Study On Geothermal Power Generation Using Abandoned Oil Wells

The current development of human civilization has been made possible by the extensive use of fossil fuels. However, fossil fuels are becoming increasingly scarce and burning fossil fuels generates carbon dioxide, which is a greenhouse gas that contributes to warming of the atmosphere. Geothermal energy is a renewable, clean and sustainable energy stored in the form of heat beneath the earth’s surface. Geothermal energy will become an important part of the new energy in the future with gradually running out of the traditional energy such as coal and oil. However the total cost of geothermal power plants is increased significantly and the wide application is restricted by the huge cost of drilling. Meanwhile there are many abandoned oil wells all round the world. These oil wells have a large amount of heat energy. Abandoned oil wells can be changed into geothermal wells for power generation. In this way the drilling costs will not only be reduced, but also problems of abandoned oil wells will be solved. It may become a new way to manage abandoned oil wells and utilize geothermal energy.Geothermal power generation using abandoned oil wells is investigated in this paper. Considering the influence of the formation heat transfer on the system, a model based on transient formation heat transfer is established. Isobutane is chosen as the working fluid. It is heated by the geothermal energy from geologic formation through the double-pipe heat exchanger and drives the turbine to generate electricity. Furthermore, a numerical simulation for an abandoned well with a depth of6000m is performed. The result shows that the geothermal power generation is largely influenced by the formation heat transfer. The outlet temperature of fluid leaving the recovery well gradually decreases with the system operating time increasing and ultimately approaches stability. The stabilized time of the system can be shortened by either the increasing formation thermal conductivity or the decreasing inlet velocity of fluid entering the injection well but is not significantly influenced by the formation heat capacity. The net power depends on the total obtained heat and the outlet temperature of fluid leaving the recovery well. And increasing the inlet velocity can increase the total obtained heat but lower the fluid outlet temperature. Consequently there is an optimal inlet velocity of fluid entering the injection well to maximize the net power for a specified geothermal heat source. Considering the heat transfer from the fluid in the injection well to the recovery well, the theoretical model is further perfected. The influence of insulation on the geothermal power generation using abandoned oil well is analyzed. The result shows that at the same thickness of insulation, the difference of the outlet temperature between with the insulation of polystyrene and with the perfect insulation decreases with the inlet velocity of fluid entering the injection well increasing, but increases with the well depth or the geothermal gradient increases. The total obtained heat and the outlet temperature of fluid leaving the recovery well gradually increases with thickness of insulation increasing. The insulation of0.03m polystyrene can prevent the loss of heat from the recovery well to the injection well effectively.On the basis of the model considering the formation heat transfer, the investigation on influences of working fluids on the power generation efficiency is performed. For abandoned oil wells with different kinds of well depths and geothermal gradients, the power generation performances using various organic fluids are analyzed. Direct power generation system (DPGS) is compared with flashing power generation system (FPGS). The results show that the geothermal energy from the abandoned oil wells with well depth less than3000m is worthless to be exploited due to low power generation efficiency. For the abandoned oil wells with well depths larger than3000m and geothermal gradients higher than0.04K/m, DPGS efficiency of supercritical working fluids leaving the recovery well is higher than FPGS efficiency of subcooled working fluids. R134a and R245fa are more suitable than R600a, R600, propylene, R290and R143a for the geothermal power generation using abandoned oil wells.