In order to deeply
study the influence of the coal bedding structure
on coal gas adsorption, low nuclear magnetic resonance (LNMR) and
a confining pressure loading system were used to carry out the LNMR
experiment of gas adsorption of high-rank coals with different beddings
under different confining pressures. The results showed that the amount
of gas adsorption of high-rank coals with different beddings increases
with time and decreases with the increase of confining pressure. In
the process from low confining pressure to high confining pressure,
the coal sample with oblique bedding (bedding angles 30°, 45°,
and 60°) has the largest average increment of gas adsorption,
followed by the coal sample with vertical bedding (bedding angle 90°),
and the coal sample with parallel bedding has the smallest increment
of gas adsorption (bedding angle 0°). The linear function relation
between the different-bedding high-rank coal gas adsorption state
and the confining pressure is
y
=
a
–
bx
. The relation between the free peak
area and the confining pressure conforms to the exponential function
y
=
a
+
b
exp(
cx
). Different-bedding high-rank coal adsorption peaks and the peak
area decrease with the increase of confining pressure, and the free
peak continues to move to the left; that is, the large pores gradually
shrink. With the increase of angle and bedding, the area of the adsorption
peak increases first and then decreases, presenting an “inverted
V” shape on the whole. The area of the free peak decreases
first and then increases, presenting a “V” shape on
the whole.
In order to understand the variation mechanism of permeability
and seepage characteristics of high-rank coal with different bedding,
we prepared cylindrical raw coal samples according to the bedding
angles of 0, 30, 45, 60, and 90° and conducted permeability tests
under two stress paths (stress path 1, unloading confining pressure
under constant axial pressure; stress path 2, simultaneous loading
axial pressure and unloading confining pressure). The results show
that the relationship between the permeability and effective stress
of high-rank coal with different bedding in the two stress paths conforms
to an exponential function, and the permeability increases gradually
with an increase in differential stress. Under the two stress paths,
the initial permeability of different bedding under the loading axial
pressure and confining pressure shows a pattern of a maximum for parallel
bedding coal samples, followed by oblique bedding coal samples, and
a minimum for vertical bedding coal samples. Under path 1, the increase
in the permeability of the oblique bedding is 21.4 times that of the
vertical bedding and 14.94 times that of the parallel bedding, and
under path 2, the increase in the permeability of the oblique bedding
is 26.45 times that of the vertical bedding and 142.11 times that
of the parallel bedding; the coal samples of the oblique bedding suffer
the greatest damage. The increase in the permeability of parallel
bedding coal samples, oblique bedding coal samples, and vertical bedding
coal samples under path 2 is 1.47 times, 13.96 times, and 11.3 times
the increase in the permeability of the corresponding coal samples
under path 1, respectively, and the damage produced by coal samples
under path 2 is greater than that under path 1.
To investigate the effect of anisotropy of coal body
on the gas
extraction effect of cis-borehole, the anisotropy permeability model
of coal based on structural anisotropy ratio and flow-solid coupling
model were established at a working face of Zhongmacun mine Henan
Province, China, as the research object, and COMSOL numerical simulation
software was used. The results show that considering coal anisotropy,
the gas pressure decreases more faster than that without coal anisotropy,
and the farther away from the borehole, the smaller the difference
between them. The extraction time was a logarithmic function of the
effective extraction radius, the negative extraction pressure was
an exponential function of the effective extraction radius, and the
borehole diameter satisfies a power function relationship with the
effective extraction radius. The variation of gas pressure with extraction
time in different stratigraphic directions was analyzed, and gas pressure
decreases faster in parallel stratigraphic directions and slower in
vertical stratigraphic directions. Considering the complexity and
safety of gas extraction at the working face, a 30% redundancy factor
is added to determine the maximum magnitude and range of gas pressure
drop when the spacing of cascade drill holes in a working face of
Zhongmacun mine Henan Province, China, is 6 m, which can avoid the
superposition of “blank zone” and ineffective extraction.
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