Biogenic gas simulation of low-rank coal and its structure evolution

Abstract

For this paper, low-rank coals with Ro,max of 0.53% were selected to carry out the simulation on the degradation of coal by the microbial action. The bacterial solution of biogenic gas simulation was derived from coal mine water containing the methanogenic bacteria. The results show that the origin of produced gas is biogenic, and its formation pathway is dominated by acetic acid fermentation, and the composition of produced gas is mainly CH4 and CO2. Changes in CH4 and CO2 content show a regularity of synchronization in the stabilization period of gas-producing, and the carbon isotopic composition of CH4 is lighter than that of CO2. Due to the inherited isotope fractionation effects, the light carbon isotopes of produced gas are fractionated into CH4 while the heavy carbon isotopes of that are fractionated into CO2. In addition, a significant negative correlation exists between δ13CCH4 and δ13CCO2. The lighter the δ13CCH4, the heavier the δ13CCO2. The enriched tendency of light carbon isotopes of CH4 is significant in the process of biogenic gas simulation. During the 90 days of simulation, both the absorption peak strength of aliphatic -CH2- stretching and that of aliphatic -CH2- and -CH3 deformation show the trend of increasing first and then decreasing. Besides, the microbial action do not only affects the aliphatic structure, but also affects the aromatic structure. It is speculated that the microbes derived from coal mine water may have the ability to make the insoluble structure of coal soluble in part. These research results have a theoretical significance for enriching the formation mechanisms of coalbed methane in low-rank coal reservoirs.