Seepage Characteristics Around Gas Drainage Borehole And Blockage Mechanism Of Powder

Methane extraction from boreholes is the main approach for prevention andcontrol of methane disasters in coal mine, and borehole sealing is an important linkwhich determines methane extraction effect. However, the growth of coal and rockcracks in the process of methane extraction leads to low concentration and fastattenuation, leaving a significant hidden danger: there is risks of explosion that lowconcentration methane in methane extraction system is easy to fall into the explosionlimit range; there is high concentration oxygen in the cracks around borehores that islikely to cause the oxidation and spontaneous combustion of coals, may inducingmethane burning. In order to improve the methane extraction concentration, this paperstudies the seepage characteristics around borehole and blockage mechanism ofpowder, which presents theoretical support for effective plugging the leakage cracksaround boreholes and the improvement of methane extraction concentration. The mainachievements are as follows:Using the combination of methods including theoretical analysis, model test andfield test to study the fracture distribution characteristics around boreholes:(1)Fracture zone induced by borehole excavation is based on the formation of thefracture zone induced by roadway excavation along borehoe axial "first increases,then decreases, then stability";(2) After borehole excavation unloading, the zonesabove and below the borehore collapse, on the left and right sides become brokenzones, and the round shape of borehoe finally comes to a "class a football" form, andits long axis and the maximum principal stress are vertical in the direction;(3) Thereare the macroscopic and microscopic straight, bending, and bifurcation slits in fracturezone, most width and length of the macroscopic slits are less than5mm and50mmrespectively.The effective stress and permeability redistributions induced by successiveroadway and borehole excavations, are studied through the methods of laboratory testand numerical calculation:(1) After roadway excavation, four zones, including thefull flow zone, the transitive flow zone, the flow shielding zone, and the in situ rockflow zone are consecutively formed along the radial direction of the roadway. Afterborehole excavation, these zones expand along the borehole direction;(2) Theeffective stress-permeability relationship in the full flow zone and the flow shieldingzone is a piecewise exponential function, and the coal permeability reduces with the increase of methane pressure. The permeability of the coal mass in the full flow zoneis two orders of magnitude higher than that of the original coal, and the permeabilityin the flow shielding zone is minimized at a value two times lower than that of theoriginal coal mass;(3) For all coal seams with low permeability when drainingmethane around boreholes, the flow shielding zone may prevent the methane in thiszone and the other zones from flowing into boreholes, whereas air in the roadwaycould easily pass the full flow zone and enter the boreholes, which eventually leads tolow methane concentration obtained from boreholes.Application of the two-fluid model to research the movement of gas-solidtwo-fluid flow in slit:(1) Before the blockage of a straight slit, at the bottom of theparticle sedimentary sequences creep travel forward in the form of integral, and theblockage time is very small;(2) The importance of the various factors affectingblockage is as follows: feeding amount> superficial gas velocity> bending angle.The bending angle of bending slit has less effect on the blocking process;(3) Themain pipe section of bifurcation slit is easy to block, and obtuse angle of bifurcationslit is easier than acute angle to block.The experiment platform for simulating the blockage of slits using powder wasestablished independently to research the blockage characteristics of straight, bendingand bifurcation slits:(1) For the bending slit, the solid mass flow rate for blockageincreases as the air velocity increases. For the straight slit, the relationship betweenthe solid mass flow rate and air velocity is also like that and it could be divided intothree typical stages in turn: medium growth stage, slow growth stage and rapid growthstage. The straight slit has stage of medium growth which the bending slit isinexistence;(2) The solid loading ratio and air velocity in the bending slit display apower function decreasing relationship. This finding is in agreement with existingtheory and experiment results. What is different, however, is that we found aminimum solid loading ratio exists. When air velocity is greater than the minimumsolid loading ratio’s corresponding velocity, the solid loading ratio and air velocityactually exhibit a power function increasing relationship;(3) The solid loading ratioand air velocity in the straight and bifurcation slits display a "S" shape relationships:with the increasing of air velocity, the solid loading ratio increases at first, thendecreases, and finally increases. Because low speed conveying of powder is easy tocause pipe blockage, straight and bifurcation slits also have the minimum solidloading ratio;(4) The formulas about the blockage condition of straight, bending and bifurcate slits are established, and the relative error of formulas is in the reasonablerange, which can be used in Engineering.The results of field experiments in No.8Mine of Pingdingshan Mines showedthat the application of the research results in this paper could increase theconcentration of methane two times as high as the original one of less than30%in theboreholes.