Experimental And Numerical Simulation Study On The Pre-Fabricated Crack Propagation Law Of The Hydraulic Fracturing Of Coal Shale

Coal shal gas is a kind of unconventional nature gas in coal measures,and hydraulic fracturing is the core technology of shale gas reservoir fracturing.Prefabricating pre-cracks is a main method to improve the efficiency of hydraulic fracturing.As a necessary preparation for hydraulic fracturing,the pre-fabricated crack can controlle the development direction and length of the crack,which directly affect the effect of the increase in the permeability of shale gas reservoirs.Therefore,it is of practical engineering significance to study the pre-crack initiation and expansion process of the coal bearing strata under the action of hydraulic fracturing.The mechanical model of hydraulic fracturing crack expansion is given,and the influencing factors of the prefabricated crack growth are analyzed.The pre-cracked specimen’s hydraulic fracturing crack propagation process was studied,and the acoustic emission energy signal and the hydraulic pressure were obtained.Using 3D pre-cracked specimen modle simulated the progration of hydraulic fracturing cracks,which verified using extended finite element method of Abaqus to simulate coal shale fracturing is feasibile.Finally,the three-dimensional numerical model of coal shale reservoir was established,and the effects of hydraulic fracturing parameters including gas reservoir depth,compression ratio,injection rate,porosity,permeability,natural fracture of coal shale gas reservoir on prefabricated crack propagation process were studied.The results show that the tense-shear propagation mode of hydraulic fracturing crack is conducted by tensile destruction of hydraulic pressure and shear failure of ground stress.The pre-fabricated cracks have a significant effect on reducing the pressure of hydraulic fracturing,and they can directly control the crack growth direction.Hydraulic fracturing pressure have progressive wedging effect to crack propagation,and hydraulic pressure change fluctuantly during the fracturing progress,which is similar with reference result of hydraulic fracturing experiment.When hydraulic fracturing water injection rate is in the range of 0.002 to 0.020m~3/s,the greater the water injection speed is,the longer the length of the crack propagation length is,the smaller the fracture pressure required,which is consistent with the conclusion of reference.In the range of0.5mD to 10mD,the larger the permeability of gas reservoir is,the longer the crack initiation time is,and the greater the crack initiation pressure is.And the effect of the shear failure of the prefabricated cracks becomes enhanced and the tensile failure effect becomes weakened.The crack length increases with the increase of permeability of gas reservoir,and it has the trend of increasing first and then decreasing.The fracture propagation is the longest in 5mD.