A low toxicity polymer crosslinking system has been developed which is tolerant of high brine salinity and hardness and has worked well in two field applications. The system involves the gelation of polyacrylamide with zirconium lactate; this is a lower toxicity system than the commonly used chromium gelling systems. Microgels prepared with the new polyacrylamide/zirconium system have been tested in two different field applications: 1) an injection well test in a large waterflood unit in northeastern Oklahoma, the North Burbank Unit, and 2) an injection well and a production well test in a waterflood unit in northern Texas, the C. B. Long Unit. In the injection well test in the North Burbank Unit, 69,000 barrels of polyacrylamide/zirconium gel were injected into an injection well. Pressure falloff tests before and after polymer gelant injection showed a reduction in fluid mobility by a factor of 5 to 6 as a result of the crosslinked polymer treatment. In the C. B. Long Unit, an injection well was treated with 65,500 barrels of microgel and injection profile surveys before and after the treatment showed a diversion of 55 percent of the injected fluid to a low permeability zone. Several offset producing wells responded to the polyacrylamide/zirconium treatment and incremental oil production has been in the range of 15 to 20 BOPD. The production well treatment in the C. B. Long Unit resulted in a reduction of the producing water-oil ratio.

This low toxicity polymer crosslinking system has been developed for in-depth diversion of injection fluid in a variety of applications. The polymer gelant uses a fairly low concentration of polymer and zirconium crosslinker so large volume treatments are economical. The polymer system is also tolerant to hard brines and low pH environments.

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