Deliverable Wellhead Temperature—A Feasibility Study of Converting Abandoned Oil/Gas Wells to Geothermal Energy Wells

Abstract

Many oil/gas wells are abandoned or approaching their end-of-life. Converting them into geothermal wells can significantly improve the economics of oil/gas field operations and reduce carbon emissions. While such conversion has proven viable in some areas, this technology has not yet been considered in many other areas. It is highly desirable to investigate the feasibility of converting abandoned oil/gas wells into geothermal energy production wells in local geological conditions. A new mathematical model was developed in this study for analyzing the feasibility of converting oil/gas wells into geothermal wells. This model predicts the deliverable fluid temperature of a well by simulating the heat transfer from the geothermal zone through the wellbore to the surface wellhead, considering pipe and wellbore insulation. Factors affecting heat transfer efficiency were investigated with the model for a generic data set. Results indicate that without pipe insulation, the temperature of the returning fluid is very close to that of the injected fluid. The use of pipe insulation can significantly increase the temperature of the returning fluid. For a system with a thermal conductivity of insulation pipe Kp = 0.03 W/m-C, the deliverable fluid temperature can be increased from 30 °C to 124 °C. Adding an insulation cement sheath can efficiently further increase the temperature of the returning fluid. For a system with a cement thermal conductivity of 0.20 W/m-C, the deliverable fluid temperature can be further increased from 124 °C to 148 °C. Increasing the length of the horizontal wellbore in the geothermal zone from 2000 m to 8000 m can further increase the temperature of the returning fluid from 148 °C to 159 °C. Merely by increasing the vertical depth of the well from 7000 m to 7800 m, the deliverable fluid temperature can be enhanced from 148 °C to 161 °C. However, vertical depth is limited by the temperature-sensitivity of drilling technologies, such as the thermal stability of drilling fluids and downhole drilling instruments.