Factors affecting the fluid temperature of geothermal energy wells converted from abandoned oil and gas wells

The transition from fossil energy to clean energy is an ongoing trend. Because geothermal energy is buried beneath oil and gas wells, it is desirable to convert abandoned oil and gas wells to geothermal energy wells. The candidate wells can be dry holes in oil and gas exploration or end-of-life oil and gas wells in depleted oil and gas reservoirs. There is a knowledge gap to fill between the oil and gas wells and geothermal wells in the well conversion engineering, that is, factors affecting the performance of the geothermal wells are not fully understood. This work investigated the factors affecting the temperature of produced water of geothermal energy wells converted from abandoned oil and 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 and gas wells to geothermal energy wells.