Oil and gas fracturing uses a suite of fluid additives to improve the outcomes of hydraulic stimulation. In addition to proppants, viscosifiers, friction reducers, surfactants, acids, scale inhibitors, gel breakers, biocides, tracers and gel stabilizers are added to water for hydraulic fracturing. Table 1 summarizes the types of fracturing fluid additives. The DOE Geothermal Reservoir Well Stimulation Program (GRWSP) ongoing from 1979-1984, extended petroleum industry stimulation technologies to geothermal wells. A total of ten stimulations were conducted in a range of hydrothermal resources on wells with low permeability. The methods worked to increase output for the first few geothermal stimulations done at Raft River and at East Mesa where temperatures were around 130-180C. However, none of the wells were economic after the stimulations. Fervo is having good success using methods developed for shale fracturing at temperatures up to 200C. However, at higher temperatures like the Baca geothermal area held by UNOCAL done as Phase 2 of the GRWSP and at commercial stimulations done at the Coso geothermal area formation damage, either at the wellbore or into the fracture system, appears to have occurred from the use of fracturing fluid additives that were not thermally stable at the well temperatures (260C-300C). (Entingh, 2000). The high temperature well stimulation tests done at very high temperatures in the Geysers used clear water for the stimulation to avoid these issues. Some fluid additives for geothermal fracturing have been tested to temperatures up to 300C. However, most high temperature additives only tested for temperatures viewed as high by the oil and gas industry (up to 260C.) Fluid additives for the FORGE project have been tested for the FORGE temperatures up to 275C. PNNL has tested specialty stimulation fluids up to 300C. However, there hasn’t been a coordinated effort to qualify fluid additives for use in high temperature geothermal fracturing. This paper outlines what is known about the thermal stability of fracturing fluid additives at temperatures above 300C and outlines a procedure for qualifying fluid additives for use in high temperature geothermal fracturing applications.

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