To research the elasticity of gas-bearing coal fluid-solid two-phase medium with seismic exploration method is critical to the prevention of gas disasters. To investigate the elasticity, the ultrasonic elastic test of anthracite samples under different gas pressures was carried out and the ultrasonic velocity and anisotropy of the samples were analyzed in this study. The results show that the velocities (P- and S-waves) decrease in turn in the strike, dip, and vertical directions. However, a negative linear correlation is proved to exist between ultrasonic velocity and gas pressure. With the increase of gas pressure, the anisotropy degree of both the P-wave and the S-wave of the samples decreases but the declining degree of the P-wave is greater than that of the S-wave. In addition, the decrease in velocity and the anisotropy degree of the P-wave is greater than that of the S-wave, indicating that the P-wave is more sensitive to gas pressure changes in terms of velocity and its anisotropy degree.
The condition of complex multiphase medium, which is composed of the material composition and structure of coal and fluid gas, is the bottleneck for the mechanism study of coal and the prevention of gas disasters [
In China, when the gas pressure in a coal seam reaches or exceeds 0.74 MPa, the coal seam can be defined as an outburst coal seam and corresponding outburst prevention measures should be taken [
In summary, many researches have been conducted on the variation law of seismic wave parameters of outburst coal through in situ seismic wave test of coal seam but it is difficult to reveal the mechanism scientifically due to the limitation of site conditions and complex influence factors. Some scholars have studied the ultrasonic elastic parameters and anisotropic characteristics of coal samples such as velocity anisotropy, Poisson’s ratio, and attenuation with different degrees of metamorphism under normal temperature and atmospheric pressure conditions through laboratory coal physical experiments. However, the ultrasonic experiments of coal samples are mainly aimed at single-phase solid coal. Considering that gas-bearing coal is a fluid-solid two-phase medium and the limitations of test equipment and conditions, the current related research only focuses on single-phase solid coal and the results are lacking of representativeness and persuasion.
The purpose of this paper is to study the influence of fluid gas on the ultrasonic velocity and anisotropy of coal samples. Therefore, the ultrasonic velocity response characteristics of coal samples under different gas pressures are tested and the effect of gas pressure on the ultrasonic anisotropy of coal samples is analyzed. The research results of this paper may provide some guidance for coal and gas disaster prediction.
The samples in this study were collected at the tunneling face of the Xinjing Coal Mine in the Yangquan coalfield (Figure
Study area location and detailed stratigraphic column of coal-bearing strata in the Yangquan coalfield.
Schematic diagram of coal sample acquisition.
In Figure
The size of coal samples is measured by a vernier caliper, and the density is measured by the drainage method [
Basic parameters of coal samples.
Sample ID | Size ( |
||||||
---|---|---|---|---|---|---|---|
#8-A | 1.62 | 2.00 | 10.32 | 10.29 | 33.73 | 1.42 | |
#8-B | 1.60 | 1.98 | 10.37 | 10.25 | 33.69 | 1.41 | |
#8-C | 1.61 | 1.99 | 10.37 | 10.18 | 33.71 | 1.42 | |
#8-D | 1.63 | 2.01 | 10.25 | 10.35 | 33.77 | 1.45 | |
#8-E | 1.59 | 1.98 | 10.41 | 10.20 | 33.67 | 1.40 |
The ultrasound pulse transmission method is used in the experiment [
Test errors are inevitable due to the experimental method and human factors [
As shown in Figure The experiment is conducted under normal temperature and atmospheric pressure (298 K, 1 atm) To ensure the coupling between the probe and the sample surface, the Shear Gel is applied on the probe surface after the prepared sample is put into the coal clamp and then covered with the top lid To check the air tightness of the system, it is observed whether the piezometer reading is changed or not. A vacuum pump is used to vacuum the sealed cylinder block. When the negative pressure in the chamber is stabilized at -0.1 MPa, the vacuum should be stopped. The acoustic signals in the When the vacuum test is finished, the cylinder body is filled slowly with the high-pressure mixture gas (CO2 As can be seen from Figure The signals collected are calculated, processed, and analyzed (refer to Section
Ultrasound anisotropy testing system of coal samples (simplified).
Gas adsorption capacity of coal samples under different gas pressure conditions.
#8-A | #8-B | #8-C | #8-D | #8-E | |
---|---|---|---|---|---|
-0.1 | 0 | 0 | 0 | 0 | 0 |
0.2 | 7.46 | 7.41 | 7.46 | 7.59 | 7.37 |
0.4 | 12.22 | 12.15 | 12.21 | 12.40 | 12.09 |
0.6 | 15.52 | 15.44 | 15.51 | 15.71 | 15.37 |
0.8 | 17.94 | 17.86 | 17.93 | 18.14 | 17.79 |
1.0 | 19.79 | 19.71 | 19.78 | 19.99 | 19.64 |
1.2 | 21.26 | 21.18 | 21.24 | 21.45 | 21.11 |
1.4 | 22.44 | 22.36 | 22.43 | 22.62 | 22.30 |
Deformation curves of isogradient pressurized adsorption and gas adsorption capacity of the coal samples.
Since the five anthracite samples are all taken from large coal samples from the same coal seam, the measured ultrasonic velocity values can denote the comprehensive response values of various material components, pore structure, and bedding plane of coal. Therefore, the average value of the ultrasonic velocity data of the five anthracite samples is analyzed and the analysis results (shown in Table
Test results of P- and S-wave ultrasonic velocities of the coal samples under different gas pressures.
-0.1 | 1793 | 1733 | 1677 | 982 | 926 | 871 |
0.2 | 1733 | 1673 | 1650 | 959 | 911 | 863 |
0.4 | 1706 | 1653 | 1635 | 951 | 905 | 858 |
0.6 | 1691 | 1644 | 1627 | 947 | 901 | 851 |
0.8 | 1683 | 1635 | 1614 | 943 | 897 | 847 |
1.0 | 1675 | 1629 | 1607 | 937 | 893 | 845 |
1.2 | 1664 | 1621 | 1600 | 933 | 889 | 841 |
1.4 | 1652 | 1613 | 1596 | 927 | 886 | 841 |
The ultrasonic elasticity test indicates that the ultrasonic velocity of coal samples shows similar downward trends with the increase of gas pressure. As shown in Figures
Characteristic charts of ultrasonic velocity of coal samples at different gas pressures.
According to the results of previous researches [
The adsorption of gas leads to a decrease in the strength of coal samples [
From Figure
Measured
Specifically, as shown in Figures
Previous studies [
According to the existing research, under the constant gradient pressure conditions, the adsorption expansion deformation of coal samples increases as time goes by but the cumulative deformation eventually tends to remain at a stable value. The isocratic pressure in the reference [
Table
Adsorption deformation constant and quantity of coal samples.
Sample ID | |||
---|---|---|---|
#8-A | 3.704 | 2.250 | 2.81 |
#8-B | 3.563 | 2.198 | 2.69 |
#8-C | 3.624 | 2.283 | 2.76 |
#8-D | 3.926 | 2.277 | 2.99 |
#8-E | 3.677 | 2.221 | 2.78 |
The gas-bearing coal sample is a typical fluid-solid two-phase and two-porosity medium, and the sample is composed of solid skeleton, pore, fracture, and gas filled in the pore and fracture [
In this paper, only the P-wave and S-wave are studied. Accordingly, the influence factor of P-wave and S-wave velocity variations only includes the solid skeleton. On the one hand, the skeleton deformation is caused by gas adsorption. In the previous study, the effect of gas adsorption deformation on anthracite samples is small, so adsorption deformation has little effect on the wave velocity of anthracite samples. On the other hand, with the increase of gas pressure, adsorbed quantity of gas increases and adsorbed quantity of gas on the skeleton surface increases, which in turn leads to the decrease of solid skeleton strength. Ultimately, the velocities of the P- and S-wave decrease with the increase of gas pressure.
It is known that with the increase of gas pressure,
Anisotropy of
As can be seen from Figure
The relation between CT image threshold and anisotropic value from different views.
With the increase of gas pressure, the anisotropy of wave velocity of coal samples keeps the same difference as that of vacuum but decreases gradually on the whole and the decrease of P-wave velocity anisotropy is much larger than that of S-wave velocity anisotropy. Specifically, the decreases of P-wave velocity anisotropy in the strike direction
Therefore, the most possible cause is that the coal samples undergo adsorption due to the existence of fluid gas [
Many achievements have been made in the ultrasonic elasticity test of single-phase solid coal seam. However, in the actual gas geological condition, coal seam is not a single solid medium but a two-phase and two-porosity. So, the ultrasonic velocity and anisotropy of gas-bearing coal two-phase and two-porosity are studied in this paper. Relevant results can be mainly applied in seismic exploration of a high-gas mine, which can provide theoretical guidance for the prediction and prevention of coal and gas disasters by identifying abnormal areas of gas enrichment in seismic exploration. Finally, the following conclusions can be drawn.
In the strike, dip, and vertical directions, there is a negative linear relationship between the ultrasonic velocity and gas pressure. It is manifested by the fact that the ultrasonic velocity (P-wave and S-wave) decreases with the increase of gas pressure. As for the cause of this phenomenon, it can be concluded that the increase of gas pressure leads to the increase of gas adsorption quantity on the skeleton surface, causing the decrease of skeleton strength, and resulting in the decrease of P-wave and S-wave velocities propagating in the solid skeleton of gas-bearing coal samples. Moreover, the correlation coefficient between the ultrasonic velocity and the gas concentration in the vertical direction is significantly higher than that in the strike direction and the dip direction The degree of S-wave anisotropy is greater than that of the P-wave. In addition, The decrease in P-wave velocity and its anisotropy is greater than that in the S-wave, which indicates that P-wave velocity is more sensitive to gas pressure changes
In the actual seismic exploration, under the constraints of drilling and logging, the above laws of the ultrasonic anisotropy of the coal samples affected by gas may provide some theoretical guidance for the prevention of gas disasters.
The data used to support the findings of this study are available from the corresponding author upon request. The data of this manuscript is based on previous studies and obtained through experiments in the laboratory. Therefore, the data in this paper are all first-hand data and the data are guaranteed to be true and reliable.
The authors declare no conflict of interest.
This paper is supported by the National Natural Science Foundation of China (nos. 41604082 and 51734009), the Independent Innovation Project for Double First-level Construction (China University of Mining and Technology) (no. 2018ZZCX04) and the National Key R&D Program of China (no. 2018YFC0807802).
Table 4: (Section