Optimizing Gas Drainage in Soft Coal Seams by Dynamic Negative Pressure Regulation

Negative pressure gas drainage is the primary measure to prevent coal and gas outburst. Aiming at the low mechanical strength and loose structure of soft coal seams, as well as the rapid attenuation of gas migration and low drainage efficiency under traditional constant negative pressure drainage, this study established a “deformation–diffusion–seepage” multifield coupling gas–air drainage model. Numerical simulations were conducted to compare dynamic drainage processes under six typical negative pressure switching strategies. The results indicate that (1) under the constant negative pressure drainage, low negative pressure (−15 kPa) performs better than medium (−25 kPa) and high negative pressures (−35 kPa); (2) the stepwise increasing negative pressure scheme, which adopts low, medium, and high negative pressures in the early, middle, and late drainage stages, respectively, can better match the dynamic evolution law of gas seepage in soft coal seams, compared with the optimal constant negative pressure; it increases the total borehole gas drainage volume by 5.23% and gas concentration by 2.6% while lowering gas pressure by 5.63%. (3) Moreover, based on the phased variation characteristics of gas flow and pressure during the drainage process, a judgment criterion for negative pressure switching time was established, thereby optimizing the initial stepwise increasing negative pressure scheme. (4) Compared with the initial stepwise increasing negative pressure scheme (plan D), the optimized plan G increases the total gas drainage volume by an additional 3.71% and reduces the gas pressure by an additional 1.73%, thereby further tapping the gas drainage potential. The findings can provide theoretical and technical references for efficient gas drainage in similar soft coal seams.