| Hydraulic fracturing is a kind of pressure relief technology widely used in impact pressure prevention and gas disaster management projects,which has the advantages of large fracturing range,good pressure relief effect,significant effect of increasing permeability and dust reduction.However,due to the lack of a test platform to monitor the fracture formation process,the problem of "invisible and incomprehensible" fracture formation process always exists in the experimental research of coal rock,which restricts the development of hydraulic fracturing technology.In order to solve this problem,a single-sided visual coal-rock body true triaxial hydraulic fracturing test platform was constructed,a single-sided visual coal-rock body true triaxial test chamber was developed,the optimal visualization test method and parameters suitable for fracture observation were explored,and the dynamic expansion law of fracture across the coal-rock interface was studied.(1)A single-sided visual coal rock body true triaxial hydraulic fracturing test platform was constructed,and a single-sided visual coal rock body hydraulic fracturing true triaxial test chamber was developed.The test platform organically combines the true triaxial loading device with the hydraulic fracturing device,and is designed with a transparent high-pressure visual window,which can observe the dynamic expansion of hydraulic fracture in real time with a digital camera,and can realize the analysis of surface strain field during hydraulic fracturing by combining with DIC analysis technology.(2)In order to make full use of the single-sided visual window to achieve clear observation of the crack expansion process,the test method of making the crack expand along the visual window was explored,and the optimal stress field,pore shape structure and water injection rate were obtained.When σ_H>σ_V>σ_h,the crack can be extended perpendicular to the visible window,which is most conducive to observation;when the pore structure is without bare pore section,the crack can be extended in the intended direction;when the water injection rate is 100 m L/min,the crack can be extended from top to bottom in the direction of ,forming a penetration crack across the interface.(3)The hydraulic fracturing test of coal rock body was carried out by using the test platform,and the fracture expansion characteristics were analyzed by combining with DIC analysis technique to obtain the dynamic evolution law of strain field in the fracture expansion process.(1)The fracture crosses the coal-rock interface,which will form a complex fracture network above the interface and presents four modes of expansion across the interface:(a)stopping expansion type;(b)communicating interface type;(c)deflecting through the interface;(d)penetrating the interface type;the existence of prefabricated fractures will make the fracture first communicate the fracture,then expand along the fracture and cross the interface,and a fracture network will form above the interface.(2)The characteristics of the fracture extension surface strain field are analyzed based on the DIC technique,and it is found that the surface strain field development presents four stages:(a)strain localization formation stage;(b)strain localization extension stage;(c)strain localization communication stage;and(d)strain localization stabilization stage.The research results of the paper are of great guidance to understand and reveal the mechanism of hydraulic fracturing in coal rocks.Figures [58] Table [8] Parameter [99]... |
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