Review of research on definition, distribution and causes of abnormal coalbed methane composition

Abstract

Although many studies focusing on the causes of the abnormal concentration of different coalbed methane (CBM) compositions were conducted, the unified standard for defining the abnormal concentration of different components is still missed. To clarify the distribution of CBM composition in China and determine the concentration boundary of abnormal composition, based on 4654 measured and collected gas data from coalfield exploration boreholes and CBM wells, which meet the lower limit of CBM reserves estimation in the standard DZ/T 0216—2020. The statistical results showed that the average concentration of CH4, C2+, CO2, and N2 in CBM in China were 91.82%、0.85%、2.04% and 5.19%, respectively, and the proportion of CH4 concentration greater than 90% was 77.44%. It can be defined as abnormal gas composition meeting any of the following four conditions: the C2+ concentration is greater than 5%; the N2/CO2 concentration is greater than 10%; the harmful gas concentration exceeds the upper limit of coal mine safety regulations; the rare gas concentration exceeds that in air. Based on the definition, the results showed that the proportion of C2+ concentration greater than 5% is 5.16% in China and the CO2 concentration greater than 10% accounts for 4.00%. Moreover, the proportion of N2 concentration greater than 10% is 13.26%, and the maximum helium concentration in the southern Junggar Basin reaches 0.97%. Based on the above results, the causes of abnormal CBM composition/concentrations are summarized as the stages of coalification, the differences in coal petrography, sulfate reduction effect, ancient weathering crust, and the migration of gas outside the source. In the subsequent CBM exploration, it is suggested to determine the total gas composition; conduct the study focusing on the gas desorption in coal reservoirs with abnormal CO2 and N2 concentration; determine the helium source rock of coal measures and discuss the law of hydrocarbon helium source and reservoirs to analyze the interaction between organic gas and inorganic gas of coal measures; comprehensively clarify the transformation of geological structure, groundwater flow, microorganism and magmatic activity on gas components to reveal the evolution mechanism of migration, enrichment, and preservation of different gas components in different regions. The results provide a guide for CBM exploitation, coal mine safety production, and CBM utilization.

Keywords

• coalbed methane
• gas composition
• definition of abnormal gas component
• causes of abnormal gas component