Abandoned wells: a source of renewable and cost-effective geothermal energy-case study

Geothermal energy is a reliable and renewable energy source, offering greater consistency compared to solar and wind energy. However, geothermal applications currently account for less than 1% of the global renewable energy production. Achieving the net-zero emissions target by 2050 requires a significant increase in the share of renewable energy in the total energy mix. Conventional geothermal projects are not sufficient to meet this demand. A significant portion of geothermal project costs stems from exploration, drilling, and facility preparation, with drilling alone accounting for approximately 40%-50% of the total cost, according to several reports and studies. Additionally, some projects fail to progress to the operational stage, further emphasizing the financial risks associated with traditional approaches. One promising solution to reduce these costs is the repurposing of existing abandoned wells, which are widespread globally. Studies suggest that reusing these wells can reduce the high start-up costs of geothermal projects by at least one-third, offering a viable and cost-effective path to scaling geothermal energy production. Abandoned oil and gas wells, along with other drilled wells for various purposes such as exploration, geological studies, geothermal energy, or waste disposal, are estimated to number between 20 and 30 million globally. Many of these wells are situated in regions with high geothermal potential. Over the past two decades, research efforts have increasingly focused on repurposing these wells for geothermal applications. Two primary systems have been explored: open-loop and closed-loop geothermal systems. Although there have been only a few successful projects worldwide utilizing closed-loop systems in abandoned wells, practical experience in this area remains limited. In this study, we review and compare both open-loop and closed-loop systems for extracting heat from abandoned reservoirs. A case study involving a vertical abandoned well in Germany is simulated, with results analyzed and compared to those from a horizontal counterpart for the same well. Additionally, challenges associated with these systems and potential cost savings are briefly discussed. The geothermal gradient of the simulated case study is about 6.3 °C/100 m. The results show that abnormal geothermal regions (above 3.5°C/100 m) have a significant energy potential even for closed-loop wellbore system if the wells are already existing. Furthermore, horizontal well outperforms vertical well owing to the large surface contact area between water and the wellbore walls. The advancement in tubing (dual vacuum pipe) insulation technologies will enhance rate of the extracted energy by decreasing loses to the annulus.