Numerical simulation of power production of a well extracting geothermal energy and the transient thermal response of the surrounding rock

A computer model was developed to simulate a geothermal well comprised of a double pipe heat exchanger system. This model determines the power production possible when utilizing the system in abandoned oil wells. The model was used to determine the suitability of using butane, propane, pentane, and refrigerant 114 as secondary fluids in the well. To determine maximum power production in 12, 8, and 6 inch diameter wells, the fluids were tested over a range of injection pressures, injection velocities, and inner pipe radii. It was determined that there is no single best choice for every set of initial conditions. Maximum power production for each fluid was found to occur at different injection pressures and velocities. Maximum power in a 12 inch well requires injection pressures between 3700 kPa and 6600 kPa, depending on the fluid. Injection velocities required range from 2.4 m/s to 3.4 m/s. As the well diameter decreases to 6 inches, the injection pressure range increases form 2200 kPa to 6700 kPa. The injection velocity range decreases to 1.4 m/s to 2.2 m/s.

Further, a second computer model was developed to work in series with the first to determine the transient thermal response of the rock surrounding the well. It was found that the size of the well has a large effect on the amount of heat that can be extracted from the surrounding rock. This has a measurable impact on the transient power production of the well. A 12 inch diameter well takes less than 10 hours to reach roughly steady state heat transfer from the rock. A 6 inch diameter well takes approximately 200 hours.