Research On Fracture Evolution Law Of Hydraulic Fracturing Coal Seam

Hydraulic fracturing technology is widely used in softening of hard coal,control of hard roof,coal seam gas drainage,prevention of rock burst and fully mechanized mining fracturing and saving energy and so on,it is to be studied that crack evolution of fracture coal seam is a common question of mining in hard-hard thick coal seam,it has great significance of learning crack evolution law of fracture coal seam and realize scientific mining.This paper combined with the hydraulic pre-splitting technique in the 5-2 coal seam of Jurassic in northern Shaanxi to control the effective rupture of coal body,combining with methods of site monitoring,theoretical analysis and numerical simulation,fracture evolution law of hydraulic cracking coal seam is studied,the main contents and conclusions are as follows:(1)By using the MTS815 electro-hydraulic servo rock mechanics test system,the determination of mechanical properties of coal seam and roof and floor.Through the monitoring process of coal seam pre-cracking fissure expansion,the formation process of coal seam complex fissure network is analyzed,and the fracture type of drilling water pressure pre-crack is given.(2)Based on the theory of linear elastic fracture mechanics,the pre-crack fracturing model was established.The stress state of the pre-cracked crack tip unit and the initiation condition of the pre-crack fracture were analyzed to determine the initiation position of the pre-crack fracture.At the same time,the mechanism of "shear-pull-pressure" suitable for pre-crack fracturing is proposed,and the shear failure,tensile failure and fracturing are analyzed under the condition of three principal stress states and coal-specific parameters.Through the three processes,to determine the pre-crack fractures of the critical water pressure parameters.The results show that the pre-cracking crack length and the coal seam hardness all affect the initiation pressure of the pre-crack fractures,and the pre-crack fracture length is proportional to the initiation pressure.The hardness of the coal seam is inversely proportional to the pre-cracking crack initiation pressure.(3)Based on the theory of fracture mechanics,the theoretical model of hydraulic fracturing expansion is established,and the hydraulic fractures are analyzed according to Irwin fissure expansion criterion.The critical water pressure of the hydraulic fractures in coal seam and rock formation is deduced and water The evolution rule of the fracture in the coal seam and coal-rock interface.The results show that the prefabricated crack length is within 1 times the pore size,and the fracture usually starts along the direction of the maximum principal stress.In the process of expansion,the coupling effect of water pressure and ground stress is extended,Zigzag shape;pre-cracked fissure length above 2 times the pore size,the fissure is basically cracked along the tip of the hydraulic fracture,and is most conducive to drilling initiation,the expansion of the pre-crack fracture length of 3 times the aperture.