Dynamic gas liberation from water-immersed anthracite coal with varying wetting states under heating conditions: Unveiling the gas breakthrough phenomena

Abstract

During coalbed methane recovery, the effective extraction of gas trapped in narrow pore throats encounters challenges due to the water blocking effect. To shed light on this phenomenon during thermal recovery of coalbed methane, this study investigated the breakthrough behaviors of CO2 traversing through a 0.2 mm diameter hole in water-immersed anthracite coal at varying temperatures (23 °C, 40 °C, and 60 °C). The measurements of breakthrough pressure (defined as the pressure at which the bubble achieves a semi-spherical shape with a minimum curvature radius), growth characteristics of CO2 bubbles, and coal wettability were conducted under varying thermal conditions and water immersion durations. The results showed that the breakthrough pressure initially increased continuously as water immersed into the coal and then gradually stabilized. During this process, the coal's water wettability increased, leading to a decrease in the diameter of the bubble's three-phase contact line, which in turn affected the breakthrough pressure. Moreover, higher temperatures accelerated the rate of water immersion, augmenting the coal's hydrophilicity. The elevated temperature also resulted in a decrease in water surface tension, consequently contributing to a lower breakthrough pressure. Interestingly, compared to CO2, He exhibited a faster increase in breakthrough pressure during water immersion. This disparity can be attributed to He's superior diffusion capacity and lower viscosity, enabling a swifter transformation of the coal's wetting state during water imbibition. This study revealed the dual mechanism of heating to reduce gas breakthrough pressure, as well as the effects of surface water content and gas properties. These findings highlight potential benefits of thermal treatment on coalbed methane recovery and provide valuable insights for its exploitation under different geological conditions.