Experimental Study On Crack Competitive Extension Mechanism During Combined Hydraulic Fracturing In Near-range Multi-coal Seam

Coal bed methane（CBM）as a high-quality clean new energy has become the focus of social concern.Speeding up coal mine gas extraction,on the one hand,can effectively reduce coal mine disasters and accidents,on the other hand,can effectively develop clean energy,is of great significance to social economic development.In recent years,the mining activities have been extending to the depth and the difficulty of gas extraction has been increasing.Hydraulic fracturing,a technique to improve the production of low-permeability reservoirs,has been widely used in oil and gas development,which plays a key role in improving the permeability of coal seam and the amount of gas extraction.Based on the engineering background of combined hydraulic fracturing for#7 and#8coal seam with the same fracturing borehole in the Shihao Coal Mine in Chongqing,China.This paper studied the crack competitive extension mechanism through theoretical analysis,physical experiment in laboratory and numerical simulation.Firstly,the physical experiment of uniaxial and triaxial compression of five-layer composite coal and rock mass and the combined hydraulic fracturing experiment of multi-seam with different strength ratios,thickness ratios and displacement are carried out.Secondly,RFPA-Flow^2Dnumerical simulation software is used to establish a five-layer"rock-coal-rock-coal-rock"model,and carry out numerical simulation experiments for different coal seam strength ratio,thickness ratio,different displacement and different principal stress difference factors.Finally,the crack competitive extension of multi-seam combined hydraulic fracturing is tested on site,and four models of competitive crack expansion are established.Mainly obtained the following research results:（1）The failure intensity rule and failure form of composite coal and rock mass are revealed.The uniaxial compressive strength of composite coal and rock specimen is slightly higher than that of pure coal specimen.The triaxial compressive strength of composite coal-rock specimen is slightly lower than that of pure coal specimen.The compressive strength and the stress curve-descending slope of the composite specimens increased first and then decreased as the height of the two coal seams approached.The height of upper and lower coal seam is 15 mm and 55 mm respectively,which is the turning point for the compressive strength of the specimen.When the height difference of coal seam is large,it is mainly tensile failure.With the height of two coal seams close to each other,the failure form develops from single inclined plane shear failure to X conjugate inclined plane shear failure.The degree of the two coal seams participating in the destruction is different,the thinner the coal seam is,the less likely it is to be destroyed,the height of the two coal seams is closer,and the destruction degree of the two coal seams is close to the same.（2）The factors influencing the crack competitive extension of hydraulic fracturing are studied.There is obvious cracks competitive extension phenomenon of in multi-seams composite specimens.The crack initiation pressure of composite specimens is lower than that of pure coal specimens.The strength ratios and height ratios of two coal seams are the key factors for crack competitive extension during combined hydraulic fracturing.When the coal seam strength ratio is greater than 2,the crack will only fracturing and extend in one coal seam.When the coal seam strength ratio is less than 2,the crack can fracturing and extend in both coal seams at the same time,and the hydraulic fracturing effect is better.Cracks tend to preferentially extend in the thinner coal seams during hydraulic fracturing of multiple coal seams.The thinner coal seam limits the propagation space of pore water pressure and promotes the crack extension in thinner coal seam.The larger the height ratios of coal seam is,the faster the frequency of"vibration"of hydraulic fracturing pump pressure is.The stress difference determines the crack extension direction and influences the crack propagation capacity.k=0.87 is the turning point of the crack extension direction.The difference of main stress also affects the crack initiation pressure and failure pressure of coal seam.Within a certain range,the smaller the difference of main stress is,the crack initiation pressure and failure pressure of coal seam will increase,and then the crack initiation pressure and failure pressure of coal seam are nearly unchanged.（3）Displacement is a important parameter for hydraulic fracturing.The change of injection pressure is affected by the displacement,and the displacement was positively correlated with the change of injection pressure and the fracturing pressure.There is a reasonable displacement interval,and the reasonable displacement of the pure sample is no more than 60 ml/min and the composite specimens does not exceed 80 ml/min,and the crack can fracturing and expand in both coal seams.With the displacement increases,Crack competitive extension is getting fiercer,which has obvious effect on the improvement of hydraulic fracturing.However,when the displacement exceeds the reasonable interval,the large displacement will cause high fracture pressure,resulting in the single crack network,small fracture expansion area and reduced hydraulic fracturing effect.（4）Field test verified the crack competitive extension in multi-coal seam during hydraulic fracturing with one fracturing borehole,and determined the influence range of hydraulic fracturing to be about 40 m.Four crack propagation models of hydraulic fracturing are established.ModelⅠ:two coal seam crack extension at the same time;The fracture initiation pressure,crack propagation velocity and hydraulic fracturing effect are not significantly different.ModelⅡ,crack extension in two coal seam;The crack will first fracturing and extend in one coal seam,then crack stop extend,another coal seam will start to extend,the two coal seams appear the pattern of alternating crack expansion.ModelⅢ,crack extension in a coal;ModelⅣ,crack will appear that jump a layer extension;Crack fracturing and expand in one seam and will extend through the intermediate segment into another seam during hydraulic fracturing.