Repurposing Idle Oil and Gas Wells to Thermal Energy Storage Systems for Long Duration, Dispatchable Electricity Delivery

Abstract There are nearly 40,000 idle oil and gas wells in California. State and local regulators apply pressure to abandon these wells, but their abandonment comes at a significant cost that is often underfunded cost, and many are the source of actual or potential methane emissions. Converting these end-of-economic-life assets to Thermal Energy Storage (TES) systems can permanently seal off emissions and provide long duration energy storage. Idle oil and gas wells are ideal environments for a TES due to their large volumes and high thermal insulation. The conversion of an idle well begins with sealing off all hydrocarbon producing zones with a cement plug per standard abandonment practices. Then a portion of the well above the plug is outfitted with a closed-loop heat exchanger surrounded by TES media. This media is a combination of sand and Phase Change Materials (PCM) designed to enhance heat storage capacity. A renewable heat source, such as solar collectors or heat pumps, heats the working fluid during the charge cycle which is circulated into the well with the TES system. During the harvest cycle, the working fluid is diverted to an Organic Rankine Cycle Engine (ORC) or other similar system for power generation. A steady-state, zero-dimensional analysis was performed to assess the potential of a repurposed oilwell-to-TES system for long duration energy storage (LDES). A 50% (volumetric) mix of PCM and sand resulted in a TES system that meets the thermal load of a 75 kW ORC power block, an established, field-tested system with over a million hours of operational uptime. The PCM analyzed is solar salt, the eutectic binary mixture comprising 60% sodium nitrate (NaNO3) and 40% potassium nitrate (KNO3) c. Preliminary analysis indicates that a TES system depth of less than 3200’ would be sufficient and this depth is well within typical oilwell operational depths. The thermal inertia of steel piping and water made a sizeable contribution to TES performance because they represent a high proportion of the total TES system volume. The nationwide inventory of idle wells is a rich resource for conversion to energy storage. TES systems provide LDES to augment intermittent and cyclic energy and balance generation and demand.