Enhanced Coalbed Methane Recovery from Lignite Using CO2 and N2

Experimental studies on enhanced coalbed methane recovery (ECBM) are crucial for advancing our understanding of gas extraction mechanisms. This investigation evaluates the feasibility of CO₂/N₂-ECBM application in lignite, a coal type that has received limited attention in prior ECBM studies. The adsorption characteristics of carbon dioxide, methane, and nitrogen in lignite were systematically evaluated, followed by comprehensive CO₂/N₂-ECBM experiments employing a self-developed experimental apparatus under various injection strategies. Key findings include the following: (1) At equivalent pressures (0.5–2.5 MPa), the adsorption capacity of carbon dioxide in lignite exceeds that of methane by a factor of 6–7, whereas nitrogen adsorption reaches 78–88% of methane levels. (2) Carbon dioxide injection predominantly occurs through matrix adsorption, achieving 70% storage efficiency with 20% displacement effectiveness. Conversely, nitrogen migration primarily follows fracture networks, yielding 20–24% storage efficiency while enhancing displacement performance. (3) Constant-pressure injection exhibits the lowest methane recovery among tested methods, particularly due to the nondrainable nature of residual methane trapped in semiclosed fracture systems.