A team led by researchers at Pacific Northwest National Laboratory (PNNL) has demonstrated a new way to monitor deep subsurface fractures. The technique, electrical resistivity tomography (ERT), gauges underground changes by measuring electrical conductivity in the rock. ERT produces 4D — that is, 3D plus time-lapse — images of the subsurface.
Conventional geothermal systems rely on water and flow pathways that are already present within hot rock. An enhanced geothermal system harvests heat trapped within dry rock by introducing water and cracks. Operators drill two underground wells thousands of feet below the surface and then inject fluid at high pressure to fracture the rock between the wells. The fracturing process for heat is similar to what’s known as “fracking” shale rock to release oil and gas.
Enhanced geothermal systems could provide an estimated 100 gigawatts of electricity — enough to power 100 million homes. But such systems involve expensive drilling, and they need better monitoring and prediction of underground changes to reduce the uncertainty and risk associated with a given project.
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