The Raft River geothermal field is the location of a Department of Energy (DOE) enhanced geothermal system test site. The field is located roughly 161 km (100 miles) northwest of Salt Lake City on the Utah Idaho border. Since 2012, the test well RRG-9 ST1 has been successfully stimulated hydraulically and thermally. The well was initially hydraulically stimulated using flow rates from 42 to 2,877 Lpm (11 to 760 gpm) at an injection pressure of 7,924 kPa (1,150 psi). The well was then shut in to allow for the construction of a 254 mm (10 inch) pipeline from the plant to the wellhead. In 2013, injection from the plant through the 254 mm (10 inch) line was initiated at a flow rate of 76 Lpm (20 gpm) at an injection pressure of about 1,937 kPa (280 psi). A second hydraulic stimulation utilizing an agricultural pump raised flow rates into the well to 643 Lpm (170 gpm). Flow rates were further increased to 1,249 Lpm (330 gpm) with the addition of a second pump. Following this second stimulation, injection was resumed through the 254 mm (10 inch) pipeline and injection improved from 189 to 454 Lpm (50 gpm to 120 gpm). Injection was maintained through the winter of 2013 and spring of 2014 when a third hydraulic stimulation was performed on the well in April 2014. Using pump trucks, a maximum flow rate of 4,769 Lpm (1,260 gpm) was achieved at an injection pressure of 6,752 kPa (980 psi). Following this hydraulic stimulation, injection was resumed into the pipeline. The injectivity of the well has steadily improved since the third stimulation. In April 2015 a pressure falloff test at the well led to further increases in well injectivity. Currently the well accepts 3,785 Lpm (1,000 gpm) at an injection pressure of 1,550 kPa (225 psi). The injectivity of the well has improved from 0.08 Lpm/kPa to 2.4 Lpm/kPa (0.15 gpm/psi to 4.4 gpm/psi) during the course of the stimulation program. Microeismic events related to plant activity have been monitored through an eight-station microseismic array deployed around the RRG-9 ST1 wellhead. Since 2010 over 180 microseismic events have been recorded. The majority of these events follow a linear trend to the northeast of the RRG-9 ST1 wellhead. This trend corresponds to the location of the Narrows shear zone. The presence of these events indicates that fluid injected into RRG-9 ST1 travels along this zone to the northeast. Additionally, a correlation between the frequency of microseismic events and increases in the injectivity of the well has been found. Prior to each major increase in injectivity, an enhanced number of microseismic events are detected. The RRG-9 ST1 stimulation program has been numerically modeled using FALCON a finite element reservoir simulation code developed by the Idaho National Laboratory. This model uses a continuum approach to simulate fluid flow through the fracture networks surrounding RRG-9 ST1. The permeability of these fracture zones is adjusted to correspond to the increasing injectivity of the well. The timing of these permeability adjustments are tied to the occurrence of increased numbers of microseismic events. Using this method, a pressure history match has been obtained for the RRG-9 ST1 stimulation program.

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