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.