Repurposing horizontally drilled oil & gas wells for geothermal systems using a coaxial borehole heat exchanger

Former oil/gas wells can be converted into geothermal wells by retrofitting them with a coaxial borehole heat exchanger (CBHE). While previous research has largely focused on direct heating applications for retrofitted vertical oil/gas wells, most onshore wells drilled in the U.S. since the 2000′s have been horizontal. We use a 2D coupled heat and fluid flow model to show that a horizontal well retrofitted with a CBHE produces significantly more energy than a vertical well drilled to the same depth. Our base case scenario assumes a vertical depth for both wells of 3 km, a bottom hole temperature of 105 °C, and an additional 2 km lateral extension for the horizontal well. The model predicts that the fluid temperature output by the vertical well will be 77 °C, hot enough to produce an estimated 36 kW of electric power from a binary organic Rankine cycle generator. The fluid temperature predicted for the horizontal well is 88 °C, raising potential electric power output to 59 kW for the base case, and up to 132 kW when optimizing design/operational parameters. This better performance of the horizontal well retrofit leads to an estimated LCOE of $64–94/MWh versus $85–127/MWh for the vertical well. A sensitivity analysis of the model shows that these outcomes will vary depending on the subsurface temperature gradient, the size of the outer and inner pipes, the fluid flow rate through the pipe, the horizontal length of the well, the thermal conductivity of the inner pipe, and the number of lateral extensions from the vertical leg of the horizontal well.

Duke Scholars

Author Bryan Higgins  

Author Lincoln F. Pratson Earth and Climate Sciences