Petroleum wells that cease to produce oil/gas economically and dry holes are referred to as abandoned wells. An abandoned well is usually plugged with cement and can take anywhere from two days up to a few weeks to cure depending on the number of plugs in the well. Since plugging a well costs capital, most wells are abandoned at the lowest possible cost and with the minimum obligations set by regulation agencies. These wells can present enduring liabilities to petroleum companies and/or governments. However, the depth and abundance of abandoned petroleum wells make them an economically attractive source of geothermal energy. Geothermal energy harvested from an oil/gas well is a renewable energy source as long as it is extracted in a sustainable manner. The energy can be used to generate electricity, used directly for heating, incorporated into a water desalination process, or used by a heat pump for heating/cooling applications. In this paper we examine the possibility of extracting geothermal energy from abandoned oil/gas wells by studying the heat transfer in underground geothermal heat exchangers installed in these wells. A double-pipe (shell and tube) design configuration is chosen for the geothermal heat exchangers embedded inside a petroleum borehole. Using in-situ gathered information from some representative petroleum boreholes in the Gulf region, the effects of key parameters such as geothermal gradient, ground temperature values, and the flow inside of the tubes are evaluated. In order to provide a constant power production the inlet temperature it is proposed to adjust the temperature of the inlet fluid, so that that the difference between outlet and inlet temperatures is kept at a desirable value. Higher differences between inflow and outflow temperatures will result in a less sustainable load put on the in-situ geothermal energy. Adding insulation to certain sections of the designs can minimize the transfer of thermal energy from the fluid to the surroundings. Insulation can also limit the thermal loss between the inflow and outflow tubes in the shell and tube design. Minimizing the energy loss of these designs leads to an increase in the outlet temperature, meaning a higher grade of heat. The sustainability of long term geothermal heat extraction is a balance between the rate at which geothermal energy is extracted and the rate at which the ground formation can replace this heat. Higher outlet temperatures can lead to a larger amount of power generation, and a higher coefficient of performance for heat pump applications. It is found that Qatar's abandoned petroleum wells can be economically reused for the purpose of sustainable geothermal energy production.


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