Surface jump mechanism of gas molecules in strong adsorption field of coalbed methane reservoirs

The gas in the coalbed methane (CBM) reservoirs is mainly in the adsorbed state, and the influence of surface jump of adsorbed gas on CBM migration and gas well productivity cannot be ignored. In this paper, firstly, the concepts of strong adsorption field and gas molecular equivalent motion volume are proposed, and the analytical relationship between gas filling degree and equivalent molecular free path is established. Secondly, an analytical model of adsorbed gas concentration and equivalent molecular free path is established, and the linkage mech-anism of " gas filling degree -equivalent molecular free path" of gas occurrence in strong adsorption field is revealed. Thirdly, the molecular dynamics mechanism of gas occurrence and surface jump in the strong adsorption field is revealed, and the coupling effect mechanism of "coal rank-pressure" on microscale surface jump is clarified. It is found that (a) the equivalent motion volume of gas molecules can effectively characterize the gas filling degree in the strong adsorption field; (b) when the activation energy is higher than the isosteric heat of the strong adsorption field, gas molecules enter the adsorption/desorption balance layer and transfer to different restricted areas; (c) when the temperature and adsorbate are determined, the coal properties and pressure are the keys to control the surface jump.