Displacement Effect Of N₂/CO₂ On Methane In Coal Samples With Different Moisture Content

In order to study the difference of CH₄replacement effect of CO₂/N₂in coal samples with different moisture content,four coal samples with 0%,0.75%,1.5%and3%moisture content were selected for gas injection experiment.In this paper,the anthracite of no.15 coal seam in Yangquan mining area in the northeast of Qinshui Basin is taken as the research object.Four coal samples with moisture content of 0%,0.75%,1.5%and 3%are selected.Isothermal adsorption experiments of CH₄,CO₂and N₂on four kinds of coal with different moisture content and high-pressure injection experiments of CO₂and N₂replacing CH₄in different coal with different moisture content were carried out respectively.According to the experimental results,the adsorption characteristics of coal samples with different moisture content for different gases were obtained,and the displacement effect of CO₂/N₂gas replacement for CH₄in coal with different moisture content was analyzed,revealing the difference of CH₄in coal with different moisture content for CO₂/N₂replacement.The specific conclusions are as follows:（1）The order of adsorption capacity of coal with the same moisture content to gases with different adsorption capacity is CO₂>CH₄>N₂.The adsorption amount of the same gas decreases with the increase of the moisture content of coal.The adsorption amount of gas is as follows:0%coal sample with moisture content>0.75%coal sample with moisture content>1.5%coal sample with moisture content>3%coal sample.（2）In the displacement experiment of replacing CH₄in coal with different moisture content by high-pressure injection of CO₂,it was found that when the equilibrium pressure of CH₄pre-adsorption was 1.0MPa,the effect of CO₂replacement decreased by 8%,13.8%and 18%in the three stages of coal sample moisture content 0-0.75%,0.75-1.5%and 1.5-3%,respectively.It can be concluded that CH₄desorption rate and CO₂injection ratio are negatively correlated with coal water content.Both CH₄desorption rate and CO₂injection ratio showed a trend of sharp increase first and then slow increase with the increase of CH₄preadsorption equilibrium pressure,and the transition point was 1.3MPa.（3）In the replacement experiment of CH₄in coal with different moisture content,N₂was injected under high pressure to replace CH₄.It was found that when the equilibrium pressure of CH₄pre-adsorption was 1.0MPa,the replacement effect of N₂decreased by 21.5%,25.5%and 23.7%in the three stages of coal sample moisture content 0-0.75%,0.75-1.5%and 1.5-3%,respectively.It can be concluded that CH₄desorption rate and N₂injection ratio are negatively correlated with coal water content.Both CH₄desorption rate and N₂injection ratio showed a trend of sharp increase at first and slow decrease with the increase of equilibrium pressure of CH₄preadsorption,and the inflection point was 0.75MPa.（4）When the equilibrium pressure of CH₄pre-adsorption is 1.3MPa and the moisture content of coal sample is 1.5%,the CH₄desorption capacity,CH₄desorption rate and CO₂injection ratio in CO₂injection experiment are 3.97cm³/g,20.45%and28.36%,respectively.In the N₂injection experiment,CH₄desorption capacity,CH₄desorption rate and N₂injection ratio were only 0.24cm³/g,1.24%and 1.71%.It is found that CO₂injection under high pressure is better than N₂injection to replace CH₄in coal.The effect of inhibiting CO₂replacement is greater than that of inhibiting N₂replacement under the same experimental conditions.（5）The effect of moisture on CH₄in gas-displaced coal is mainly affected by water content,surface free energy of coal,intermolecular force and pore structure of coal.The research results are expected to provide data reference and theoretical guidance for underground gas injection to replace coal seam gas and other engineering practices.There are 30 figures,33 tables and 96 references.