Research On Time-varying Characters Of Displacement Coalbed Methane By Gas Injection From Coal Bedding Bore

As a new type technology of strengthening gas extraction, there existed someinsufficiencies in the mechanism, reasonable gas injection parameters and other aspectsfor the technology of displacement coalbed methane technology by gas injection. Thispaper used the means of theoretical analysis, numerical simulation and field test, on thebasis of researching its mechanism, choiced the suitable source correlation, researchedon time-varying characters of displacement coalbed methane by gas injection and gasinjection pressure, drainage negative pressure, drilling interval influence on them,carried on the field tests underground, and verified the consistency of measured resultsand the simulation results.First of all, through analyzing and summarizing the research results at home andabroad, five aspects of the mechanism, namely the displacement of the injected gas, thereduced partial pressure of methane, displacement by sieving adsorption, improvementof coalbed methane diffusion-filtration velocity and enhancement of permeability byfracturing coalbed, were puted forward. Through analyzing the efficiencies ofdisplacing coal methane’s mechanism by CO₂and N₂injection and the influence of thecoal permeability and outburst hazard after gases adsorption, it can be concluded thatalthough the efficiency was better by CO₂injection, the coal seam injected by CO₂would be swelling deformation, which resulted in the permeability decreased and theoutburst hazard increased. Futher, the CO₂injected would lead to non-ignition ofmethane in utilizing system and thus the gas would be of no applicable value. Therefore,N₂would be the optimal gas source.Secondly, the mathematical model of displacement coalbed methane technology bygas injection was established, and the time-varying of this technology and effects of gasinjection pressure, drainage negative pressure and drilling arrangement type on thetime-varying were analyzed by using the COMSOL Multiphysics software. The resultsshow that methane content gradually decreased with time increaseing under certainconditions, and the position near the bore of gas injection was the weak area ofdisplacement technology; the greater the gas injection pressure, the faster the methanecontent decreased and the shorter the elimination outburst cycle of the same location in the coal seam, and the reasonable gas injection pressure was0.5Mpa; the effects ofnegative drainage pressure on displacement methane by gas were not sharp, and thereasonable drainage negative pressure was5-13kPa; the shorter the drilling interval, thedecreasing the methane content; according to the reasonable gas injection pressure andthe drainage negative pressure,obtained that the mathematical relation of the weak areaof displacement technology and the time.Carried on the field tests of displacement coalbed methane technology by gasinjection underground in Shigang company, by comparison with the numerical resultsobtained that under the same conditions, the methane contents of field measurement wasalways slightly smaller than the simulation results in the same position of coalbed, andthe reasons for this difference were also analyzed.