Mechanism of moisture influence on methane adsorption/desorption in coal under radio frequency heating

Electromagnetic (EM) heating has been recognized as an emerging and promising thermal technique for enhancing coalbed methane (CBM) recovery. This work develops a coupled multiphysical numerical model to assess the effectiveness of radio frequency (RF) heating in promoting water-phase transitions and enhancing CBM recovery. For the assumption of homogeneous and isotropic reservoirs, a two-dimensional axisymmetric model is established to simulate temperature distribution, moisture evolution, and methane desorption behavior. Model validation against laboratory experiments shows that the proposed model accurately predicts RF-induced temperature variations, while neglecting moisture leads to overestimation of heating efficiency and adsorption capacity. Results indicate that water saturation is a key parameter governing RF heating efficiency. Moderate moisture accelerates early heating, whereas high saturation prolongs evaporation and suppresses temperature rise. RF heating significantly reduced the Langmuir adsorption constant, dropping by 94.4% near the wellbore within 76 min. Moisture evaporation expands the dry zone, resolves the “Water lock effect”, and increases the diffusion coefficient by approximately 1.5 times. Competitive adsorption analysis reveals that water vapor generated during heating occupies adsorption sites, thereby enhancing desorption, with a maximum desorption rate of 98.4% near the wellbore. Lower initial water saturation leads to higher methane desorption and gas concentration, while increased RF power expands the effective heating radius and sustains high desorption levels, though excessive power may pose operational risks. Overall, water phase change plays a decisive role in RF-assisted CBM recovery and provides a quantitative basis for optimizing moisture and power input.