Repurposing depleted hydrocarbon reservoirs for geothermal energy: A case study of the Vilkyčiai Cambrian sandstone in Lithuania

Lithuania, located on the eastern edge of the Baltic sedimentary basin, hosts a geothermal anomaly in its southwestern region. This anomaly comprises two main geothermal complexes: the Cambrian and Devonian aquifers. The Cambrian formation, consisting of tight sandstones, extends beyond 2000 m in depth and reaches reservoir temperatures of up to 96°C. The Devonian formation, composed of unconsolidated sands from the Pärnu–Kemeri formation, lies at depths exceeding 1000 m with reservoir temperatures up to 46°C. Several depleted Cambrian oil fields in Lithuania operate at 99 % water-cut, making them promising candidates for geothermal heat utilization. This study explores the repurposing of depleting oil and gas reservoirs for geothermal energy development with potential application of horizontal wells for geothermal production in the Vilkyčiai field. From the simulation results it is confirmed that a single horizontal well is capable of producing 447 m³ /day of water, supporting a power output of 1148 KW over a 25-year period. Furthermore, findings suggest that increasing horizontal well length up to 4500 m and application of fracturing can significantly enhance water production by 23 %, thereby boosting power generation. A full-field simulation over 30 years confirmed that no thermal breakthrough occurred between wells, validating a 2000 m well spacing as the optimal scenario for field development. These results highlight the advantages of horizontal well technology in geothermal applications, demonstrating increased water production, improved energy recovery, lower drilling costs, and greater operational efficiency—mirroring advancements in hydrocarbon extraction. In addition, the requirement for home heating at a considerable distance from the old oilfield to residential neighborhoods presents a significant logistical challenge. This study also underscores the potential for substantial heat loss through both surface and subsurface pathways during long-distance hot water transportation, emphasizing the necessity for effective insulation to minimize thermal losses. Thus, this study shows that old oil and gas reservoirs can be reused to produce geothermal energy. Moreover, it also supports Lithuania’s goals for using more renewable energy. In addition to that it also highlights how horizontal wells can help develop geothermal resources.