LCO2-ECBM technology for preventing coal and gas outburst: Integrated effect of permeability improvement and gas displacement

Abstract

More than 60% of China’s coal seams have the feature of low permeability, which severely restricts the gas drainage efficiency when using traditional drilling method. The development of enhanced coalbed methane (ECBM) technologies is essential to prevent coal and gas outburst (CGO) disasters caused by high levels of coal bed methane (CBM). This paper proposes an ECBM technology by liquid carbon dioxide (LCO2-ECBM), based on permeability improvement by thermal damage and CO2-CH4 displacement effect. In-situ experiment conducted in the No. 6 coal seam of Zhangji mine shows the effectiveness of LCO2-ECBM technology, which had drainage efficiency 1.9 times higher than that of the traditional drilling method. The mechanisms of LCO2-ECBM recovery were theoretically deduced and experimentally verified. The evolution of adsorbed pores (ADP) and seepage pores (SEP) in coal samples before and after LCO2 saturation were revealed. The ADP specific area, SEP volume, and permeability in the saturated coal samples were both greater than those of the raw ones. Moreover, the periodic features of binary-gas competitive adsorption in ADP and SEP were revealed by the variation of the CH4 concentration in the displacement test. The feasibility of LCO2-ECBM technology was proved by in-situ experiment and laboratory tests, and it provided a new means for preventing CGO hazards.