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Concept of a High-Temperature EGS Plant in Central Oregon
Daniel DICHTER, Trenton CLADOUHOS, Quinlan BYRNE, Victor RUSTOM, Greg SZUTIAK
[Quaise Energy, USA]
Quaise, a geothermal development and drilling company, has broken ground on a high-temperature geothermal power plant in Central Oregon called Project Obsidian. It is sited outside of and adjacent to the southern boundary of the Newberry National Volcanic Monument. The geothermal gradient at the site is believed to be about 100 °C/km on the basis of nearby well temperature logs, and the targeted vertical depth range is about 4.3-4.9 km. The subsurface at this depth is expected to have very low natural permeability. Thereby, the project is expected to be completed by fracturing between wells using modern EGS (enhanced geothermal systems) techniques, rather than as a conventional hydrothermal project. Two reservoirs are planned, with targeted average feedzone temperatures of 315 °C and 365 °C. Each reservoir is to be connected to one injector well and two producer wells, with the wellfield comprising six flowing wells altogether. The injectors have a tie-back design with a 7” outer diameter casing throughout; the producers have a similar casing in the feedzone, stepping up to a 9 5/8” outer diameter casing around 2.5 km TVD to improve the flow characteristics of the more-compressible produced geofluid. The wells are planned to be inclined at 45° in the feedzone, to balance the challenges of directional drilling at high temperatures with the need for significant horizontality with which to connect the wells using vertical fracture planes. The geofluid enthalpy at the producer feedzones is considered as a key unknown, and a finite-element compressible flow model was developed to assess its significance, and its relation to geofluid pressure and phase. This model was used to generate a set of productivity curves, showing that liquid-dominated production and vapor-dominated production represent lower and upper bounds, respectively, for exergetic power at the wellheads. Several plant configurations were considered, including dry steam, flash, cyclopentane-binary, and water-binary. The best-performing cycle showed a strong dependence on the phase of the geofluid at the producer wellheads. Overall, it was found that the proposed system has the potential to deliver at least 50 MWe net from a total of six flowing wells using a binary cycle regardless of the geofluid phase.
Topic: Enhanced Geothermal Systems