Fluids based on chelating agents have been developed for matrix stimulation of high-temperature sandstone formations. These fluids dissolve sizeable amounts of calcite and clays and maintain high levels of dissolved metal in solution over time with minimal precipitation. A series of field samples from high-temperature (149°C) sandstone reservoirs in a West African formation bear carbonate concentrations ranging from 2 to 37% (w/w). The effects of matrix treatment using a chelating agent-based system on these field samples were studied using coreflood and slurry reactor experiments.

Linear coreflood test data show dramatic increases in the formation permeability after treatment with the chelating agent-based fluid. The improvement in permeability is ascribed to the removal of carbonate minerals and soluble clays, without secondary metal-precipitation. Slurry reactor tests elucidated the kinetics of mineral dissolution in mechanically ground field samples. Treatment with acidic chelant fluids generated high levels of dissolved calcium, silicon, and aluminum that remained in solution over time. For comparison, conventional mineral-acid treatment of the field samples generated high levels of metals in solution that declined over the same period of time, indicative of secondary precipitation. The effectiveness of the chelant fluid for stimulation of this high-temperature formation was confirmed through increased formation permeability and high levels of dissolved minerals.

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