Washout of Cr(III)-Acetate-HPAM Gels From Fractures: Effect of Gel State During Placement

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.