The gas loss in sampling is the root of coalbed gas content measurement error. The pressure and particle size have a significant impact on the gas loss. Using the self-developed coal particle pneumatic pipeline transportation experimental system, this study investigated the pressure and particle size changes in the sampling pipeline. It is found that the sampling process can be divided into four stages: no flow field stage, sample outburst stage, stable conveying stage, and tail purging stage. The extreme pressure in the sampling pipeline appears at the sample outburst stage; and the pressure in the pipeline has levelled off after sharp decrease in the stable conveying stage. It is also found that the extreme pressure increases first and then decreases with the increase of particle size. The duration of outburst stage is negatively correlated with particle size, and that of stable conveying stage is positively correlated with particle size. In addition, the results show that the loss rate of 1–3 mm particles is the smallest after the test but that particles less than 1 mm increase by about two times and particles greater than 3 mm decrease by more than three times. The study also shows that the particle size distribution of coal samples is a single peak with left skew distribution, and the gas reverse circulation sampling test does not change the location of the peak but makes it higher and sharper. The single size coal sample is more likely to collide than the mixture. This study can help to advance the understanding of impact factors on gas loss during reverse circulation sampling.
Gas content has been recognized as the basic parameter of gas disaster prevention as well as coalbed methane resource development and applications [
In direct methods for measurement of gas content, the gas desorption amount and residual gas content from a coal sample are firstly measured, and then the gas loss in the sampling process is assessed. The gas loss in the sampling process has been recognized as the error source of this method [
In the process of sampling, the gas loss comes from the gas desorption and diffusion of granular coal. Gas desorption of granular coal is the gas transfer process in porous media, and this process is affected by particle size, pressure, temperature, and other factors [
The influence of particle size on gas loss is reflected in the influence of particle size on desorption speed. The larger the particle size of the coal sample, the smaller the initial kinetic diffusion parameter, and the smaller the amount of gas desorption at the same time [
The air reverse circulation sampling technology is a method that brings the coal sample at the bottom of the hole to the surface from the central channel of the drill pipe with the help of compressed air [
This study mainly discussed the factors affecting the gas loss during sampling rather than desorption behaviors in a closed space. Therefore, the temperature, moisture content, forming pressure, and other factors were not investigated. Instead, the change rules of pressure and particle size were experimentally studied, aiming to reveal the change rules of pressure and particle size during reverse circulation sampling and provide the basic theory for the establishment of a more accurate gas loss compensation model.
To study the change rules of pressure and particle size in the process of sampling, an experimental reverse circulation sampling device was designed. The experimental system included air compressor, pressure gauge, gas flowmeter, hopper, conveying pipeline, and high-precision pressure sensors. The experimental system is shown in Figure
The experimental system. (a) Schematic diagram of the experimental system. (b) Physical map of the experimental system: ① Conveying pipeline. ② Particle analyzer. ③ High-speed camera. ④ Bunker. ⑤ Concentrator.
The parameters of the main parts are outlined as follows: Air compressor: maximum power of 110 Kw, exhaust volume of 17.1 m3/min, and exhaust pressure of 1.0 MPa. Detecting system: a pressure gauge with range of 0–1.6 MPa was used to monitor the output pressure of the air compressor. The flowmeter was used to monitor the instantaneous flow and velocity in the pipeline. In addition, eight pressure sensors were arranged on the pipeline. The pressure sensors were installed at 0.1 m, 4 m, 8 m, 12 m, 16 m, 24 m, 65 m, and 80 m from the feed port. The sampling frequency of the pressure sensor was 2400 times in 1 s, and the accuracy level was 0.5. In addition, the concentrator and software were designed for storing measurement data. Coal sample conveying system: the design volume of the bunker was 15 L, which can hold a coal sample of about 12 kg. The inner diameter of the pipeline was 40 mm, which was consistent with the inner diameter of the double-barreled drilling rod used in the current air counter circulation method. The total design length of the pipeline was 80 m, and a mesh bag was used to collect coal samples. Granularity analysis system: the OCCHIO ZEPHYR ESR2 particle analyzer was used to determine the particle distribution of coal samples before and after the air reverse sampling test. This allowed easy and rapid analysis of the particle size parameters, shape parameters, and number of particles in the range of 30
The coal samples used in this experiment were collected from No. 4 coal Seam of Xintian Colliery, which is located in Qianxi County of Guizhou Province. The location of the mine is shown in Figure
Geographical location of the coal sample.
The process of sample preparation is shown in Figure
The process of coal sample preparation.
According to the coal sample size commonly used in gas content measurement, the original coal sample size was classified as ≤1 mm, 1–3 mm, 3–4 mm, 4–5 mm, 5–6 mm, 6–7 mm, and 7–8 mm. The particle size distribution of the original coal sample drilled by using the cone bit and PDC bit is shown in Table
Particle size distribution of the original coal sample.
Sample size | ≤1 mm | 1–3 mm | 3–4 mm | 4–5 mm | 5–6 mm | 6–7 mm | 7–8 mm |
---|---|---|---|---|---|---|---|
Cone bit | 34.61 | 35.10 | 8.27 | 9.10% | 4.51% | 5.45% | 2.96% |
PDC bit | 30.87 | 43.29 | 10.10 | 5.61% | 5.32% | 1.25% | 3.56% |
After the equipment and the coal sample were prepared, the numbered coal samples were successively loaded into the bunker. Then, the air compressor was started. When the output pressure of the air pressure was stable at 0.6 MPa, the valve was opened. This allowed the compressed air to draw the coal sample into the pipeline and start transmission. At the same time, the sensors collected the pressure data in the transmission pipeline. When all the coal samples had been transported, the air compressor was turned off and the coal samples in sampling mesh bag were collected. Finally, the size distributions of coal samples collected were tested by particle analyzer.
The pressure change in the air reverse circulation sampling pipeline is the result of the kinetic energy transfer between gas and particles, as well as the conversion of the gas phase kinetic energy and pressure potential energy. Pressure variation in the pipeline during the tests of the mixture sample is shown in Figure
Pressure variation in the pipeline during the tests of the mixture sample. Point “e” represents the extreme pressure point of 1 # sensor, and point “f” represents the extreme pressure point of 8 # sensor.
The whole test process can be divided into four stages: no flow stage, sample outburst stage, stable conveying stage, and tail purging stage. The no flow stage refers to the stage from energizing the detecting system to opening the valve. At this stage, there is no fluid in the pipeline, so the indication of the sensor is zero. The sample outburst stage refers to the completion of transportation of coal sample accumulated at the feed port. In the sample outburst stage, most of the pressure potential energy of the compressed air was rapidly converted into kinetic energy, and part of the kinetic energy was transferred to the coal sample at the feed port, so that the coal sample could achieve acceleration and move with the compressed air. Currently, the moving speed of the coal sample was less than that of the compressed air. Therefore, the movement of compressed air is the main element in the pipeline, and the pressure in the pipeline increases from atmospheric pressure to compressed air pressure. It can be seen from Figure
In the stable conveying stage, coal sample enters the pipeline evenly from the feed port. The kinetic energy and pressure potential energy of the compressed air were converted into a dynamic equilibrium. Therefore, there was relatively little pressure change in the pipeline, which is reflected in that the curve of the stable conveying stage tends to be stable after a sharp drop in Figure
In the tail purging stage, the coal sample is no longer supplied to the pipeline from the feed port, so the remaining coal sample in the pipeline is reduced. Therefore, the movement resistance of compressed air is reduced, and more pressure potential energy is converted into kinetic energy, resulting in the reduction of static pressure in the pipeline. When the particles are completely transported, the flow field returns to pure air flow, and the pressure in the pipeline returns to a lower level. It is shown in Figure
The pressure values of each measuring point in the pipeline at 35 s, 55 s, 75 s, and 95 s are shown in Figure
Pressure value in pipeline at different time of stable conveying stage.
Pressure variation in the pipeline during the tests of single size coal sample is shown in Figure
Pressure variation in the pipeline during the tests of single size coal sample. The point “e” represents the extreme pressure point of 1 # sensor, and the point “f” represents the extreme pressure point of 8 # sensor.
The relationship between the pressure characteristics and the particle size. (a) Duration of sample outburst stage and stable conveying stage. (b) Extreme pressure of 1 # sensor and 8 # sensor.
In the test of 1–3 mm coal sample, the duration of sample outburst stage is 2.4 s, and that of the stable conveying stage is 20.6 s. Meanwhile, in the test of 7–8 mm coal sample, the duration of sample outburst stage is 0.8 s, which is only one-third of the 1–3 mm coal sample. The duration of stable conveying stage is 46.8 s, which is increased by 127%. This shows that the larger the coal sample size, the shorter the sample outburst stage, but the longer the duration of the stable conveying stage. In the test of 1–3 mm coal sample, the extreme pressures of 1 # and 8 # sensors are 520.53 Pa and 104.73 Pa, respectively. In the test of 6–7 mm coal sample, the extreme pressures of 1 # and 8 # sensors are 520.53 Pa and 104.73 Pa, which increased by 6.4% and 13.0%, respectively. However, the extreme pressures of 1 # and 8 # sensors during the test of 7–8 mm coal sample are 547.78 Pa and 104.95 Pa, respectively, which are 1.1% and 12.8% lower than those of the test of 6–7 mm coal sample.
Further analysis of the results showed that if the particle size is too small, the density of small particle size sample at the feed port will be uneven, which will lengthen the duration of sample outburst stage. On the other hand, the smaller the particle size, the larger the total surface area of coal sample with the same quality and the more contact area with compressed air. As a result, more pressure potential energy of compressed air will be converted into kinetic energy of coal sample. The extreme pressure of the sample outburst stage is reduced. The movement speed of coal sample is improved, and the duration of stable conveying stage is shortened.
The coal samples before and after the gas reverse circulation sampling tests were mixed evenly, and a certain amount of coal samples was taken from each coal sample twice. Then, the coal samples taken each time were divided into two parts: one for backup and the other for particle size distribution measurement using the particle analyzer. The particle size distribution (Feret’s minimum diameter volume distribution) after the mixed coal sample tests is shown in Table
Particle size distribution after the mixed coal sample tests.
Sample size | ≤1 mm (%) | 1–3 mm (%) | 3-4 mm (%) | 4-5 mm | 5-6 mm | 6-7 mm | 7-8 mm |
---|---|---|---|---|---|---|---|
1 # coal sample | 62.32 | 32.54 | 2.06 | 1.83% | 1.24% | 0 | 0 |
2 # coal sample | 65.91 | 29.42 | 3.97 | 0.70% | 0 | 0 | 0 |
3 # coal sample | 64.74 | 29.41 | 3.61 | 0 | 2.24% | 0 | 0 |
4 # coal sample | 64.82 | 28.05 | 5.33 | 1.11% | 0.69% | 0 | 0 |
By comparing the original particle size distribution (shown in Table
According to Tables
Figure
The histogram of particle size distribution before and after the tests. The volume fraction represents Feret’s minimum diameter volume distribution. (a) 1 # and 2 # coal sample. (b) 3 # and 4 # coal sample.
The cumulative distribution before and after the tests. (a) 1 # coal sample. (b) 2 # coal sample. (c) 3 # coal sample. (d) 4 # coal sample.
A straight line can be obtained by plotting ln{−ln[1−
The percentage of particle volume between any two particle sizes can be calculated as follows:
According to formula (
Cumulative particle size distribution and fitting results for 1 # coal sample after the test.
Fitting parameters of Rosin–Rammler distribution.
Samples | Before test | After test | ||||||
---|---|---|---|---|---|---|---|---|
1 # coal sample | 0.8577 | 6.6383 | 2.2972 | 0.9906 | 0.9525 | 6.5341 | 0.9533 | 0.9989 |
2 # coal sample | 0.9781 | 6.6194 | 0.8695 | 0.9961 | ||||
3 # coal sample | 0.8994 | 7.0159 | 2.4433 | 0.9976 | 0.8834 | 5.9926 | 0.8835 | 0.9984 |
4 # coal sample | 0.8352 | 5.6107 | 0.8273 | 0.9974 |
As mentioned,
Using the same method as the mixed coal sample, the gas reverse circulation sampling tests with single particle size of 1–3 mm, 3-4 mm, 4-5 mm, 5-6 mm, 6-7 mm, and 7-8 mm were conducted. The distribution of particle size after the tests is shown in Figure
The distribution of particle size after the tests of single particle coal sample. The volume fraction represents Feret’s minimum diameter volume distribution.
The cumulative distribution of particle size after the tests of single particle coal sample. (a) Original size: 1−3 mm. (b) Original size: 3-4 mm. (c) Original size: 4-5 mm. (d) Original size: 5-6 mm. (e) Original size: 6-7 mm. (f) Original size: 7-8 mm.
Fitting parameters of Rosin–Rammler distribution.
Samples | After test | |||
---|---|---|---|---|
1−3 mm | 1.7953 | 11.7079 | 0.6795 | 0.9998 |
3-4 mm | 0.7717 | 4.5987 | 0.3873 | 0.9685 |
4-5 mm | 1.1094 | 7.4188 | 0.8021 | 0.9726 |
5-6 mm | 1.6574 | 10.5177 | 0.5701 | 0.9969 |
6-7 mm | 1.6961 | 10.3262 | 0.4406 | 0.9905 |
7-8 mm | 1.2022 | 7.5682 | 0.5420 | 0.9742 |
The results show that the particle size distribution after the test presents a single peak with left skewed distribution, and the cumulative distribution after the test conforms to the R–R distribution. However, there is no obvious correlation between the particle size distribution and the original particle size. The fitting parameters of cumulative distribution vary widely and are not correlated with the original particle size. When comparing the particle size distribution of the mixed coal sample and the single particle, it can be seen that the number of coal samples with a particle size greater than 3 mm after the single particle size test is lower. This indicates that the impact crushing of particles in the reverse circulation sampling pipeline is a random process and the impact crushing degree of uniform single size particles is more serious than that of mixed coal samples.
In this study, change rules of pressure and coal particle size in an air reverse circulation sampling pipeline were experimentally evaluated. The results show that the sampling process could be divided into four stages: no flow field stage, sample outburst stage, stable conveying stage, and tail purging stage. In the actual sampling process, the sample collection can be started during stable conveying stage, as the pressure in the pipeline tends to be stable in this stage. The duration of outburst stage is negatively correlated with particle size, and that of stable conveying stage is positively correlated with particle size. The extreme pressure in the pipeline occurs in the sample outburst stage, and the extreme pressure increases first and then decreases with the increase of particle size.
The particle size changed significantly in the process of gas reverse circulation sampling due to particle-particle and particle-tube wall collision. Comparing the particle size distribution before and after the test, it is found that the proportion of 1–3 mm coal sample changes the least. Therefore, coal sample with particle size of 1–3 mm is recommended for gas content measurement.
The particle size distribution presents a left skewed distribution, and the cumulative distribution follows Rosin–Rammler distribution. After the test, the value of
These results are helpful to understand the factors affecting gas loss during gas reverse circulation sampling and thus provide insights for establishing a more accurate compensation model of gas loss.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors acknowledge the relevant coal mine for providing the desired coal samples for this study. This work was supported by the National Key Research and Development Plan of China (2018YFC0808001), Major National Science and Technology Project of China (2016ZX05045-006-001), and Innovation Project of Tiandi Science & Technology Co., Ltd. (2020-TD-QN014).