Modeling and analysis of co-axial borehole heat exchanger for the heat production from the depleted/exploratory hydrocarbon wells.

The heat beneath our feet, as a geothermal energy resource, seems to have the potential to provide a portion of the world's energy demands, whether by direct heat offer or power generation. The major research challenge has been to develop a technique to economically and efficiently collect thermal energy from the ground. Drilling new geothermal wells, which cost over half of the plant's total budget, is a major limitation in allocating money. There are millions of abandoned/exploratory hydrocarbon wells, however, that can be used to collect heat from the ground. These existing hydrocarbon wells can be regarded as a viable option since most of them have the capacity to generate enough heat for direct consumption and power generation. However, we must determine which wells can produce enough heat energy in a safe, flexible, and cost-effective manner. Both the scientific community and industry have shown an increased interest in geothermal energy in recent years. In this thesis work, we chose two exploratory hydrocarbon wells located in northern Italy as case studies: CASTEGGIO Well in the province of Pavia, Lombardia, and TURBIGO Well in the province of Milan, Lombardia. In order to draw heat from the bottom of the well, a simplified co-axial wellbore heat exchange model is used, with water flowing downward via the exterior pipe and returning through the inner tube. The influence of several typical characteristics of the rocks, such as thermal conductivity, specific heat capacity, and density, on the quantity of heat extracted, was investigated using an existing simplified analytical model by using MATLAB software. A sensitivity study was also performed by changing the fluid's inlet flow rate and temperature, outer pipe diameter, insulating material and thickness of the inner pipe. As the wells' locations are already known, this study could make use of accessible geological data, a temperature profile, and a geothermal gradient. The goal of this study is to determine if the identified hydrocarbon wells have enough potential to be turned into geothermal wells, either for power generation or direct consumption, using co-axial borehole heat exchangers technology. Additionally, in order to tap the geothermal energy at the surface to its complete potential, an analysis is done on the parameters influencing the heat extraction rate for different scenarios in both wells of varied geological formations, depths, and geothermal gradients. The inferences drawn based upon results have shown that the thermal energy extracted from the wells depends primarily on the circulating inlet fluid flowrate and temperature, depth of the well (temperature of rocks varies and increases with depth), geothermal gradient, thermal conductivity and thickness of insulation of the central pipe. The research also outlines that in comparison to the other factors, the impact of outer diameter is important but slightly less significant in our study; and the influence of heat conductivity of rocks rises with the increase in depth of the well that has a greater impact on TURBIGO Well. Basis comparison of the above parameters, and the direct effect of differences in depth and geothermal gradient on the outlined parameters, we observed that TURBIGO is more effective in terms of electricity generation as opposed to CASTEGGIO. For an in-depth assessment to determine the viability of electricity generation and direct use of thermal energy would require further cost analysis.