Hydrocarbon wells potentially suitable for geothermal repurposing in Italy. A first assessment

Global changes urge a radical transformation and improvement of the energy production systems to meet the very ambitious and challenging decarbonisation targets of the European economy by 2050. In this context, the possible repurposing of depleted hydrocarbon wells for geothermal heat production represents a promising contribution, with an approach inspired by the circular economy, to the decarbonisation of energy production systems. The mature stage of oil and gas wells is often characterized by the production of hydrocarbons and associated formation waters, which must be treated continuously being often reinjected into the reservoir. Usually, the volume of produced water increases with the maturity of the fields until the production of hydrocarbons becomes uneconomic and the wells are cemented and closed. However, according to their depth and the local geothermal gradient, these oil and gas wells could have bottom-hole temperatures high enough to sustain the geothermal exploitation of the reservoirs fluids. Therefore, when the hydrocarbon wells are going to be depleted and where a geothermal potential exists, the conversion into geothermal wells could be a reasonable alternative to the mining closure, which will compromise the possibility to repurpose the existing wells for geothermal applications. Many existing Italian oil and gas (O&G) wells are approaching the end of their economic lifespan. This irreversible process necessarily requires an in-depth and systematic assessment of their conversion potential for heat production. To identify the most promising hydrocarbon wells suitable for geothermal repurposing we have combined information on onshore wells and fields made available by the Italian National Mining Office for Hydrocarbons and Georesources, which have been integrated with data retrieved in the scientific literature, with the estimated temperatures at depth derived from the Italian National Geothermal Database. According to the latest available data, there are 892 active wells in Italy located onshore in the existing mining licenses with different operational status (i.e., productive wells, potentially productive wells, reinjection, monitoring or other purposes), With this simplified approach, we have selected wells, associated with 42 hydrocarbons fields with depths deeper than 2000-3000 m and with temperatures higher than 60-70 °C, which have the potential to be repurposed for geothermal application. For five of these oil & gas fields, the geothermal potential has been evaluated, by applying the volume method. An estimation of the geothermal energy that could be recovered by repurposing the exiting hydrocarbons fields has been obtained. Finally, we discuss how the use of a screening matrix, based on a review of the approach already proposed by Soldo and Alimonti (2015), can allow an analysis of the existing wells to define the more appropriate technology for their geothermal repurposing