I saw an interesting report today from MIT on The Future of Geothermal Energy in the U.S. It got me thinking back to the first ‘real job’ I ever had – in UNOCAL’s Geothermal Division. Our petroleum engineering curriculum required sophomores to work as roustabouts, the theory being that exposure to ‘real work’ (and real workers) out in the oil patch would give us a better appreciation for field operations before we assumed a lifelong career staring into a computer monitor from a cozy office in Houston.
My internship saw me assigned to UNOCAL’s geothermal operation in Brawley, California 10 miles north of the Mexican border. Besides producing steam, Brawley is best known for producing lettuce. Lots and lots of lettuce. The lettuce requires lots of water to grow and to survive Brawley’s 115+ degree summer days. When it gets that hot, water in the open irrigation pits evaporates, adding just enough humidity to the air to remind me of the most oppressive August day in Houston. When you’re working outside in that kind of heat, adjacent to 24” pipes carrying 400F super-heated steam, well, you learn to appreciate a career behind a desk in an air-conditioned office!
The reason Brawley has all that steam below it relates to the San Andreas fault, the shaper of so much of California’s landscape. Brawley sits almost directly on top of the San Andreas in an area where the fault (or more precisely, at least six subordinate faults) have caused the crust to thin as the Pacific and North American plates pull apart from each other. Given the thinner crust, and bolstered by magma upwelling in the subsurface, the area’s geothermal gradient is exceptionally high. High enough, in fact, to superheat groundwater to nearly 500 degrees Fahrenheit (under pressure so it doesn’t boil away to steam until it’s brought to the surface).
Geothermal operators drill wells to tap this superheated water, convert it to steam (directly or indirectly), and run the steam through a turbine to generate electricity. Figuring out where to target these wells – whether producers or injectors – requires some fairly precise science. And this is where NEOS can help.
Generally, the operators want the wells to be located near fault and fracture systems, as it will be easier to produce (and inject) fluids in these areas. According to our VP of Exploration Solutions (who did his Master’s thesis on geothermal fracture detection), “geothermal production largely stems from hydrothermal systems containing high-salinity fluids present along major, deep-seated faults. These fault systems typically can be defined by lineaments characterized by low resistivity, magnetite-destructive alteration, water-transmitted radioactivity, and hydrothermal clay alteration.” Several of our sensors, including radiometric and hyperspectral but especially magnetic and magneto-telluric (MT), can help detect and map these fault-fracture networks from the basement to the surface.
Others sensors, such as gravity, can help geoscientists map the thickness and density of the rock packages above the basement. Interpretation of these measurements can be used to identify zones of crustal thinning or those possessing igneous intrusions, either of which may lead to higher thermal gradients within the subsurface.
Many of us in the oil & gas business don’t think too much about geothermal. But if you have a project that involves geothermal exploration, think about NEOS and how our multi-measurement methodology might be able to help.