Delayed HPAM gelation via transient sequestration of chromium in polyelectrolyte complex nanoparticles

Heterogeneity of oil reservoirs often leads to unproductive cycling of injected oil recovery chemicals, resulting in the loss of significant quantities of reserves. To maximize recovery efficiency, a blocking agent may be placed deep into high-permeability channels so that the subsequently injected chemicals can be redirected into previously unswept regions. Cr(III)-polyacrylamide gels have been used extensively in field applications as blocking agents for sweep improvement; however, the gelation time of the current state-of-the-art is too short to achieve in-depth placement. This paper describes a novel approach of using polyelectrolyte complex nanoparticles to entrap and control the release Cr(III) to effectively extend gelation time. Self-assembly of polyethylenimine (PEI) and dextran sulfate (DS) resulted in the formation of similar to 100-200 nm particles that efficiently entrapped chromium while maintaining colloidal stability in water or gelant. Although the addition of chromium chloride to HPAM typically produced gels in minutes, chromium was efficiently sequestered in nanosuspensions of polyelectrolyte complexes, resulting in a significant delay in gel formation that was dependent on pH, ionic strength, and temperature. The gel formation kinetics of PEI, polyelectrolyte complexes (PECs) of PEI and DS, and PECs loaded with chromium were compared. PEI, a known cross-linker of HPAM, produced a steady increase in gelant viscosity over time. PECs without chromium demonstrated a delayed gel formation compared to PEI but possessed a similar creeping increase in viscosity. In contrast, PECs loaded with chromium typically showed minimal viscosity increase over time followed by an abrupt viscosity increase, resulting in gel formation. This study suggests that PECs offer a flexible nanotechnology platform that may enable novel chemical delivery schemes in the oil and gas industry.