Research On Permeability Improvement And Methane Driven Effect Of Hydraulic Fracturing For Low Permeability Coal Seam

Hydraulic fracturing for increasing permeability in coal seam is an effectivetechnological approach to gas extraction in low-permeability coal seam and outburst removalin outburst coal seam, especially in single low-permeability coal seam. Based on a lot ofexperiments and filed tests, this paper takes hydraulic fracturing driving for gassy coal seamas research object and carries out a systematic study on permeability improvement andmethane driven effect of hydraulic fracturing for low permeability coal seam with the methodof theoretical analysis, laboratory and site experiments, which is provided with importanttheoretical significance and widely engineering application prospects.Study on pore and fracture structure characteristics of coal seam are conducted byscanning electron microscope, mercury penetration test and hydrophilic experiment, etc. Theinternal structure of coal seam is constituted by jiont fissure, cleat and bedding, microfractureand pore levels. Those factors including soft coal seam, gas adsorption and deabsorptioneffect and developed natural fissures make hydraulic fracturing for coal-rock mass complex.In22coal seam of Xuehu colliery, fissures are very developed and most of them are1~10μmwidth. Average porosity is4.73%, average contact angle51.15°, wettability good. Porevolume mainly consists of micro pore and then small pore, superficial area is mainlycomposed of micro pore. If the porosity rises, fractal dimensions of skeleton reduce. Fractaldimensions of percolation are close related to porosity, namely that when the porosityincreases, fractal dimensions of percolation augment. Coal wettability affects capillarypressure, and then pressure difference of two phase interface when two kinds of immisciblefluid, water and gas, flow in pore and fracture system of coal mass during increasingpermeability and extracting gas by hydraulic fracturing for gassy coal seam.Research on crack propagation law of hydraulic fracturing for coal mass is performedwith the help of true triaxial hydraulic fracturing experiment system. Hydraulic crackpropagates like elliptic form on the whole and the propagation direction parallels to themaximum principal stress direction. The less the difference value of maximum and minimumprincipal stress is, the higher the rupture pressure of corresponding sample is. The less theprincipal stress difference of confining pressure is, the easier spatial turn of hydraulic crackoccurs. Hydraulic crack generates along drilling hole radial direction must satisfy that theminimum principal stress direction is parallel axial direction of drilling hole at first and localstress concentration exists along the axial direction of drilling hole fracturing segment. When the output volume of hydraulic fracturing rises, dynamic effect of crack propagation becomesobvious, hydraulic crack is easier to bifurcate and turn, crack surface roughness also increases.Hydraulic fracturing for gassy coal seam should adopt fast fracturing technology with bigoutput volume in order to create more hydraulic cracks with big range and then be beneficialto water drainage and gas extraction, etc.Fluid-solid coupling structure and permeability evolution law of coal mass is researchedon MTS test machine with transient pressure pulse method, including overall stress and strainof coal mass, test on permeability variation during confining pressure relief and seepagewater pressure difference change. The permeability of coal mass is close related to its internalstructure. Under the condition of fluid-solid coupling without damage, transient permeabilitycoefficient is proportional to hydraulic gradient as a whole. Depend on fluid-solid coupling,seepage water pressure can cause dislocation and closure of structural plane or make rupturedebris gather and block seepage channel. Under the condition of false triaxial and constantaxial compression, the deformation of coal sample is mainly radial expansion deformation inthe process of confining pressure relief. During confining pressure relief, volume straincomprehensively reflects coal sample damage of crack growth and expansion. When axialpressure is above uniaxial compressive strength and below triaxial compressive strength, coalsample typical curve of confining pressure relief and volume strain in the process ofconfining pressure relief contains three stages: elastic deformation recovery stage, plasticdeformation stage and failure stage. The variation rule of permeability coefficient accordswith coal sample volume strain. Coal sample damage and permeability coefficient variationhas confining pressure relief threshold and inflection point in exist. Above this threshold,permeability coefficient rises rapidly. Mechanical properties and initial stress state of coalsample determine the value of confining pressure relief threshold. When confining pressurerelief exceeds the inflection point, coal sample gets macro rupture and permeabilitycoefficient increases sharply.Gas driving phenomenon of hydraulic fracturing for gassy coal seam is found. Whenhydraulic fracturing for gassy coal seam creates hydraulic crack, pressure water permeatesboth sides of crack at the same time. Pore water pressure contacts free gas inside pore andfissure, leading to the variation of gas pressure and pore water in crack surrounding rock.Uneven distribution of pore water can generate pore pressure gradient. Free gas migrates fromthe location of high pore (gas) pressure to that of low pore (gas) pressure. Gas pressuregradient is the fundamental power for gas driving of hydraulic fracturing for gassy coal seam.Water injection into gassy coal seam underground mining often makes gas density in returncurrent increase to a certain extent, which is induced by gas driving under coal seam injection. After deep hole hydraulic fracturing in driving face of outburst coal seam is carried out, gascontent in coal mass presents the phenomenon of "low-high-low", verifying the presence ofgas driving phenomenon of hydraulic fracturing for gassy coal seam. The inhomogeneity ofhydraulic crack propagation and time effect of gas driving can lead to the inhomogeneity ofgas driving. In practice, fostering strengths and circumventing weaknesses of gas drivingphenomenon is in demand.The technology of increasing permeability and removing outburst by hydraulicfracturing driving in deep hole and gas extraction in shallow hole of outburst coal seam isproposed and put into effect. By means of optimizing drilling hole arrangement of hydraulicfracturing, alternating pressure relief hole and observation hole, reasonably determining thedeep hole sealing position and short hole sealing length, the safety of hydraulic fracturingdriving in leading deep hole of tunneling place of outburst coal seam is guaranteed. Asexperience proves, in combination with hydraulic fracturing in deep hole and gas extractionin shallow hole, the technology of removing outburst by hydraulic fracturing driving inleading deep hole of tunneling place of outburst coal seam and gas extraction in shallow holerealizes the dynamic integration of permeability increasing, weakening, gas driving,extraction and water injection humidification, effectively improves gas extraction efficiencyand eliminates outburst danger. Tunneling driving in outburst coal seam for Xuehu collierysaves work amount and time of10regional holes, and reduces time of gas extraction, andobtains prominent technical and economic benefits.