PRIORITY CONSIDERATIONS FOR ASSESSMENT OF GEOLOGIC ENERGY STORAGE

As the United States (U.S.) transitions away from fossil fuels, its economy will rely on more renewable energy. Because current renewable energy sources may be intermittent, it is important to store energy for use when power supply drops below power demand. Geologic (underground) energy storage may be able to retain vastly greater quantities of energy over much longer durations compared to typical electric battery storage, and geologic energy storage does not rely upon limited availability of critical minerals. Geologic energy storage is also versatile; different types of subsurface formations can be used to store energy in a variety of underground configurations. The U.S. Geological Survey (USGS) is positioned to conduct research and to assess domestic geologic energy storage resources to help prepare the U.S. for the energy transition.

Geologic energy storage methods may be divided into three broad categories: Chemical, mechanical, and thermal. Chemical methods store potential energy in chemical bonds in the form of methane or natural gas, natural gas liquids, ammonia, and hydrogen. Mechanical methods store non-chemical potential energy using materials or fluids, and include techniques such as compressed air energy storage, with constant or variable temperatures; gravity energy storage using suspended loads; and pumped hydroelectric energy storage. Thermal methods store energy as heat in materials. Thermal energy storage includes underground hot- or cold-water storage for heating, cooling, or industrial processes such as drying. Geologic energy storage settings may include depleted hydrocarbon reservoirs, abandoned mines, solution-mined caverns in salt, purpose-drilled shafts, or non-potable aquifers.

As one of the first steps toward assessing domestic geologic energy storage resources, the USGS is identifying depleted oil and gas reservoirs that could be used for chemical energy storage. Factors such as abandoned well locations, pore volume, and pressure gradients are being considered. The Illinois Basin is an initial focus area. Complementary assessment work has started to identify pore space in target formations that could be used for multiple energy storage applications such as geothermal or mechanical methods.