Experimental Study On The Anti-reflection Effect And The Desorption Mechanism Of Low Permeable Coalbed Gas

Generally speaking, the coalbed of China is characterized by its low permeability,which is not conducive to the exploitation of coalbed gas. In order to improve thepermeability of coal, new measures are being studied and used, such as the gas explosiontechnology, the ultrasonic penetration incentive technology, and the high voltage pulsetechnology. The high voltage pulse technology was originally applied to oil exploration,and later it was grafted to the coal seam gas extraction. As a new technology to improvecoal seam permeability, it is still in its initial stage of research, and many relative problemsneed to be solved. In this paper, theoretical mechanics, experimental research and finiteelement analysis software are used to analyze the effect of this technology in increasing thepermeability of coal seam of drilling water injection. The results are as follows:(1) The theoretical analysis shows that the more the coal seam fracture develops in itsdensity, length, width and perforation, the higher the permeability of coal will be, and theeasier methane desorption the coalbed will get. It is derived from the rock mechanicstheory that in the experiment, the hole wall to produce new fracture initiation pressureshould not be less than5.5MPa, and this theoretical calculation results are in goodagreement with the experimental results. And based on the theory of fracture mechanics,the process of the static pressure and pulse pressure variation of crack tip stress intensityfactors are analyzed, so as to understand the evolution of crack propagation.(2) On the basis of the above theoretical analysis, the experiment of permeabilityimprovement by high voltage electrical pulse on coal seam of drilling water injection isconducted in the laboratory. Simulation of3MPa hydrostatic pressure and hydrostaticpressure under the condition of3MPa with10kv High voltage pulse are applied to studythe coal fracturing effect. Then the coal samples are scanned by the CT scanner. Thescanning results are as follows: the original coal is of high density, high hardness, with nocrack distribution. Under3MPa hydrostatic pressure, the coal wall and its surrounding have no cracks. While10kv High voltage pulse is cast in, the fissure number increasessignificantly. It indicates that the high voltage pulse technology in the water can cause anideal permeability increase effect.(3) With the use of ABAQUS in the extended finite element method, the experimentof coal seam permeability improvement by high voltage pulse on the borehole waterinjection is simulated, to analyze the expansion of coal sample fracture. Simulation resultsshow when the hydrostatic pressure gets to3MPa, crack length does not change, but thereis a change of the width; when the pressure reaches7.541MPa, crack begins to extend.After the pulse is loaded, the extension length of crack in the hydrostatic pressure increasessignificantly, and the extended length increases with the increase of initial dischargeenergy. This indicates that the fracturing effect of high voltage pulse on the coal is betterthan the hydrostatic pressure, and the higher the initial discharge energy is, the betterfracturing effect there will be. Accompanied by the hydrostatic pressure and pulse pressure,there is tensile stress at the crack tip, and the tension increases with the increase of stress,which is the main reason for the crack extension.