Closed-loop systems for geothermal resources exploitation from hydrocarbon wells.

Below the surface of the earth, there exist geothermal resources with the potential to make a significant contribution to the increasing demand of power consumption and environmental sustainability. However, the expensive capital costs of drilling the geothermal well still represent the main challenge to the implementation of this resource in energy systems. Dismissed hydrocarbon wells in mature oilfields can be considered good candidates for the geothermal energy exploitation as they usually have high bottom-hole temperature, reliable wellbore integrity and large production capacity. Existing borehole and perforation data can be also useful to have geological, geophysical and geochemical information about the sub-surface reservoirs and allow direct access to the sub-soil heat. In different studies of the past years, authors were oriented in the direction of the implementation of open loop systems by using directly oil or gas reservoir as a geothermal reservoir. Another way to recover heat is to convert the considered borehole into a heat exchanger using closed-loop system: this kind of system allows to extract heat from the ground without requiring any water to be abstracted or re-injected at all. In the developed thesis work, the attention was centred on two different closed-loop-type technologies: U-tube and coaxial Wellbore Heat Exchanger (WBHE). The selected case study is located in one of the largest European oil fields: Villafortuna Trecate oilfield, active since 1984. The extracted heat it has been hypothesized to be used either for district heating or other direct uses. Two different heat exchange models (U-tube and coaxial WBHE) have been implemented in MATLAB software with the main aims to 1) find the best configuration that allows to maximize the heat recovery from the soil, examining the main factors affecting the extraction efficiency; 2) understand the technical feasibility of such retrofitting project from a hydrocarbon well to a geothermal one. The results have shown that thermal power extracted from this type of applications depends mainly on geological parameters, such as site stratigraphy, and on technical parameters related to the plant, such as depth, diameter of the well and insulating material.