Study On Heat Transfer Performance Of Single Fractures In Fracture Reservoir Of Enhanced Geothermal System

Enhanced geothermal system is the key technology to extract heat from fracture network of hot dry rock.The single fracture is a microscopic component of the fracture networks,and its heat transfer efficiency directly determines the heat transfer efficiency of the enhanced geothermal system.Therefore,the investigation of the factors influencing the heat transfer in a single fracture is important for enhancing heat extraction.This study takes a single fracture in the artificial fracture networks as the research object.A cylindrical single fracture heat transfer model is constructed to explore the factors affecting the heat transfer.Combined with theoretical derivation and numerical simulation,the overall heat transfer coefficient formula is deduced and optimized.The boundary conditions(fracture inlet flow,fracture aperture,cylinder surrounding temperature),fracture morphology(composite fracture aperture,fracture inner wall area,roughness)and fluid working medium(water,supercritical carbon dioxide,supercritical nitrogen and mixed fluid)on temperature distribution and overall heat transfer coefficient in single fracture are explored.The following conclusions were obtained:The influence of fracture morphology on the overall heat transfer coefficient is divided into two parts: the influence of composite fracture aperture and the influence of fluid flow area.The decrease of CFA and the increase of the fluid flowing through the area will both lead to the growth of the overall heat transfer coefficient.For actual rough fractures,the composite fracture aperture and fluid flow area usually increase simultaneously with the surface roughness,this results in a local maximum global heat transfer coefficient in a single fracture.In addition,it is found that the global convective heat transfer coefficient increases significantly with the increase of flow rate,and the increase of flow rate also amplifies the influence of fracture morphology on the global convective heat transfer coefficient.The overall heat transfer calculation formula and simulation model proposed in this paper can explore the fracture morphology with the best heat transfer performance,so as to guide or optimize the construction process of fracture networks.When fluid medium for elemental(water,supercritical carbon dioxide and supercritical nitrogen),it is found that the change of pressure in a single fracture is not very obvious to the overall heat transfer coefficient in a single fracture with water as a fluid,and has a downward trend.The pressure of the enhanced geothermal system is inversely proportional to the global convective heat transfer coefficient when supercritical carbon dioxide and supercritical nitrogen are used as the fluid.The sensitivity to pressure is the highest when supercritical nitrogen is used as the fluid,while the global convective heat transfer coefficient is the lowest.The comparison of three groups of the single working medium as fluid shows that water has the highest heat transfer efficiency and heat extraction quantity,followed by supercritical carbon dioxide and supercritical nitrogen.In this study the influence of fracture morphology and fluid working medium on fluid heat transfer is researched,which can provide a reference for improving the heat extraction efficiency of single fracture and contribute to the heat extraction research of EGS reservoir.