Investigation On The Propagation Of Hydraulic Fracture Under The Influence Of Coal Macrolithotypes In Shallow Coal Seam Of Hancheng Block

The mechanics properties,microfractures development and reservoir fracability of different coal macrolithotypes are significantly different.These discrepancies have a great influence on the propagation of hydraulic fractures in different coal macrolithotypes,which results in different morphology of hydraulic fractures.The evaluation and optimization of hydraulic fracturing for different fractures morphology is also different.It is important to understand the influence of coal macrolithotypes on the hydraulic fracture propagation,which is directly related to the fracturing effect of coal seam.Therefore,several hydraulic fracturing experiments considering the classification of coal macrolithotypes are carried out by taking the shallow coal seam of Hancheng block,China.We also conduct the evaluation of reservoirs fracability.A three-dimensional(3D)H-type fracture model of dull-type coal seam is established combined with fluid-solid coupling theory and cohesive element model.And a 3D fracture network model of bright-type coal seam is also established combined with fluid-solid coupling theory and MShale discrete fracture network model.The propagation of H-type fracture in dull-type coal seam and fracture network in bright-type coal seam is explored,and the evaluation and optimization of hydraulic fracturing is also carriedThe results show that:(1)The H-type fracture dominated by horizontal fractures is formed in dull-type coal seam.The horizontal fractures are the major contributor of fracturing effect.The difference of the minimum horizontal principal stress between layers mainly affects the fracture length of horizontal fracture.The greater the stress difference is,the larger the length of horizontal fracture is.With the increase of coal seam permeability,the horizontal fracture develops from stubby-type to slender-type.There is a certain critical displacement,which makes the horizontal fracture develop from slender-type to thick-long type,and achieves better fracturing effect.With the increase of fluid viscosity,the fracture length of horizontal fracture increases first and then decreases.The fracture length of horizontal fracture reaches the maximum when the viscosity is 15 m Pa.s.The perforation location has a significant effect on the propagation of H-type fracture.(2)The fracture network dominated by main fracture is formed in the bright-type coal seam.When the elastic modulus difference between layers is 10 GPa,the conductivity and SRV of fracture network reach the maximum.The difference of Poisson's ratio between layers,and the perforation length have little influence on fracture network.With the increase of displacement,the fracture length,fracture height and SRV of fracture network increase linearly,but the conductivity decreases.The excessive high or low viscosity is not conducive to the effect of fracture network,so it is very important to optimize the viscosity of fracturing fluid.When the perforation location is selected near the formation interface,the fracturing effect of fracture network is better.The research results provide theoretical guidance for efficient fracturing of coalbed methane.