Hungarian researchers proposed to use decommissioned oil and gas wells for storing heat energy. Such abandoned wells are sources for methane leakages, and methane is known for its greenhouse effect 25–34 times exceeding the carbon dioxide effect. At the same time, liquidation of such wells is very expensive, while their conversion into heat storage facilities turns out cost-efficient.
The operating principle for the underground heat storage technology under temperature not lower than 90 °C is known as HT-ATES (High-Temperature Aquifer Thermal Energy Storage); it is based on pumping the redundant heat into aquifer during the summer season, and then recovering it and using for heating purposes during the winter season. To explore the efficiency of this technology application in the conditions of Hungary, the scientists studied Bekesi depression — one of the deepest and the most well-studied sections of Pannonian basin in the South-West of the country. This region features a massive network of old wells earlier used for producing oil and gas. The pay zones there occur at a depth of about 2,000–2,500 meters, and high temperatures (from 70 to 120 °C) is achieved on account of abnormally high geothermal gradient. Local sands feature high porosity (up to 25%) and permeability (up to 2,000 millidarcy), which makes them especially fit for pumping in and then recovering heat.
Hydrogeological simulation was developed to assess the performance of the system; it allowed for re-creating the movement of underground flows. The heat transfer simulation was also developed reflecting the distribution and accumulation of heat in the rock. The simulation was based on two wells at a distance of more than 500 m between them – to avoid a thermal outbreak. During the simulation the change of temperature in the direct proximity from the wells was modelled with the analysis of the system efficiency at every stage of the annual cycle. Then, the machine learning algorithm was “trained” based on the obtained data. This algorithm is capable of forecasting the thermal efficiency of other wells, including those beyond the initial simulation. The results of simulations and forecasts were visualized in the forms of temperature maps, graphs of seasonal heating and cooling, as well as the maps of forecasted efficiency growth for all the studied wells in the region.
The ATES technology has already proved its efficiency — there are about 2,800 similar systems operating all over the world, mainly in the Netherlands, where they provide for cumulative heat supply of circa 2.5 TW*hour per annum. In Hungary there are thousands of depleted wells in the geologically fit layers. The scientists estimate that introducing at least 100 HT-ATES systems each capable of accumulating from 8 to 12 GW*hour of heat per annum will allow to accumulate from 800 up to 1,200 GW*hour of thermal energy annually. This is enough to cover the winter heating needs of 100–150 thousand households, simultaneously decreasing the greenhouse gases emissions and reducing the old wells disposal costs.
