Dottorato in INGEGNERIA ELETTRICA, DEI MATERIALI E DELLE
NANOTECNOLOGIE

Titolo della tesi: Improvement of the performance and applicability of the WellBore Heat eXchanger

The subject of this work is the WellBore Heat eXchanger, a zero-mass extraction device that can be applied in geothermal systems at medium and high temperatures. The studies on this device started in the ’90s but increased in the last 20 years, due to the interest of the geothermal sector and the hydrocarbon one. The WellBore Heat eXchanger is composed of two coaxial pipes, where a working fluid circulates and extracts heat from the ground in contact with the borehole wall. It has low efficiency in heat recovery respect to the conventional geothermal plants, but it avoids the risks and costs related to fluids production. For instance induced seismicity, encouraging a positive social response to geothermal plants. The interest of oil companies is due to the possibility to apply the WellBore Heat eXchanger in depleted oil and gas wells, avoiding the costs for drilling, exploration and facility. In this thesis, the applicability of the WellBore Heat eXchanger and the improvement strategies have been evaluated. The work includes a literature review, with simulations and pilot test results, which demonstrates the feasibility of the WellBore Heat eXchanger. Then the study has been focused on the optimization of the design, demonstrating that the optimal design in the classical coaxial layout is with constant diameters for the entire depth of the well. The internal coaxial diameters and the internal insulation seem to be the geometrical parameters that affect the most the heat transfer. The investigation on the suitable uses for the WellBore Heat eXchanger applied exergy analysis. The results indicated a good potential of district heating application when the geothermal source temperature is in the range 100 ÷ 300 °C, whereas the production of electricity could be suitable only with very high bottom-hole temperature (>300 °C) at relatively shallow depth. An important part of the thesis is focused on the identification of the most correct simulation method to study the thermal influence of the WellBore Heat eXchanger on a geothermal reservoir with convective structures. The work has demonstrated that only the coupling of a numerical model of geothermal reservoir and WellBore Heat eXchanger can depict the thermal field and the effects on heat recovery. Furthermore, the simulations regarding the application of a single WellBore Heat eXchanger in the volcanic area of Campi Flegrei, have shown that a thermal power of 850 kW can be produced for 10 years without affecting the thermal field of the reservoir, indicating the plant is sustainable in time. A future implementation of this work could be the study of the effect of the nanoparticle on the heat transfer effectiveness of the working fluid. Thus, the most promising nanofluid in terms of heat transfer improvement, the risk of instability and changes in thermal properties, and the evaluation of costs will be evaluated.