Analysis on adsorption capacity of coal, gas content and methane carbon isotope characteristics in coal: A case study from Southwestern Qinshui Basin, China

The methane adsorption capacity, gas content, and carbon isotope characteristics of coal are crucial parameters that determine the productivity of coalbed methane (CBM) wells and their development potential. In this paper, test analyses of methane adsorption, gas content and carbon isotope of methane were carried out using 89 samples from the No.3 coal seam in the southwestern part of the Qinshui Basin. Their characteristics and correlations were analyzed. A relationship model between methane adsorption, gas content, carbon isotopes, coal metamorphism and material composition were established, and its controlling mechanism was investigated. The results indicate that the distribution patterns of Langmuir volume and Langmuir pressure in No.3 coal seam are mainly determined by the material composition and the thermal evolution level. The methane gas content in coal is mainly affected by the burial depth, microcosmic composition, mineral content, moisture content and ash yield, adsorption capacity and metamorphism of the coal. The methane carbon isotope ( δ 13 C 1 ) values in the natural desorbed gas from No.3 coal seam range from −26.95% to −57.80‰, with a mean value of −34.53‰. δ 13 C 1 in coal shows a two-stage variation pattern with increasing in vitrinite reflectance ( R max o ). When R max o is blow 3.0%, δ 13 C 1 values of methane in coal become progressively heavier with increasing R max o . When R max o reaches or exceeds 3.0%, δ 13 C 1 values exhibit a lightning trend with further increases in R max o , which is primarily controlled by the carbon isotope fractionation effects during thermal evolution.