Reusing abandoned oil wells as deep closed-loop geothermal systems: FE multiparametric sensitivity analysis and the role of heat convection in fractured reservoirs

The decarbonization of communities and their energy supply is considered as a contemporary priority, although it poses many challenges. In this scenario, geothermal energy can cover a pivotal role in the energy transition and the possibility of reusing or modifying existing wells for geothermal purposes is becoming a hot and promising topic. In Italy for example, there are more than 4000 abandoned/inactive hydrocarbon wells, abandoned either for the end of the resource (exhausted well), or for the lack of finding the resource (barren well).These wells can represent a huge opportunity for geothermal resource exploration and exploitation, as historical well data can provide useful information on the underground conditions, reducing mining risk, and sometimes allow a direct access to the sub-surface heat energy.

This work aims to analyse the feasibility of retrofitting abandoned oil and gas wells to understand which the benefits of reusing old wells are compared to drilling new ones.

A finite element numerical model of a deep U-shape closed-loop Borehole Heat Exchanger (BHE) was implemented to evaluate the performance and efficiency in terms of energy production of this solution. Sensitivity analysis allows highlighting the main operational and environmental parameters involved in the heat exchange processes between the working fluid and the surrounding reservoir rocks, and particularly to quantify how the variation of design and geological parameters influences the outcome temperature of the working fluid and thus the energy efficiency and production of the BHE system.

After an initial round of simulations using purely conductive models, we investigated the potential for convective heat transfer within a geothermal reservoir, considering both porous and fractured media. Using the Nelson Diagram as a foundation, we assessed various permeability/porosity ratios to better understand how the interplay of these parameters influences system efficiency. Specifically, we explored whether fluid circulation within the reservoir enhances heat exchange over long-term simulations, potentially leading to improve system performance, particularly following extended periods of geothermal source exploitation.