Experimental Study Andapplication Of Radial Gas-flow-characteristics Around Hydraulic Borehole

For controlling the coal and gas outburst of a single coal seam with low gaspermeability, variety of hydraulic drillings such as hydraulic punching, water injectionand hydraulic fracturing etc. were developed. However, the mechanism of moistureeffecting on characteristics of Coalbed Gas Seepage (CGS) in gassy coal is still lackof in-depth study. In this paper, the properties of Gas Radial Seepage Around theHydraulic Drill of (GRSAHD) were studied by laboratory tests, numerical simulation,field tests and application. The results are as follows.The experimental set-up of the GRSAHD was developed. Experiments of radialsteady/unsteady flow, water injection and gas dispersing, isothermal adsorption ofcoal with wetting and high-pressure water injection can be conducted. Meantime, thisequipment can meet the experimental conditions of different overburden pressure, gasseepage pressure, gas type, borehole diameter and the moisture content, as well asmonitor the water inlet pressure and boundary pressure under different flow.Gas pressure, adsorption, overburden pressure and the influence of drilling reliefon coal permeability were researched based on steady radial-seepage-experiment.Kinetic processes of water injection and gas dispersing were analyzed. Inner and outerboundary pressure evolution during the water injection and gas dispersing were tested.Four stages of water injection and gas dispersing were raised. Overburden pressure,raw gas pressure, water flow and other factors on the coal seam gas seepagecharacteristics of water injection and gas dispersing were researched. In addition, themoisture content distribution of coal around the borehole was also obtained.The physical model of GRSAHD was raised. In this model, coal isothermaladsorption characteristics and pore characteristics affected by the moisture werestudied. The influence of water/CH4competitive adsorption on the permeability ofcoal was tested. Laboratory determination of coal seam gas permeability affected bythe moisture was established. Finally, the prediction model of CH4permeability in wetcoal was built and verified by experiments.Distribution of radial flow field around borehole under different pre-pumpingtime, borehole diameter, original gas pressure and permeability model was simulated,which also verified the GRSAHD.For the first time, the radial deformation of hydraulic drilling with differentborehole diameter was measured by the FBG technology. The deformation of radialexpansion was used as the indicator for evaluating the effect of drilling relief. The field test shows that expansion of deformation of coal which away from the center ofdrilling1.35m~4.35m is40.47‰~1.92‰. The diameter of coal increases from160mm to579mm. The diameter of drilling relief increases by1.71m. The fieldexperiment of regional outburst prevention technology by hydraulic drillings inQINGDONG coal mine suggested that the coal permeability coefficient increased61.1~144.7times after treated by this method. In the heading face, the stressconcentration is moving to the deep, stress concentration factor is reducing, and thegas desorption index is significantly lower. The effect of Regional outburst preventionis remarkable.