achieved after placement and in-situ crosslinking of immature gel (gelant) were comparable with rupture pressures achieved after mature-gel placement, but were less predictable. Placing immature gel in the adjacent matrix and in the fracture increased the resistance to gel rupture compared with placing gel in the fracture volume only. In some cores, gel did not form after placement in its immature state. Interactions between Bentheim rock material and gelant were observed, and believed to be the primary cause for lack of gelation. Significant permeability reduction was achieved during subsequent waterfloods after placement of either immature or mature gel in open fractures. Residual resistance factors for cores treated with gel and gelant were comparable initially. After significant water throughput, substantially greater pressure gradients were observed in cores treated with formed gel rather than gelant crosslinked in situ, and the permeability reduction averaged 5,000 for mature gel and 600 for gelant-treated cores.

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.