Study On Mechanism And Technology Of ECBM By Gas-injection Via Gas Drainage Hole In Da-ping Coal Mine

To enrich the theory of ECBM by gas-injection Under Mine,and to solve the current difficulty that the gas production of drainage borehole is low in the later period in Da-ping Mine “three-soft” coal seam.Firstly we studied the ECBM mechanism by Promotion of Desorption and by Energization.Then we analyzed Gas Migration Rules in Low Permeability Coal Seam and the feasibility of ECBM by gas-injection via Gas Drainage Hole in Da-ping Mine.At last an industrial test of ECBM by gas-injection(0.6MPa compressed air)was implemented in 21161 Bed Plate Tunnel,which obtained certain result.Research found that both N2 and CO2 can decrease the Partial Pressure of CH4,and promotion of CH4 desorption,that is the ECBM mechanism by Partial Pressure and Promotion of Desorption;In terms of Adsorption Potential Theory,N2 cannot replacement of CH4,and CO2 takes effect under a given condition;Based on the principle of Experimental Methods for testing Threshold Pressure Gradient,we create the Field Method,and got Methane Threshold Pressure Gradient of Coal Seam in a state of nature and in the state after Hydraulic Flushing,they are 0.46 MPa/m and 0.12 MPa/m,at the same time we got the permeability under considering Threshold Pressure Gradient.In the later period of Gas-drainage,because of the decrease of Coal Reservoir Energy,a part of methane cannot break Threshold Pressure Gradient and stay at diffusion state.After injecting other gas whose pressure is higher than Coal Seam Methane Pressure,the Mixture-Gas Pressure Gradient increase,which makes diffusion state gas can break Threshold Pressure Gradient to become low-flux nonlinear flow state.As a result,the gas production increases,that is the ECBM mechanism by Energization.Through detailed analysis of gas features before and after gas injection we found that: the process of ECBM by gas-injection was slow and persistent;As for this test,the daily gas production increased every day during gas-injection,and it could be up to 434 m3,which increased 3 times more than the 100 m3 before gas-injection;It`s different each time in four gas-injections,which speculated that ECBM by gas-injection have something to do with position of gas-injection borehole.And gas concentration was trending down slowly during gas-injection,but return to previous level when stop gas-injection.