The effective extraction radius of borehole is the main parameter for determining the design and layout for coalbed methane extraction. This paper is aimed at the problem that the criteria and calculation methods for the effective extraction radius of coal seams are not comprehensive. With Wudong Coal Mine in China taken as an example, the critical gas pressure (0.441 MPa) for the calculation of effective extraction radius is first defined. Next, the gas seepage unit model around the borehole is developed, and the theoretical mathematical expression to calculate the effective extraction radius is derived. Then, the numerical simulation of gas extraction is carried out through a computational fluid dynamics program, and the effective extraction radius is obtained numerically. Furthermore, the influence of original gas pressure, borehole diameter, and negative pressure of gas extraction on gas extraction is analyzed. Finally, the results from the theoretical calculation and numerical simulation are validated by the field engineering measurement and the average relative error is small (less than 10%). The results indicate that the effective extraction radius has a linear relationship with the extraction time. Moreover, it is found that the original gas pressure is the main factor affecting the effective extraction radius and the effective extraction radius is proportional to the borehole diameter and negative pressure through analysis. Therefore, the above findings can provide a theoretical basis for determining parameters such as borehole spacing and extraction time of gas extraction in coal mines.
In the environments of various open coal storage sites, mining-affected coalbeds, and goafs, etc., some coal bodies are often affected by external environmental factors. They are highly prone to spontaneous combustion in low moisture content (≤8%). In order to examine the effect of low moisture content on the spontaneous combustion tendency of coals with different metamorphic grade, we conducted a temperature programmed oxidation (TPO) experiment and differential scanning calorimetry (DSC) experiment to study the spontaneous combustion characteristics of coals with different metamorphic grade at four different low moisture contents. The change laws of the characteristic parameters of four different metamorphic grade coals at four different low moisture contents were comparatively analyzed. The experimental results indicate that: (1) Compared other low moisture content, anthracite and fat coal at a low moisture content of 1.2 % show a stronger tendency for spontaneous combustion, and long flame coal and lignite at a low moisture content of 3.4% and 5.6% are more prone to spontaneous combustion. (2) Four different metamorphic grade coals at a low moisture content of 7.8% are less prone to spontaneous combustion.
The gas released
from the bottom coal of the horizontal slicing
mining face in steeply inclined and extra-thick coal seams seriously
threatens the safety of the upper slicing mining face. To explore
the seepage characteristics of bottom coal gas, the coal deformation
and gas permeability evolution law of four coal samples in different
stress zones of bottom coal in the working face were analyzed through
true triaxial fluid–solid coupling seepage experiments. At
the same time, the seepage capacity of bottom coal gas was partitioned
according to the field test. The results show the following: (1) The
gas permeability of the bottom coal stress concentration zone first
decreased and then increased with axial pressure loading and confining
pressure unloading. The gas permeability of the bottom coal stress
relief zone increased rapidly with decreasing axial pressure and confining
pressure. The gas permeability of the bottom coal stress recovery
zone gradually decreased with the cyclic loading and unloading of
axial pressure and tended to be stabilized. (2) The evolution law
of gas permeability in the bottom coal was closely related to the
damage and deformation of coal. (3) From the original stress zone
to the stress recovery zone, the gas seepage capacity of bottom coal
can be divided into four zones, namely, the original seepage zone,
the seepage reduction zone, the seepage sharp increase zone, and the
seepage reduction zone. The gas seepage capacity in the stress concentration
zone was more substantial than that of the stress recovery zone. The
results of this study are of great significance for strengthening
the dynamic disaster prevention and control of bottom coal gas in
the horizontal slicing mining face of steeply inclined and extra-thick
coal seams.
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