Comparative Study On Composition And Content Of Coalbed Methane In Different Coal Rank Reservoirs

In order to explore the variation law of composition and content of coalbed methane（CBM）with coal rock,burial depth and coal rank in different coal ranks in China,based on cluster analysis,principal component analysis（PCA）,redundancy analysis（RDA）and correlation heat map analysis（spearman）,the composition and content data of CBM in the’coal/CBM geological exploration report’of the main CBM blocks in 13 CBM provinces and regions in China were studied.The results show that CH₄ and heavy hydrocarbon(C₂₊)in coal are positively correlated with vitrinite in low rank coal,and negatively correlated with vitrinite in medium rank coal.The medium and high rank coal is mostly affected by magmatic activity,resulting in weak correlation between CO₂,N₂ concentration and maceral composition.The higher the vitrinite content in coal in different regions,the higher the gas content of coal seam,the higher the inertinite content,the lower the gas content of coal seam;the concentration of CH₄ in different coal rank and different buried depth coal is the highest,while the concentration of CO₂,N₂ and C₂₊in different coal rank coal increases first and then decreases with the increase of buried depth.The burial depth affects the gas content of the coal seam by affecting the temperature and pressure of the coal reservoir.The positive effect of the reservoir pressure is mainly in the low-rank coal,and the positive effect of the reservoir pressure is mainly in the middle-rank coal below 500 m.In the coal seam of 500-1200 m,the negative effect of the formation temperature is equivalent to the positive effect of the reservoir pressure.The positive effect of the reservoir pressure is mainly in the high-rank coal below 400 m.In the coal seam of 400-1200 m,the negative effect of the formation temperature is equivalent to the positive effect of the reservoir pressure.With the increase of coalification degree,the concentration of CH₄ in coal increases first,then decreases slightly,and then increases with the increase of coal rank.The concentration of C₂₊reaches the highest in the stage of gas fat coal.The concentration of CO₂ and N₂ is the highest in the middle and low coal rank reservoirs,and in the high coal rank stage,a large number of carbon,methyl and nitrogen-containing heterocycles fall off,which also increases CO₂ and N₂ in the coal reservoir.The gas content of coal seam shows a trend of sharp increase-slow increase-sharp increase-sharp decrease with the increase of coal rank.Coalification is the main exploration factor affecting the composition and content of coalbed methane.The research results mainly clarify the coalification control of coalbed methane composition and content under different buried depths,which is of great significance to the development and utilization of coalbed methane and coal mine safety production.