Free and adsorbed gas control P-wave velocity in coal through effective stress and adsorption-induced swelling

Understanding how free and adsorbed gas affect seismic-wave velocity in coal is important for evaluating gas-bearing reservoirs, but their individual roles remain difficult to separate. Here, ultrasonic measurements with helium and methane were combined with simulations on three-dimensional coal models reconstructed from X-ray computed tomography to distinguish the effects of free gas, gas adsorption and pore pressure on P-wave velocity. The results show that free gas mainly changes P-wave velocity by reducing effective stress as pore pressure increases, whereas adsorbed methane alters the coal framework through adsorption-induced swelling. Under constant confining pressure, methane adsorption dominates at low pore pressure and increases P-wave velocity. At higher pore pressure, effective-stress reduction becomes dominant and P-wave velocity decreases. The numerical simulations reproduce these experimental trends and show that the observed non-monotonic velocity response arises from competition between adsorption-induced strengthening of the coal framework and pressure-induced weakening associated with lower effective stress. These findings clarify the distinct but coupled roles of free and adsorbed gas in controlling acoustic properties of coal and provide a physical basis for interpreting P-wave measurements in coalbed methane reservoir evaluation and gas monitoring.