Molding Condition Optimization Of Coal Containing Gas And Its Application In Physical Simulation Of CBM Extraction

China is a country which is very abundant in coalbed methane resources and the large-scale exploitation and utilization of CBM are very significant for mine gas disaster control,energy comprehensive utilization,environment pollution prevention and energy structure improvement.Study on the dynamic evolution law of coal reservior parameters and gas migration during CBM exploitation can provide theoretical support for CBM exploitation and it is significant for improving CBM production.But the coal reservior and occurrence condition of CBM are both very complex, most of the coal( especially the coal in southern China)is loose texture,the bedding and joints are well developed,so it is very difficult to make the raw coal specimens.Instead of using raw coal specimens,optimizing the molding condition for making suitable briqutte to complete tests is particularly important.The coal is taken from Songzao mine coal K3 seam.Using the self-developed heat-fluid-solid coupling trixial servo experimental apparatus for coal containing gas,the mechanical and seepage characteristics experiments of coal containing gas under different particle ratio,gypsum powder content,constant pressure time and forming pressure are carrid out.On the basis of comparative analysis and optimal selection of the results,Using self-developed multi-field coupling test system for CBM exploitation,apllying the most opimal molding condition brequtte,physical simulations on CBM exploitation are carried out to further study the influence of different seepage characteristic coal on dynamic evolution law of coal reservior parameters and gas migration. The research obtains the following conclusions:1)With the increasing of fine particle content,the principal stress difference at the same strain is near but the permeability decrease apparently.With the increasing of the biggest particle in brequtte, the principal stress difference at the same strain increases but the permeability decreases befor increasing a little.Considering the mechanical,seepage characteristics and difficult degree of making brequtte,the ratio B2=40~60:60~80:80~100:>100:gypsum powder=23:12:6:52:7(total volume of particle is seen as 100)based W.B. Fuller’s grading curve theory is selected for CBM exploitation.2)With the increasing of gypsum powder content,fixed carbon content decreases but ash and volatile matter content increase linerly.The principal stress difference at the same strain increases.The permealility decreases but the decreased amplitude and rate decrease.Considering the industrial analysis results and the mechanical,seepage characteristics,the 6% gypsum content is selected for next studying.3)With the constant pressure time increasing,while the principal stress difference at the same strain increases,the permeability is influenced little.With the forming pressure increasing,the principal stress difference at the same strain increases apparently.The permeability decreases but the decreased amplitude and rate decrease.Considering the working parameters of test equipment and all the results studed above,the molding condition applied into the physical simulation of CBM exploitation is:the particle ratio is B2,the gypsum content is 6%,the constant pressure time is 60 min and the forming pressure is 7.5MPa.4)During CBM exploitation,as the permeability of coal seam decreases after the brequtte molding condition opitimization,the decreasing rate and decreased amplitude of gas pressure which is monitored by gas pressure sensors in different places decrease.Along the axial and radial direction of exploitation hole,gas pressures in different places show more apparent otherness.With the analysis of gas flow field characteristics,while the gas migrates to the exploitation hole fast in all places in high permeability coal seam at the beginning,the gas pressure in different places in low permeability coal seam are influenced little by the CBM exploitation but decreases delayed as a whole.The decreasing rate and decreased amplitude of temperature lows much.The increasing rate of deformation in different principal stress directions and deformation at the same time decrease a lot.5)The effective exploitation region occurs first around the exploitation hole orifice and shows as a circle in the cross-section,while showing as a funnel which is symmetrical about the exploitation hole in the level and longitudinal surfaces.So we can infer that the effective exploitation region evolutes as a cone surface which is axled the exploitation hole to the deepth. As the permeability of coal seam decreases after the brequtte molding condition opitimization, the effective exploitation region occurs much later. Comparing the increasing rate of effective exploitation region, low permeability coal seam is much slower than the high one.