| Aiming to achieve clean heating in winter,and make full use of the existing large number of abandoned oil and gas wells in oilfields and geothermal wells that are about to be shut down,with the goal of improving the heat transfer rates and exploiting the heating potential of a single geothermal well,A mid-depth single well circulation with pumping and recharging(SWCPR)system involved packer was proposed and studied.Firstly,a numerical model of flow and heat transfer in SWCPR system was established.The model was verified by field test,and the data are in good agreement.The Fluent platform was used to study the influence of parameters,and it was found that the wellhead outlet temperature was positively correlated with the groundwater flow rate and recharging temperature,and negatively correlated with the permeability coefficient of aquitard layer and the thermal conductivity of inner pipe.However,heat transfer rate was only positively correlated to the flow rate.If only high heat transfer rate is pursued,the operation mode of large flow rate should be adopted Thus,the operation mode with a small flow rate and large temperature difference is quite recommended for long-term continuous operation of SWCPR system.The downhole heat exchanger is divided into a conductivity zone and the pumping and recharging zone.The heat transfer rate in conductivity zone can reach 137.8 W/m.But the proportion of heat transfer rate in the pumping and recharging zone stabilizes at 89.66%,amounting to 8.67 times that of the conductive zone.Secondly,the heat transfixion of the downhole heat exchanger in SWCPR system is discussed.Three heat transfixion paths,namely,thermal short-circuit of the inner and outer pipes,direct mixture in the wellbore,and leaking between the pumping and recharging layers,are respectively proposed to strengthen the thermal insulation performance of the inner pipe,the strict sealing of the packer,and the exploration of multi-aquifer geological structure area,which can all achieve the expected results through simulation calculations.Focusing on the packer,which plays a key role in the heat transfer effect in the SWCPR system,a fluid-solid coupling model is constructed to explore the sealing performance.Using the ANSYS Workbench platform,the feasibility of applying the packer technology to the geothermal system was verified through the variable flow numerical simulation.In the selection of the flow rate of the system,it is necessary to consider the shear strength of the rubber material,and it is appropriate to select a larger flow rate within the shear strength range.In practical applications,the desired effect can be achieved by adjusting the friction coefficient of the rubber,and the optimal packer type can be selected in combination with the completion process of the geothermal well to improve the sealing effect.If a compression packer is selected,the packing effect is better when a larger axial load is selected within the strength range that the rubber material can bear.Finally,a heat transfer enhancement method by installing the enlarged diameter inner pipe at the end of the downhole heat exchanger in the single aquifer system is proposed.Analogous to the principle of the orifice meter,the pressure is adjusted by throttling to achieve enhanced heat transfer.Through numerical simulation,as the blockage ratio increases,the heat transfer rate of a single well increases.When the blockage ratio increases to 100%,the outlet temperature and heat transfer rate of a1751.5 m deep well can gradually stabilize at 58 ℃ and 995.46 k W respectively.Compared to the single geothermal well circulation without enlarged-diameter inner pipes,the heat transfer rates can be increased by 84.71%.Furthermore,compared with the downhole heat exchanger,the ratio of the heat transfer rates varies from 3.57 to 6.60 under different operation conditions. |
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