Method of Characteristics Based Model for a Coaxial Borehole Heat Exchanger

Mature petroleum fields provide readily accessible wells for geothermal energy extraction in many parts of the world. Likewise, the major hydrocarbon-producing region of New Zealand, the Taranaki region, has many mature wells with high temperatures (∼150 °C) measured at the bottom. Borehole heat exchangers could be a cost-effective way of producing geothermal energy from these wells. Therefore, the purpose of this study is to present a mathematical model capable of evaluating the potential of geothermal energy production from unused wells. We first set up our mathematical model using the Method of Characteristics for the wellbore heat exchangers. A co-axial borehole heat exchanger is selected for our study due to its simple design and popularity in use. The model is then validated against the analytical results available in the literature. For the first time, a study utilizing borehole heat exchangers to produce geothermal energy is being conducted in New Zealand. This study will help boost interest in reusing unused wells for residential and industrial direct use applications. Based on our model, the capability of alternate fluids such as supercritical CO2 to further enhance the performance of heat exchange will be analyzed in the future.