Study On Occurrence Of Coalbed Methane In Situ And Ch₄ Compressibility Factor Effect On Ch₄ Adsorption In Coal

Degassing amounts and its compositions of exploration coal sample from fields and laboratory tests were employed to investigate occurrence of coalbed methane (CBM) in situ. The CH₄ adsorption in coal obtained from laboratory and the degassing amount were used to compare the difference between test data and true value of CBM. The multiple linear regression equation of Langmuir adsorption constants were built based on CH₄ adsorption with SPSS software. The CH₄ compressibility factor for different Equation of State (EOS) was obtained with iterative method and VFP software, and used to investigate the effect on CH₄ adsorption in coal. Finally, the influence of buried depth of coal bed, coal quality, adjacent formation of coal bed and hydrological geology on occurrence of coalbed methane was studied based on their deposited conditions.The studying results show that: (1) the degassing amounts from exploration coal sample in different degassing steps could characterize recovery of coalbed methane and gas outburst trend. The essential component of CBM is methane, CO₂ in CBM could completely be desorbed with heat method, and N₂ in CBM is stored in micropore of coal. (2) The calculated reserves and recovery of CBM based on CH₄ adsorption isotherm in coal is obviously higher than CBM content and real recovery obtained with degassing data in situ. (3) The compressibility factor of CH₄ could be obtained with universal Equation of State (EOS) and special EOS methane by means of iterative method, and its value with different EOS is obviously a great diversity under high pressure. The CH₄ adsorption isotherm obtained with the calculated the compressibility factor of CH₄ is obviously different. The error source of CH₄ adsorption isotherm is the error of pressure measurement. (4) The CBM amount in situ increase with an increase of depth of burial and coal rank, the loosen adjacent-rock of coal bed is adverse to the storage of CBM. The CH₄ composition in CBM increase and N₂ composition in CBM decrease with increase of depth of burial. Heavy hydrocarbons in CBM often are determined for anthracite.The studying results and methods in this paper would provide a basic reference of estimation of CBM resource potential, prediction of CBM developing prospect, establishment of CBM exploitation plan, and prevention and control of coal gas, in theory and practice.