Authors: Jacob Bradford John McLennan Joseph Moore Robert Podgorney Mitchell Plummer Greg Nash
Publish Date: 2017/01/05
Volume: 50, Issue: 5, Pages: 1279-1287
Abstract
The Raft River geothermal field located in southern Idaho roughly 100 miles northwest of Salt Lake City is the site of a Department of Energy Enhanced Geothermal System project designed to develop new techniques for enhancing the permeability of geothermal wells RRG9 ST1 the target stimulation well was drilled to a measured depth of 5962 ft and cased to 5551 ft The openhole section of the well penetrates Precambrian quartzite and quartz monzonite The well encountered a temperature of 282 °F at its base Thermal and hydraulic stimulation was initiated in June 2013 Several injection strategies have been employed These strategies have included the continuous injection of water at temperatures ranging from 53 to 115 °F at wellhead pressures of approximately 275 psi and three shortterm hydraulic stimulations at pressures up to approximately 1150 psi Flow rates wellhead and line pressures and fluid temperatures are measured continuously These data are being utilized to assess the effectiveness of the stimulation program As of August 2014 nearly 90 million gallons have been injected A modified Hall plot has been used to characterize the relationships between the bottomhole flowing pressure and the cumulative injection fluid volume The data indicate that the skin factor is decreased and/or the permeability around the wellbore has increased since the stimulation program was initiated The injectivity index also indicates a positive improvement with values ranging from 015 gal/min psi in July 2013 to 173 gal/min psi in February 2015 Absolute flow rates have increased from approximately 20 to 475 gpm by February 2 2015 Geologic downhole temperature and seismic data suggest the injected fluid enters a fracture zone at 5650 ft and then travels upward to a permeable horizon at the contact between the Precambrian rocks and the overlying Tertiary sedimentary and volcanic deposits The reservoir simulation program FALCON developed at the Idaho National Laboratory is being used to simulate and visualize the effects of the injection The simulation model uses a discrete fracture network generated for RRG9 using acoustic borehole imaging and analysis of microseismic activity By adjusting the permeability of the fractures a pressure history match for the first part of the stimulation program was obtained The results of this model indicate that hydraulic fracturing is the dominant mechanism for permeability improvement for this part of the stimulation program
Keywords: