| Low gas permeability is a common property as to the coal seam gas occurrence inour country. In order to reasonably and effectively extract and discharge the coal seamgas, some measures must be adopted to increase the permeability for high-gas andlow-permeability coal seam. Hydraulic fracturing, as one of the valid hydrodynamicmethods lifting coal seam permeability level, have been applied into many respects inthe mines. In this paper, the similar material was used to simulate the coal seamsituation. The influence of the ground stress to the propagation rules of hydraulicallycreated fracture was studied, combining with theoretical analysis and field application.Meanwhile the method of optimized layout on gas extraction drilling has been proposedbased on the crack propagation rules under the effect of in-situ stress, which providesthe appropriate reference for the application of coal seam gas extraction in later.(1) The mechanical characteristics of coal rock, roof and floor were tested by usingthe MTS815Electro-hydraulic Servo-controlled Rock Mechanics Testing System,which provide a valuable basis for the design of similar material in terms of coal rock,roof and floor. The mercury injection test was used to study the pore and fracturecharacteristics of M6-3coal seam. The pore structure of the coal seam is mainlyfeatured by fine pore and micro pore, the porosity is2.7793%and structuralmorphology displays the opening characteristic and good connectivity, which isconductive to the take corresponding measures to increase coal seam permeability. Thedistributing rule of ground stress was measured by Kaiser Effect of Acoustic Emission.The coal mine in FengChun is featured as tectonic stress field model, the verticalprincipal stress is the intermediate principal stress, the horizontal principal stress is themaximin principal stress and the maximum horizontal stress orientation is in thenorthwest.(2) The hole bottom hold pressure model was proposed by hold pressurecharacteristics of coal seam drilling hydraulic fracturing. The encountering model ofhydraulic fracture and natural fracture was built, based on the mechanical relationshipbetween hydraulic fracture and natural fracture. The conditions of extension and stretchwere analyzed after the encounter between hydraulic fracture and natural fracture, andcoal seam hydraulic fracturing was considered to be the main reason for the alternateextension and stretch between two kinds of fractures. The fracture network is made of main cracks and branch crack in the end. The hydraulic fracture extension modalitywaspredicted, the continuity equation of fluid in the cracks, the pressure drop equations, slotwidth equation and fracture height control equation and algorithm method of the modelwere given based on the pseudo-three-dimensional model of hydraulic fracturingverticalfracture.(3) The similarity material of simulation coal rock was studied to determine thesimilar materials proportion of coal rock, roof and floor. Physical simulation test of coalseam hydraulic fracturing was designed by the application of roof-coal rock-floorlayered casting method. The expanding regularity of coal seam hydraulic fracturingcracks was systematically explored by true triaxial hydraulic fracturing test.The resultsindicate that coal seam is the major area for the extension of hydraulic fracture, theoverall shape is elliptic and the seam height changes according to condition of the crackpropagation. The extended direction of the crack is perpendicular to the orientation ofthe minimum horizontal principal stress and is parallel to the road of the maximumprincipal stress. The ground stress is the determining factor in fracture propagation. Thenumerical change of the angle between the directions of hydraulic fracture and themaximum horizontal principal stressis contrary to the changing trend of ratio ofhorizontal maximum and minimum principal stresses. The direction of the crack isdeflected and crack fracture pressureisgradually increased. The extension direction ofhydraulic fracture is randomunder the isotropic ambient pressure condition. Once thefracture plane is formed, cracks will expand along the dominant fracture surface andextend along one direction of the horizontal principal stress. The intersection anglebetween hydraulic fracture and prefabricated crack has become one of the key factors todetermine the possibility of a hydraulic fracture through the prefabricated crack. Whenthe angle is less than45°, the hydraulic fracture will successfully slip through theprefabricated crack and go further place. Hydraulic fracture can be extended in two coalseams in the process of joint fracture in two coal seams. Coal seam drillingcan play aguiding role on hydraulic fracture propagation to some extent. The concentrated stressof drilling will locally influence the extended orientation of hydraulic fractures. Thehydraulic fracturesstill will continue to propagate through the drilling hole under theeffect of ground stress.(4) The calculation formulas used in the plastic zone around the oval was givenbased on the final propagation pattern of the hydraulic fracture, under the joint effect ofthe ground stress and coal seam roof-and-floor. The plastic zone was considered to be the pressure relief increased permeability zone around coal seam hydraulic fractureregion and also to be emphasis area of optimized layout on gas extraction and coal seamdrilling. Borehole layout optimization model was established and the distance betweenholes of the effective radius of extraction was gotten,3R, according to the request ofno blind area. The drilling borehole arrangement optimization design of stratapenetration and consequent landslide were carried out based on the fracture morphology,the extension direction and distribution of plastic zone around the developed cracks. Torealize the optimization design of the gas extraction drilling, the extended direction ofhydraulic fracture was investigated by the drilling gas extraction volume from thesingle-hole. Hydraulic fracturing and gas extraction was combined and a set ofselect-judgment-fracturing-drilling-extraction integrated process of hydraulic fracturingand gas extraction was proposed according to the characteristics of ground stress andgeology tectonic.(5) The industrial application of hydraulic fracturing in FengChun coalminedemonstrate that single coal seam hydraulic fracturing can improve thepermeability of coal seam,increase gas production and speed up the drivage speedof thecoal seam roadway. Multiple coal seamscombined fracturinghas obtained the favorableeffectsincross-measure cutting coal, not only increased the extractionconcentration,extractionscalar, and also shortened the time limit for cross-measure cutting coal andguaranteed the excavation replacement of mine and safety production. |
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