| Since the 20th century,the demand for fossil energy has been increasing year by year,traditional fossil energy resources such as coal and petroleum are depleting,and the energy problems caused thereby have become increasingly serious.At the same time,the greenhouse effect,smog and other environmental issues caused by the widespread use of fossil energy have posed serious threats to the sustainable development of human society.Therefore,searching for alternative clean energy has become imminent,and it has become the focus of research in countries around the world.In addition to clean energy such as wind energy and solar energy,geothermal energy is becoming a kind of sustainable clean energy that has received extensive attention due to its rich reserves and no pollution.However,the huge drilling cost has limited the exploitation and utilization of geothermal resources to some extent.At the same time,with the massive exploitation of fossil fuels such as natural oil and gas,a large number of abandoned oil and gas wells have been formed on a global scale.If these abandoned oil and gas wells cannot be effectively managed,it will bring serious environmental problems due to leakage.Therefore,using the geothermal resources of abandoned oil and gas wells can not only achieve low-cost geothermal utilization,but also solve the environmental problems caused by the comprehensive treatment of abandoned oil and gas wells.Aiming at the problem of low energy utilization efficiency in the process of using abandoned oil and gas wells,this paper proposes a plan for enhanced geothermal utilization of abandoned oil and gas wells,and establishes a theoretical model for the enhanced utilization of geothermal energy for enhanced abandoned oil and gas wells.Based on this,the thermal properties and economics of enhanced abandoned oil and gas wells geothermal energy utilization systems have been studied.Based on the wellbore flow and heat transfer model,this paper analyzes the effects of the mixed organic working medium HFC437a and pure working medium HFC134a,butane and pentane on a simple single well power generation system.The analytical results show that the temperature-changing phase transition properties of the mixed organic working fluid HFC437a can help reduce the irreversibility of the heat transfer process and increase the net power generation of the system;at the same time,increasing the thickness of the wellbore insulation layer and decreasing the thermal conductivity of the thermal insulation layer can increase the outlet temperature of the circulating organic working medium and the net power generation of the system,however,the excessively large thickness of the heat insulation layer will also restrain the net power generation of the system to a certain extent.In order to solve the problem of limited heat transfer area between working medium and formation in the single wellbore system,this paper proposes an enhanced abandoned oil and gas wells geothermal utilization system and establishes an analytical model for single well double working medium enhanced geothermal power generation utilization system.And the effect of geothermal reservoirs on the heat production and power generation are analyzed using this model.The results show that in enhanced geothermal energy systems,geothermal reservoirs can significantly increase the efficiency of geothermal energy systems,and with the increase of geothermal reservoir depth and injection velocity of the first cycle working fluid,and with the decrease of geothermal reservoir porosity,the heat production power and power generation can be further improved;at the same time,there is an optimum geothermal reservoir length for obtaining maximum heat production power and power generation.Within the range of calculation conditions,the optimum length is 300 m.In order to further improve the thermal performance and power generation performance of geothermal energy systems,a double wells enhanced geothermal power generation system and its numerical analysis model are developed based on a single well enhanced geothermal power generation system,and numerical analysis of the system was conducted.The analysis results show that,under the same conditions,comparing with the single well enhanced geothermal system,the heat production power and power generation of the double wells enhanced geothermal power generation system have been significantly improved about 20%,however the change trends influenced by the key parameters are similar.Since the cost is one of the important factors to be considered in the actual development of geothermal systems,this paper establishes an economic analysis model of an enhanced geothermal energy system combining the heat transfer model and analyzes the changes of net cost of the enhanced geothermal energy comprehensive utilization system during the operation.Study results show that using abandoned oil and gas wells for geothermal energy exploitation and utilization can not only effectively treat abandoned oil and gas wells,but also save a lot of development costs and realize net profit earlier;during the 10 years of system operation,there are optimum injection flux making the lowest net cost,and the optimal injection flux increase approximately linearly with the well spacing within a certain range;the direct use system has the best well spacing making the system net cost the lowest at the well depth of 6,000 m,in addition,the geothermal energy direct utilization system has higher heat utilization rate and investment return rate than the indirect power generation system. |
