Abstract
To study the shrinkage rule of borehole diameter and its effect on gas extraction, a visco-elastoplastic model for boreholes considering strain softening and the dilatancy characteristic was established to obtain the expressions of the coal stress, variation in diameter, and pressure relief range. The stress distribution and pressure relief effect of the boreholes in soft and hard coal seams were comparatively analyzed. The shrinkage rule of the borehole diameter was studied. The reasons for the rapid reduction in the extraction concentration of the borehole in soft coal seams were described. A technology of improving the gas extraction effect in soft coal seams was developed. The research results showed that the radius of the plastic softening zone is 0.405 m for a borehole in a soft coal seam and 0.224 m for that in a hard coal seam. This indicates that the borehole in a soft coal seam has a better pressure relief effect. The boreholes in both hard and soft coal seams will incur a shrinkage phenomenon; however, the soft coal seam has low strength and a weak ability to resist damage, and thus the surrounding coal will have a more intense creep deformation, leading to an instability failure during a short period of time and thus, a blocking of the extraction channel, thereby causing a rapid reduction in the gas extraction concentration. The borehole in a hard coal seam also shows a shrinkage phenomenon, but remains in a stable state without a blockage; thus, high-concentration gas can be extracted from this borehole for a long period of time. The geo-stress and coal strength are the two main factors controlling the amplitude of borehole shrinkage. From an increase in stress, the borehole in a hard coal seam shows a more intense creep deformation in a deep mine, which may lead to blockage. The key to improving the gas extraction effect in soft coal seams is to maintain a smooth extraction channel. The full screen pipe is installed through the drill pipe to retain an extraction channel, leading to an average gas extraction increase from 0.043 m3/min to 0.12 m3/min, an increase of 2.77 times. These research results are consistent with actual production, and can provide theoretical guidance for determining the gas extraction parameters.
Highlights
Gas extraction is the most important prevention and control measure of coal and gas outburst [1–4]
A hard coal seam has a strong capability to resist destruction, and a borehole in such a seam can remain in a stable state despite a certain amount of shrinkage, and a high concentration of gas can be extracted from the borehole during a long period of time
The gas extraction concentration curves of the soft and hard coal seams in Ligou Mine at different times is shown in Fig 6, which shows that the gas extraction concentration from a borehole in hard coal seams remains at over 40% after 1 months of extraction, whereas that in soft coal seam is reduced to 5% and below after only 15 days
Summary
Gas extraction is the most important prevention and control measure of coal and gas outburst [1–4]. It is very important to establish a visco-elastoplastic model for boreholes considering considering strain softening and the dilatancy characteristic, and to study the borehole diameter shrinkage rule. For the coal yield in the plastic softening zone, the Mohr-Coulomb strength criterion is employed, namely, spy 1⁄4 Kpspr þ spc ; ð3Þ where Kp = (1+sinφ)/(1−sinφ); φ is the internal friction angle of the coal; spr and spy are the radial and tangential stresses of the coal in the plastic softening zone, respectively; and spc is the compressive strength of coal in the plastic softening zone. The volume deformation of coal in the viscoelastic zone is negligible, and the dilatancy characteristic exists in both the plastic softening zone and the damaged zone. Dεbr þ Z2Dεby 1⁄4 0; ð7Þ where Dεbr and Dεby are the radial and tangential strain increments of coal in the damaged zone, respectively; and η2 is the dilatancy coefficient of coal in the damaged zone
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