Research On Seepage Characteristics Of Coal Containing Gas Under Complex Loading-unloading And Its Application

As one of basic technical methods for gas control, gas drainage plays a vital role in outburst mine, high gas mine and deep mining area of gas mine. After coal seam is influenced by mining, the stress of coal seam is redistributed, local coal mass is injuried or damaged, the structure of pore and fracture in coal and the ability of permeability are changed. And the changes in porosity and permeability not only cause the variation of the gas seepage velocity and the distribution of pore pressure, but also result in the changes of displacement and stress of coal seam. For the drainage boreholes after hydraulic reaming, the coal mass around boreholes is also influenced by the coupling effect between mining stress field and gas seepage field, and the influence of excavation effect due to hydraulic reaming on coal permeability is much obvious. Thus, in order to thoroughly discuss the influence of excavation effect due to hydraulic reaming on the permeability of coal around the borehole, the in-seam hydraulic reaming technology is regarded as engineering background in this dissertation, and the experiment of the seepage characteristics of coal mass containing gas under complex loading and unloading is carried out, then the fluid-solid coupling model of coal mass around drainage borehole is established. In addition, the stress field and gas seepage law of general drainage boreholes and hydraulic reaming boreholes are numerically simulated, and finally the numerical simulation results are validated by the data of the parameters such as residual gas content of coal seam, gas flow of boreholes, which can reflect the gas drainage effect of working face.Firstly, on the basis of the evolutional characteristics of stress for coal mass around boreholes that the tangential stress increases from original stress to peak strength and then reduces to residual strength while the radial stress reduces from original stress until the coal mass damages due to unloading, the gas seepage characteristics experiment for coal containing gas under the stress paths of regular tri-axial loading and complex loading-unloading is carried out by the help of the platform that is the equipment for testing seepage characteristics of loaded coal mass containing gas, and qualitative and quantitative relationship of confining stress, axial stress, stress path, pore pressure and permeability of coal samples. Among them, regular tri-axial loading path is that confining stress is fixed and the axial stress increases until the coal mass is in residual stress stage, but complex loading-unloading path is that axial stress increases gradually while confining stress decrease until the coal mass is in residual stress stage. Especially, complex loading-unloading path is very similar with the stress variation process for coal mass around borehole. The complete stress-strain curves can be divided into five stages including coal dense stage, elastic stage, yield stage, post peak softening stage and residual strength stage, for seepage experiment of coal mass containing gas under the stress paths both conventional tri-axial loading and complex loading-unloading. Under the conditions of constant axial stress and confining stress, the permeability of regular tri-axial loading coal samples sharply decreases and then increases with the increase of pore pressure and presents the "V" shape variation. Under the conditions of constant axial stress and pore pressure, the permeability of regular tri-axial loading coal samples exhibits exponential decline with the increase of confining pressure. Before and after the yield strength, the relations between volumetric strain and permeability of coal samples under conventional tri-axial loading are both exponential. The peak strength and the axial derformation with peak strength of coal samples under complex loading-unloading is less than the one of conventional tri-axial loading, but the radial deformation with peak strength of coal samples under complex loading-unloading is more than the one of conventional tri-axial loading. And along with the continuous increase of axial strain, permeability of coal samples under complex loading-unloading varies as firstly decreased and then increased. The increasing amount of permeability after yield of coal samples due to unloading confining pressure under complex loading-unloading is much greater than the one under conventional tri-axial loading. The changes of initial axial stress in the beginning of unloading can cause the differential evolution of permeability for coal samples in loading-unloading stage. The permeability of coal samples at the initial time of unloading exhibits exponential decline with the increases of initial confining pressure at the initial time of unloading. While the initial confining pressure and axial stress at the initial time of unloading keep constantly, with the increase of pore pressure, the peak strength of coal samples under complex loading-unloading decreases linearly, thereby leads to the accelerated emergence of coal samples failure, and then reduces the drastic change time for permeability of coal samples.Secondly, the determination testing of porosity for coal samples under the stress path of complex loading-unloading is launched. On the basis of the analysis on the effective stress mechanism for the coal samples under complex loading-unloading, starting from the basic definition of porosity and considering the swelling deformation of coal matrix due to adsorption, compression deformation due to pore gas pressure and the thermal expansion deformation, the porosity evolution model of coal sample under complex loading-unloading is estabilished. Then comparing various kinds of domestic and abroad permeability evolution models, making the cubic law and the stress-strain constitutive equation of loaded coal as the bridge, the permeability evolution model of coal samples under complex loading-unloading is also estabilished. Considering Klinkenberg effect and mass transfer characteristics in the process of gas adsorption-desorption-diffusion, coal expansion effect due to gas adsorption, the nonlinear gas seepage equation of coal mass containing gas under complex loading-unloading is deduced. By elastic-plastic analysis on coal mass around boreholes, the analytical expressions of stress field and displacement field for coal mass around boreholes are solved and obtained. Through the implementation of simultaneous equations, multi physical field coupling of stress field, deformation field and seepage field is eventually achieved, meanwhile fluid-solid coupling model of coal containing gas around the boreholes is established. Thereinto, under the conditions of different initial axial stress with unloading, the porosity evolution of coal samples under complex loading-unloading shows as decreased firstly and then increased along the unloading stress path step by step, but the relationship curves of average effective stress and pore pressure have great difference. Under the combined action of outer stress and pore pressure, coal samples under complex loading-unloading are subject to the constraints of effective stress in different loading stages, but the distinction lies in the difference of the proportion in different loading stages for three kinds of effective stress. For the coal samples under complex loading-unloading, there exsit the expressions including αâ†'Φ in compression and linear elastic stage,Φ≤α≤Φd in the yield and post-peak softening stage, Φd≤α≤Φc in the residual strength stage. And the evolutional functions of porosity and permeability for the coal samples under complex loading-unloading can be expressed respectively as coal samples under complex loading-unloading as the theoretical basis, and combining the stress redistribution law of coal mass around drainage boreholes after caving, the permeability evolution characteristics for coal mass around drainage boreholes is analyzed. As the coal mass around drainage boreholes is regarded as a visco elastic-plastic medium, it can be divided into residual strength zone, plastic softening zone, viscoelastic zone and the original stress zone, along the direction away from the borehole. The coal mass in the residual strength zone and the plastic softening zone experience the action of peak stress, and constitute the ultimate stress zone, the scope size of which determines the effect of gas drainage with boreholes good or bad. Considering the rheology, shear dilatancy and plastic softening characteristics of coal mass around boreholes, the visco elastoplastic model is estabilished and the stress-strain solutions in different regions of coal mass around drainage boreholes are deduced. Considering Klinkenberg effect and mass transfer characteristics in the process of gas adsorption-desorption-diffusion, coal expansion effect due to gas adsorption, thermal expansion effect, coal compression effect due to pore gas, the nonlinear gas seepage controlling equation of coal mass containing gas around drainage boreholes is established, including the nonlinear flow equation for gas flow, the evolution equation of permeability and the equations of tangential stress, radial stress and volumetric strain for coal mass around boreholes. Using numerical simulation software COMSOL, the fluid-solid coupling model of coal containing gas around the boreholes is introduced, and gas extraction in working face is regarded as the background, the change rules of stress and strain for coal mass around common drainage boreholes, the change laws and influence factors of gas content and permeability in coal seam are analyzed. Thereinto, along the direction away from boreholes, tangential stress of coal around boreholes overall exhists as increased first to peak, and then dropped to original rock stress, but radial stress of coal around boreholes is characterized by the evolution law of increasing up to the original rock stress. In the same stress environment and under the same occurrence condition of coal seam, along with the increase of the radius of boreholes, stress-relief range of boreholes increases. But under the conditions of different extraction time, permeability of coal mass along the direction away from borehole appears to decrease first and then recover. With the extension of extraction time, residual gas content of coal mass around boreholes in constant area shows a trend of gradual decline. In addition, the simulation result that the effective influence radius for borehole extraction of 30 days is 2.77 m, which has the smaller difference compared with measured value.Finally, according to the characteristics of the hydraulic reaming technology, typical working face is selected for application, and the effect of hydraulic reaming technology application is on-site inspected especially for gas drainage effect after hydraulic reaming in working face. And the excavation model of in-seam reaming boreholes is set up. With the aid of fluid-solid coupling model of coal mass containing gas around boreholes, distribution of stress and gas permeability of coal mass around in-seam reaming boreholes are analyzed and compared with the ones of ordinary drainage boreholes. Meanwhile, the correctness of numerical simulation for the permeability evolution law of coal around the boreholes is validated with field investigation results. After the completion of hydraulic reaming for drainage boreholes, radial peak stress of coal mass around boreholes goes down, and tangential peak stress of coal transfers along the direction away from boreholes. The implementation of the hydraulic reaming technology expands the effective stress-relief range of drainage borehole, and after a period of extraction time for ordinary borehole and reaming borehole, the residual gas content of coal seam around these two kinds of boreholes decrease in different degree, but the decline degree and range of reaming borehole is greater and wider than the others. Although the gas drainage amount of reaming boreholes have no obvious increase compared with the ordinary drainage boreholes, the gas flow attenuation coefficient decreases and the effective extraction time increases. The gas drainage amount for hydraulic reaming boreholes in one month within the influence scope of reaming effect of common drainage holes is more significantly improved than the regular drainage boreholes unaffected by hydraulic reaming.The innovation of this dissertation is mainly reflected as follows:(1) According to the effect of borehole cavation on the changes in stress of coal mass around the borehole, the experiment of gas seepage characteristic for coal mass containing gas under complex loading-unloading is designed. And the influence of the initial axial stress at the initial time of unloading, the intial confining stress at the initial time of unloading, pore pressure and stress path on the evolutional permeability of coal mass is discussed from the experimental data.(2) The determination experiment of porosity for coal samples under the stress path of complex loading-unloading is designed. And the evolutional models of porosity and permeability of coal samples under complex loading-unloading are established on the basis of considering the factors such as effective stress mechanism for coal samples, swelling deformation of coal matrix due to gas adsorption, compression deformation due to pore gas and the thermal expansion deformation.(3) Considering Klinkenberg effect and mass transfer characteristics in the process of gas adsorption-desorption-diffusion, coal expansion effect due to gas adsorption, thermal expansion effect, coal compression effect due to pore gas, the nonlinear gas seepage controlling equation of coal mass containing gas around drainage boreholes is established. According to the comparision between the numerical simulation results of gas migration law and in-field tested residual gas content for surrounding coal mass around drainage boreholes, the nonlinear gas seepage controlling equation is verified.