Glimpses of my lecture notes, that may turn into a book.
Wednesday, March 5, 2025
Monday, March 3, 2025
Notes on Stanford 50th Geothermal Workshop
The 50th Stanford Geothermal workshop was held on the Stanford University campus 10-12 Feb 2025. This was my second consecutive SGW. This meeting was sold out with 450 attendees (up 32% from last year).
Professor Roland Horne gave the welcome address and had to fill in for the scheduled DOE-GTO speaker who bowed out due to funding and other chaos back in Washington DC. Roland gave an interesting overview of the history of SGW, which you can read here. Key faculty figures were Paul Kruger, a self-proclaimed 'nuclear civil engineer' who worked on early concepts to make geothermal reservoirs by thermo-nuclear explosion (an idea again under discussion, though not at this meeting), and Henry Ramey Jr who is famous for, among many other things, writing a seminal 1962 paper on wellbore heat transmission of injected fluids in the presence of a linear geothermal gradient (link to abstract).
Next up was Professor Joe Moore giving an update on the UTAH FORGE project (paper here). Very good talk on an impressive and important EGS project. Some key takeaways: project start in 2014, max well depth 9500 ft, max temperature 250 C. The well laterals are at 65 degree dip to allow wireline logging, later work by Fervo has shown pure laterals can also be wireline logged without issue. Basic power element is a 2-well doublet (producer/injector) in granitoid and metamorphic rock. First doublet well: drill, stimulate, map microseismic cloud, drill 2nd well into the cloud. Goal is commercial flow rates at commercial temperature. Distributed acoustic+ sensing (DAS) in a monitoring borehole, applied outside casing, is key technology for microseismic mapping (reservoir extent), temperature (reservoir heat energy), and strain. Tracers are injected with each frac to demonstrate connection between injector and producer. Seismic monitoring is critical, surface and downhole; deployable to 150 C but high failure at greater temperature. Flow detail from pressure-temperature-spinner (PTS) logs (Weatherford tool link, no endorsement implied).
Final talk of the lead-off section was Jack Norbeck of Fervo Energy giving and update on the 400 MW Cape Station project in Utah (paper link). An excellent talk. Some of the key points: Fervo has > 550 MW in secured power purchase agreements (PPAs), investors Devon Energy, Capricorn, Google, over $200M debt secured. Project Red (Fervo Blue Mountain pilot project in Nevada) a full year of operation with no thermal decline. Project Cape: adjacent to UTAH FORGE site, 20 horizontal wells drilled from 3 pads, total drill length 13500 ft (4115 m), depth 8500 ft (2590 m), lateral length 5000 ft (1525 m); reservoir temperature 225 C; fastest well drill to date 15 days (approximately 37.5 ft/hr); reservoir fluid pressure 3632 psi (25 MPa), inter-well pressure differential 170 psi (1.17 MPa) for single-well flow rate of 110 kg/s (calculates out to reservoir impedance of Ir = 10636 Pa/(kg/s)); after 30-40 day flow test produced water temperature still rising; first power will be from three 30 MW ORC plants; Gringarten type curves have been updated to modern EGS lateral/frac practice (see the paper, section 4) and used to estimate fracture surface area 30M ft^2 (2.8M m^2) per well -- that is about one square mile of surface area developed in the reservoir by completion and stimulation methods -- which all leads to new thinking about the thermal recovery factor for EGS projects: traditional geothermal projects assumed about 30% of the heat energy in place could be recovered, but the new analysis suggests 60%; since power (MW) is proportional to the thermal recovery factor, this implies more power, slower reservoir thermal decline, and improved economics; translates to power density of about 30 MW/km^2 or 85 MW/mi^2. Fervo drilled 204K ft in 2024, first year the company had a rig working every day, this calculates out to 1 MW per 1000 ft (300 m) of drilling, or 100 MW/rig/yr.
I truly admire the transparency and level of detail Fervo puts out in these papers, it is a service to the fledgling industry and is helping all of us move toward a better, more sustainable energy future. Key figures from the paper below. Great stuff!
