Study On Mechanism And Controlling Of Borehole Collapse In Soft Coal Seam

The soft and methane-rich coal seam, widely distributed in China, is the suitablebed for disasters related with methane and coal dynamic activities, the main treatmentfor which is drilling boreholes into the panel area prior to long-wall mining to reducethe methane content of the coal bed. With such conditions, borehole collapse oftenoccurs because the coal is too soft, resulting in a loss of gas drainage. The originalobject of this paper is to investigate the instability of the gas-drainage borehole in softand methane–rich coal seam. Based on mathematical model, numerical simulation,experiments and field test, collapse mechanism and control techniques for collapseborehole were studied in this paper.The linear elastic model coupled with gas seepage and the failure of hole-collapsewas established. The relation between gas seepage and the collapse pressure wasdiscussed. Effects of basic mechanical parameters of soft coal on borehole collapse,such as gas pressure, drilling track, drainage time and so on, were analyzed.Conclusions show that:The instability of borehole in soft and methane-rich coal seam is serious if the gascontent is huge, especially at the initial period of gas drainage. But it will declinequickly after a short time of drainage. As a result, it is necessary for theprevention of collapse in initial drainage.When the inclination angle of borehole is less than60°, the instability ofborehole is serious when the azimuth angle is equal to35°. At the same time,when the inclination angle of borehole is above60°, the stability is worst as theazimuth angle equal to90°.For boreholes across weak intercalation and complicated geological structures,the enhancement of hole stability is workable by keeping drilling at the angle of22.5°.Taking damage mechanics into account, the visco-plastic coupled with nonlinearcreep model of soft coal was established. Based on creep experiment of soft coalsample, rheological parameters were obtained by fitting conic to scattered data. Theeffect of the key elements on creep properties was also investigated.Creep properties of boreholes both with and without support were studied. Meanwhile,effects of parameters such as depth, gas pressure and coefficient of lateral pressure oncreep characteristics of soft coal were researched upon the visco-plastic coupled with nonlinear creep model. It is concluded that:When the depth exceeds600m, the creep rate of borehole starts to speed up,resulting in the increase of vertical displacement and plastic zone range aroundborehole. Then, the borehole will suffer variable diameter.When coefficient of lateral pressure is above1.2, the amount that remains afterthe maximum principal stress is subtracted from minimum principal stress is thebiggest on both the top and the bottom of borehole, where will suffer from themost serious shear failure.The pore pressure is proportional to the vertical displacement of borehole, whichdoesn’t have an influence on both the maximum vertical stress and the plasticzone.With the enhancement of borehole supporting, the reductions of verticaldisplacement and the intensity of acoustic emission are found in the physicalexperiments.According to the decrease of quantity of gas flow, the instability of borehole wasseparated into four grades. At the same time, three methods for the installation of linerto enhance the stability of boreholes with the fourth, the third and the second degrade.Then, the synergism of drilling and inserting of liner integrated with the matched bitand boring pipe was test in the field, showing that the quantity of gas flow fromboreholes completed by this test was improved to twice than that by no supporting.