| Abstract |
A non-productive well at Desert Peak (well 27-15) has been the subject of various analyses as part of a project co-funded by Ormat and the US Department of Energy’s Geothermal Technologies Program. After consideration of data from several disciplines, well 27-15 was recompleted in 2009 with a series of plugs to leave only the Tertiary “Rhyolite Unit” open and accessible for stimulation activities. The available interval is relatively short (from the casing shoe at about 3,000 feet to the top of the uppermost plug at about 3,500 feet) and is hosted in welded ash-fall tuffs with varying clay content and composition. Pre-stimulation injection testing indicates that this interval has low injectivity (0.04 gpm/psi), and that the Rhyolite Unit around the well has low transmissivity (~60 md-feet) and storage (~0.001). Considering 27-15’s location (0.5 to 1 mile north of several active production wells that supply the 15 MW Desert Peak binary power plant), the stimulation program developed by the project team anticipates conducting a series of at least five stimulation steps, with real-time seismic monitoring, tracer testing, and monitoring of active wells throughout the field during each step. The first five steps are designed to inject at constant rates with wellhead pressures not exceeding specified levels (200 psi for the first step and 600 psi for the fifth, ramping up at 100 psi increments for each step), and with a pressure fall-off period and temperaturepressure- spinner (TPS) survey after each step. Injectivity will be constantly measured during the five steps by monitoring the reservoir pressure downhole along with the corresponding injection rate. If the permeability improvements induced by the five steps are not considered sufficient, and if there is no indication of adverse effects on active production wells (as indicated by well behavior, tracer testing results, and seismic monitoring data), two additional steps are contemplated: 1) a chemical stimulation using a chelating agent; and 2) a true “hydrofrac” in which the minimum principal stress (?3 or SHmin) would deliberately be exceeded. Based on the results of an extended leak-off test (or “mini-frac”) conducted in July 2009, ?3 at the middle of the stimulation interval would be exceeded at a wellhead pressure of approximately 750 psi. The results of this work would be directly applicable to a second Ormat-sponsored and DOE cost-shared EGS project at the nearby Bradys Hot Springs geothermal field, where another non-productive well (15-12 RD) has access to about 850 feet of the same Rhyolite Unit. |