The transition from fossil energy to clean energy is an ongoing trend. Because geothermal energy is buried beneath oil /gas wells, it is desirable to convert abandoned oil/gas wells to geothermal energy wells. The candidate wells can be dry holes in oil/gas exploration or end-of-life oil/gas wells in depleted oil/gas reservoirs. There is a knowledge gap to fill between the oil/gas wells and geothermal wells in the well conversion engineering, that is, factors affecting the performance of the geothermal wells are not fully understood. The mathematical model developed by Wei and Guo (2023) was used in this work to investigate the factors affecting the temperature of produced water of geothermal energy wells converted from abandoned oil/gas wells. Both vertical and horizontal well options were considered. The result of the field case study using the data for a well in the Songliao Basin of Northeastern China shows that, without pipe insulation, the temperature of the returned water is very close to that of the injected water, regardless of vertical or horizontal wells. With pipe insulation, the temperature of the returned water in the horizontal well is higher than that in the vertical well. The temperature of the returned water declines quickly as the thermal conductivity of pipe insulation increases in the low-thermal conductivity region. The temperature of the returned water in horizontal wells is affected by the horizontal hole section length for heat transfer. But this effect levels off after about 1,000 m of horizontal hole section is reached, meaning that 1,000 m of horizontal hole section is adequate for heat transfer from the geothermal zone to the injected water. This paper provides an analytical method for the technical feasibility assessment of converting abandoned oil/gas wells to geothermal energy wells.

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