- A gas–air compositional model is developed to evaluate the pre-mining gas drainage processes.
- The model was verified against field data of gas concentration profiles.
- The verified model was applied to evaluate the effectiveness of an in-situ control technology.
Fully coupled compositional (coal seam gas and air) model of coal deformation, coal gas flow and transport, and air flow in coal seams is developed to better understand the gas drainage processes and the coal gas–air mixing mechanisms during pre-mining coal seem gas extraction. The model was first verified by showing that the modelled gas concentration profiles match reasonably with the in-situ measured ones. The verified model was then applied to evaluate how the drained gas concentration could be controlled under different conditions of the sealing length, the leakage rate and the leakage fracture width. These modelled results provides the basis of a new in-situ control technology of gas–air mixing, which uses fine expansive particles to seal the leakage fractures around the borehole. This technology has been commercially applied to enhance the concentration of the pre-mining gas drainage. The field test shows that the characteristics of the leakage fractures are greatly changed after the particles are injected into the in-seam drainage borehole when the gas concentration declines. Once the leakage fractures are blocked with the particles, the outside air is prevented from entering the coal seam. Thus, an ideal gas concentration can be maintained, and the duration of higher gas concentration is extended. For boreholes with an originally low gas concentration (0–30%), the new technology can increase the gas concentration by 10–65% and extend the production time by approximately two to three months. The total amount of gas drainage increases to 2000–3000 m3 per borehole.