Research On Failure Mechanisms Of Gas Drainage Through Drilling In Coal Seam And Efficient Sealing Technology

Gas drainage through boreholes is a critical technique of eliminating coal-seam-gas hazards.Not only can the extracted gas be utilized as clean energy, but also it plays a positive role inrelieving the global greenhouse effect. In spite of having abundant gas resource in China, theextracted gas concentration, however, reveals apparent fluctuation and consequently the gasdrainage efficiency is relatively low. The quality of borehole sealing determines drainageperformances directly, therefore, it’s urgent that the study on gas-drainage borehole failuremechanism and efficient borehole-sealing technique should be conducted.In this thesis, firstly, based on extensive investigations on the gas-drainage status andcorresponding borehole-sealing materials&construction process home and abroad, theborehole-sealing mechanism was analyzed by making use of the theory of fluid sealing in themachinery industry. Then the effects of different factors on sealing and the sealing difficultieswere illustrated in two aspects of coal’s double pore texture (fracture and pore) and the changesin coal-seam fracture and permeability caused by the re-distribution of mining-induced stress.Secondly, this thesis put forward the main reason of gas drainage borehole failure is theinfiltration of roadway air. The borehole air leakage not only affects the concentration of gasextraction and causes the loss of negative pressure, but also leads to the spontaneous combustionof coal seam. This thesis theoretically analyzed the air leakage pathways and reasons of theinseam gas drainage borehole, mainly including the coal wall leakage on the side of the roadwayand the borehole sealing section leakage. Furthermore, the influence on gas drainage of boreholedeformation was studied. Results show that the borehole buckling deformation reduces theeffective drainage area, enlarges the gas flow resistance and increases the difficulty of boreholesealing, which affect the drainage efficiency seriously.Thirdly, this thesis put forward the sealing technology of solid-liquid mixed fluid, anddiscussed its sealing mechanism from two aspects of water lock effect and solid particlesclogging effect. Based on the research of its basic characteristics, the material ratio wasdetermined. Then we tested the sealing property of the solid-liquid mixed fluid through physicalsimulation and field tests. It turned out to be that the solid-liquid mixed material is an idealsealing medium. The technology could make the hole sealing simple and effective whichguarantee efficient gas drainage for a long time.Fourthly, aiming at the problem of air leakage on the side of roadway, we simulated theprocess of gas drainage in both cases: with spraying and without through the Fluent software. Itcan be concluded that the wall spraying can reduce the negative pressure loss effectively and itshould be conducted with all effort in gas drainage. Fifthly, aiming at the adverse effect of borehole instability and deformation on gas drainage,a method by filling materials to support boreholes in soft coal seam was put forward, whichadopted the process of "drilling punch, drill back filling and hole sealing extraction". Throughanalysis on the performance of porous materials, we finally chose foam light soil to be the bestfilling material.Finally, the three techniques of spraying on roadway wall, solid-liquid mixed boreholesealing and filling materials to support borehole were refined and combined into the“separating-sealing-supporting” trinitarian technology. What’s more, the superiority andreliability of fully blocking the air leakage passage and efficiently extracting gas of the newtechnology were also discussed.