Study on Mechanism of Pressure Relief and Permeability Enhancement in Soft-hard Composite Coal Seam by Directional Hydraulic Flushing Technology

primary structural coal has the problems, such as high gas content,
high pressure, poor permeability and difficult extraction, which
seriously affect the production safety in the mining. To study the
evolution characteristics of pressure relief in soft-hard composite
coal seam based on directional hydraulic flushing technology,
taking Hudi coal mine in China as the engineering background, and
the pressure relief effect of directional hydraulic flushing on 'two
soft and one hard' composite coal seam was studied by theoretical
analysis and numerical simulation. The stress evolution of the
whole coal seam under a single hole and porous interaction of
different flushing radii were analyzed. The field test of directional
hydraulic flushing in tectonic soft coal seam similar to protective
layer mining was conducted. Results show that when the flushing
diameter is 3-4 m, the influence range of stress disturbance covers
the whole coal seam, and the vertical stress relief of the primary
coal seam is the largest, which provides a theoretical basis for coal
seam pressure relief and permeability enhancement technology. The
average daily gas extraction (ADGE) concentration has doubled,
and the ADGE purity has increased from 0.03 m3
/d to 1.07m3
/d.
The target period of gas extraction is shortened from the expected 6
months to 1 month, and the average daily driving speed of coal
roadway increases from 2.32 m/d to 5.37 m/d. The gas extraction
efficiency is significantly improved, and the risk of gas outburst is
effectively reduced, which has important theoretical and practical
significance for improving the pressure relief and permeability
enhancement efficiency of the soft and low permeability coal seam.