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As main heat exchange channel in enhanced geothermal system, the evolution of hydraulic conductivity in fracture is significance for efficient heat mining. For the thermal stress or thermal cracking spontaneously induced by the temperature difference between low-temperature fluid and hot rock in heat mining stage, it is necessary to explore the damage mechanism along EGS fracture and the corresponding permeability evolution. Firstly, the long-term permeability tests under high temperature (50–200 ℃) were conducted by the self-developed high temperature seepage experimental device. Then, a coupled THM-D model was constructed to describe the damage distribution along fracture. Combined with experimental and simulation results, relationship between the thermal stress/cracking and the evolution of fracture permeability is revealed. The results indicate that during high-temperature (200 ℃) experiments, the fracture permeability first increases rapidly under the low-temperature induced thermal stress/cracking, then decreases due to the blockage effect induced by the debris particles generated in thermal cracking along fracture. The enhancement of injection velocity and heterogeneity are all conducive to the emergence of thermal cracking in matrix along fracture. Simultaneously, high confining pressure has a negative effect on the migration of debris particles of thermal cracking, which contribute to prevent the blockage of debris particles. [ABSTRACT FROM AUTHOR]

As main heat exchange channel in enhanced geothermal system, the evolution of hydraulic conductivity in fracture is significance for efficient heat mining. For the thermal stress or thermal cracking spontaneously induced by the temperature difference between low-temperature fluid and hot rock in heat mining stage, it is necessary to explore the damage mechanism along EGS fracture and the corresponding permeability evolution. Firstly, the long-term permeability tests under high temperature (50–200 ℃) were conducted by the self-developed high temperature seepage experimental device. Then, a coupled THM-D model was constructed to describe the damage distribution along fracture. Combined with experimental and simulation results, relationship between the thermal stress/cracking and the evolution of fracture permeability is revealed. The results indicate that during high-temperature (200 ℃) experiments, the fracture permeability first increases rapidly under the low-temperature induced thermal stress/cracking, then decreases due to the blockage effect induced by the debris particles generated in thermal cracking along fracture. The enhancement of injection velocity and heterogeneity are all conducive to the emergence of thermal cracking in matrix along fracture. Simultaneously, high confining pressure has a negative effect on the migration of debris particles of thermal cracking, which contribute to prevent the blockage of debris particles. [ABSTRACT FROM AUTHOR]

The radial well fracturing has been applied and achieved effective output. However, the fracture morphology and propagation law of it are still remain unclear, which limits the development and application of it. A 3D numerical model is established by ABAQUS to solve the current problems. The stage of fracture propagation is simulated by the extended finite element method. The conclusions stated that in situ stress field distribution was changed during the radial well fracturing and the induced stress field along the radial well is established at the same time. The propagation of major fracture is orientated by the induced stress in a certain area. The guiding function of fracture orientated propagation is evaluated by included angle η between the axis of radial well and equivalent fracture. The smaller the value of η is, the better effect of fracture orientated propagation is. The increase of horizontal stress difference and decrease of vertical density of radial wells are unfavorable to decrease the value of η. The failure pressure grows up with the azimuth angle of radial well increasing. The increasing of elastic modulus of formation is not conducive to the effect of radial well fracturing. The increasing of Poisson’s ratio of formation is conducive to the failure pressure and maximum fracture width, but not as helpful as to decrease the value of η. Larger diameter of radial well is better for fracture oriented propagation. Operation parameters of fracturing fluid viscosity and construction displacement have less effect on the value of η. Also, the degree of different parameters influence on value of η, failure pressure and maximum fracture width is ranked by the grey correlation method. The results can be referred to prediction of fracture propagation and design of radial well completion or fracturing. [ABSTRACT FROM AUTHOR]