| The geothermal energy is an important clean and renewable energy,particularly medium-deep geothermal energy is abundant in China.Accelerating the development of medium-deep geothermal energy can provide key support to adjusting the energy structure,saving energy and reducing emissions,and realizing the peak carbon dioxide emissions and carbon neutrality in China.The open-loop geothermal system in a single well has the advantages of circulating fluid in the same layer by one well and enhancing the heat transfer.Therefore,it is expected to achieve high-efficiency “heat extraction without water consumption” and promote the sustainable development of medium-deep geothermal energy.In this paper,the methods of numerical simulation and theoretical analysis are comprehensively employed to implement in-depth research on the flow and heat transfer mechanisms and the parameter effect characteristics of the single-well openloop geothermal system.According to the dynamic heat transfer processes of the inner tube,the annulus and the formation around the wellbore,a 3D thermal-hydraulic wellbore model is established by introducing the effect of natural convection in the aquifer.Based on the local nonthermal equilibrium and dual-porosity model,a 3D thermal-hydraulic-mechanic reservoir model is presented by considering the dynamic heat transfer coefficient between the fluid and rock and the influence of the reservoir deformation on porosity and permeability.The wellbore-reservoir model is established through the bidirectional data transfer at the wellbore bottom boundary.Based on the coupling model,the influence of the fluid friction and thermosiphon process on the pressure distribution in the wellbore are revealed.It is found that the noninsulation pipe in the wellbore causes considerable heat loss and hinders the heat transfer in the reservoir,indicating the necessity of full insulation.The flow and heat transfer changes in the formation near the well are obtained.The “dual heat transfer”(non-thermal equilibrium in the fracture and near thermal equilibrium in the matrix)",“dual flow”(high velocity in the fracture and low-velocity source/sink in the matrix)" and “dual deformation”(significant deformation of the fracture and limited deformation of the matrix)mechanisms in the reservoir are deeply clarified.The influences of the wellbore and reservoir on production are quantified.The characteristics of the balanced injection and production are revealed.The influences of the operation parameters,wellbore parameters,reservoir scale and reservoir properties on the heat extraction are comprehensively studied.The sensitivity factors are determined by the Grey Relational Analysis: The injection flow rate and the reservoir thickness are the most vital factors in the operation parameters and the reservoir scale,respectively.The thermal conductivity of the inner pipe and the injectionproduction spacing in the wellbore parameters have an important impact on the heat production of the system,while the length of the production section and thicknesses of the inner pipe plays vital role in the injection-production pressure difference of the system.Also,the influence of the well type on heat transfer,energy,flow and economic characteristics of the system is analyzed.Taking the energy and economic efficiencies as indicators,the applicable conditions of the single-well open-loop system under various well types are obtained.The research results provide a scientific reference for scheme design and field application of the single-well open-loop system in the exploitation of medium-deep geothermal energy. |
