The capacity and importance of solar and wind power have been growing dramatically over the last few decades and now account for a material fraction of energy on our electric grids. These resources are intermittent, and large-scale, long-term storage methods are required. Here we describe Geomechanical Pumped Storage (GPS), and first results from feasibility studies performed in the field. GPS is a new approach for storing electric energy at scale that is conceptually similar to pumped hydro storage (PHS), which currently accounts for 94% of global energy storage. Like PHS, water is transferred from a surface pond to a high-pressure reservoir, and high efficiency pumps/turbines are employed to transfer the working fluid, effectively storing and extracting electric power by hydraulic means. Unlike PHS, which needs elevated terrain and gravity potential to realize a high- pressure reservoir, GPS injects water into a storage lens in the subsurface; the lens volume is pressurized by a combination of elastic deformation of the rock strata and overburden lift. GPS is under evaluation in four separate basins across North America. In each site, optimized completion and stimulation methods have been deployed and shown to consistently generate horizontal storage lenses with excellent connectivity to vertical wellbores. Novel methods have been developed and demonstrated for effectively increasing the fracture toughness of the rock matrix; this results in net pressures and lens widths about an order of magnitude greater than those used in stimulating unconventional reservoirs. These water-propped lenses have so far allowed injection and production rates at powers up to 100 kW with hydraulic efficiencies up to 90 percent. The completion and stimulation methods are being further developed towards commercially viable installations with powers above 1 MW for 10 hours or more.

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