Stimulation of a High-Temperature Granitic Reservoir at the Utah FORGE Site

Abstract

AbstractUtah FORGE (Frontier Observatory for Research in Geothermal Energy) is a field laboratory for developing, testing, and prototyping technologies that could be adopted for commercializing Enhanced Geothermal Systems (EGS). The principle of developing an EGS is to use multiple hydraulic fracturing stages to interconnect an injection well and a production well – forming the surface area within a large heat exchange system. At the Utah FORGE site, near Milford, Utah, an injection well (65° to the vertical) has been drilled and a three-stage fracturing treatment was carried out at the toe of this well. A production well will be drilled into the stimulated domain determined from microseismic measurements. The objectives of the treatments were to establish if the created fracture networks will form independent flow networks between the injector and the producer, and ultimately to test long-term connectivity between the two wells. In addition, the mechanics of isolating stages and developing fracturing fluid viscosity in a naturally fractured granitic reservoir at 435°F [224°C] were evaluated.Three stages were pumped. Geophones in three offset wells and shallow distributed acoustic sensors (DAS) and surface monitoring devices tracked the fracture evolution. The first stage was slickwater in a barefoot section, pumped at rates up to 50 bpm [7.95 m3/min]. Bridge plugs were used in 7-inch [177.8 mm] casing to isolate the next two stages, which each used a single long perforation cluster (20 ft [6.096 m] long, with six shots per ft at 60° phasing for each of the latter two stages). The second stage was slickwater pumped at rates up to 35 bpm [5.56 m3/min]. The final stage was crosslinked carboxymethyl hydroxypropyl guar (CMHPG) polymer fluid pumped at rates up to 35 bpm [5.56 m3/min] with low concentrations of microproppant. The well was flowed back between each stage to mitigate the potential for stage interference, facilitate running bridge plugs, and reduce the possibility of undesirable microseismicity.Isolation technology had been a significant concern before the treatment. However, bridge plugs successfully functioned at these high temperatures – isolating stages 1 and 2 and stages 2 and 3. Treatment records show a significant morphological difference between pumping in the openhole section (stage 1), and in the two cased and perforated zones (stages 2 and 3). Microseismic data suggest nominally planar growth orthogonal to the wellbore for the two cased and perforated zones – favoring intersection with the soon-to-be-drilled production well.These treatments superficially seem mundane. However, they successfully demonstrated the viability of hydraulic fracture creation in a cased well in hot, low permeability granitic rocks, a prerequisite for EGS development, revealed conditions for limited natural fracture interaction, and this was one of the few high-temperature granitic stimulation treatment programs since Fenton Hill in the 1980s.