Effect of Ash Yield of Deep Coal Reservoirs on Methane Migration Performance

Ash yield affects the pore–fracture structure of deep coal seams, yet its source and constraint on CBM productivity remain unclear. This study investigates coal seam 8 from five wells in the Benxi Formation (eastern Ordos Basin), studying the effects of ash yield and moisture contents on pore structure, the controlling mechanisms of ash yield on pore–fracture and adsorption capacity, and productivity performance via numerical simulation. Results show that samples are classified into three types by ash yield, with Type A exhibiting the largest micropore volume, confirming ash yield as a primary control on micropore development. A coal facies classification integrating TPI, GI, and ash yield is established. Ash yield variations are mainly governed by sedimentary environmental changes and gangue input during coal formation. Productivity simulations reveal that the impact of ash yield on CBM production depends on reservoir saturation state: In oversaturated reservoirs, lower-ash coals yield significantly lower productivity due to reduced initial free gas contents; in undersaturated reservoirs, low-ash coals exhibit higher initial adsorbed gas content and outperform higher-ash coals in both maximum and average gas production.