Gas Drainage Theory And Technology In Underground Coal Mine Based On Hydraulic Fracturing

Gas drainage is a fundamental measure for coal and gas outburst prevention, gas overflow at working face and mine ventilation support ability. Coal reservoir permeability is the key factor for gas drainage, so how to improve the permeability of coal seam has become an important research subject.In no relief state gas drainage depends entirely on original coal seam permeability. The current drainage in low permeability coal reservoir are mainly dependent on the decrease of drilling spacing, increase of drilling works, the extension of drainage time and a variety of hydraulic measures, but the drainage effect is not ideal, and regional outburst removing is quite difficult. With great cost of the project, gas drainage effect is limited. Improving coal reservoir permeability in no relief state is to be solved, so this paper mainly discuss how to"transplant"hydraulic fracturing technology for the ground coalbed methane(CBM) wells to the underground coal mine and increase the coal reservoirs permeability.By comparing acoustic velocity, the fractal dimension and geological strength index(GSI) quantitative characterization of coal structure, GSI is the optimized means according to the relationship between parameter and permeability and access method. The permeability prediction model and mechanical properties acquisition of nonholonomic coal body is established based on GSI parameter. According to control factors of the coal deformation, rock mass strength and the fractal dimension are selected as indicators to predict the spatial distribution of coal structure, which is a foundation for hydraulic fracturing layer and process selection. It also provides an important direction for CBM development-"one district one policy" and "one well one method".The diffusion and low non-linear flow in addition to linear flow are revealed in the light of start-up pressure gradient. With mercury injection and gas natural desorption experiments, the pore size distribution of GSI>45 and GSI<45 coal reservoir is confirmed of significantly different and the latter shows the pore structure of "double peaks" feature. Acorrding to start-up pressure gradient and the pore structure, gas migration constitutive equation based on the starting pressure gradient and bidisperse pore model is established, i.e. low non-linear flow and two-level diffusion model, which provide a theoretical basis for process select of the underground drilling hydraulic fracturing.Two group parallel samples are chosen by acoustic velocity and permeability experiments. Under the acoustic emission monitoring, the permeability change trend with stress-strain is tested, and GSI value is record at different stages of coal structure. According to the time of each group parallel samples, the experimental data(acoustic emission, stress-strain, permeability and GSI) is full of docking, and the coupling relationship of stress and strain-coal structure-permeability is built up. Due to coal reservoir permeability-GSI value and the Reynolds number-permeability, the relationship model between coal structure GSI and the Reynolds number was established. Based on Reynolds number cut-off point of linear flow-nonlinear flow-diffusion, GSI interval of different flow patterns is calculated. Hydraulic fracturing is equivalent to decreasing coal structure GSI value, which promotes gas flow pattern from diffusion to nonlinear flow (with start-up pressure gradient) to linear flow, so increasing permeability mechanism by hydraulic fracturing lies in the flow pattern change.Hydraulic fracturing has multiple effects in addition to increasing permeability. By experiment comparison of coal containing water or not about the start-up pressure gradient, permeability and initial speed of methane diffusion, gas emission inhibition mechanism after water flooding lies in increasing the start-up pressure gradient of coal matrix. By mechanical properties test of coal samples with different saturation, the strength of"hard coal"decreases, but which of"soft coal"improves based on powder mechanics. At the same time, hydraulic fracturing can homogenizate gas pressure and in-situ stress field, which is helpful to reduce rockburst and gas outburst disaster.Based on isotherm adsorption experiment and CBM development practice, surface modification to coal reservoir via chlorine dioxide reduces the capacity of adsorption methane. According to permeability test and surface etching observation under the microscope when before and after coal and chlorine dioxide, the mechanism of chemical enhance permeability is identified. Under adsorption constants and initial speed of methane diffusion impact of guar gum, coal reservoir damage and remove by chlorine dioxide experiment, fracturing fluid system with chlorine dioxide injection is established for underground drilling.With quantitative characterization GSI of coal structure, the hydraulic fracturing parameters such as the tensile strength, fracture pressure, filtration coefficient and fracturing radius are optimized, which provides the appropriate calculation model of construction parameter for different GSI coal reservoir. And hydraulic fracturing effect evaluation method is preliminarily formed.By many hydraulic fracturing field tests in Hebi Zhong Tai Mining Co.,Ltd and Sijiazhuang Mine in Yangquan, and ultimately hydraulic fracturing process based on coal structure is achieved, which includes drilling arrangement, sealing technology, pumping procedure and effect testing. Hydraulic fracturing realizes six targets, namely, "enhance permeability, gas emission inhibition, the balance of in-situ stress and gas pressure field, coal strength change and dust fall", which lays theoretical and technology foundation for engineering application.