Characterizing Narrows Zone at the Raft River EGS site using 2010 VSP data and microseismic data

The Raft River geothermal field was selected in 2010 by the U.S. Department of Energy (DOE) as an Enhanced Geothermal System (EGS) demonstration project. Prior to the drilling of sidetrack RRG-9 ST1, the stimulated well, a vertical seismic profile (VSP) survey was conducted in 2010 in the original well for subsurface imaging. The survey included eight walkaway VSP lines and 40 three-component receivers in well RRG-9. Fluid injected in RRG-9 ST1 exits the well through a single fracture zone in the Elba Quartzite at a depth of 5640-5660 ft. The fracture zone is located on the southeast side of the low permeability Narrows Zone that separates the injection wells from production wells located on the northeast side of the zone. To image the subsurface structures, we process the raw VSP data to separate the up-going PP and converted PS wavefields and build a velocity model using sonic logs. We then apply reverse-time migration (RTM) to the up-going waves of the VSP data and obtain high-resolution subsurface PP and converted PS images along the eight walkaway lines at the Raft River geothermal field. Microseismic activity related to the stimulation program suggests the injected fluid travels northeastward along the fracture zone encountered in the well and then northeastward along the Narrows Zone. We project the microseismic event locations onto a walkaway VSP image along a 2D line approximately perpendicular to the Narrows Zone, and find that the microseismic events are mostly located along a region with VSP image discontinuities.