Pulverization characteristics of coal affected by magmatic intrusion and analysis of the abnormal gas desorption index on drill cuttings

Abstract

The gas desorption index of drill cuttings is a basic index that measures the initial desorption capacities of coal seams and predicts coal seam gas outbursts. Following a long period of gas drainage in the No.7 coal seam of the No.86 mining area in the Haizi coal mine, the gas desorption index of drill cuttings was still found to be much higher than the threshold value for outburst risks. This abnormal phenomenon led to the present study of the rational selection of test methods and objects in this context. In this study, particle size distribution, pore structure and gas desorption characteristics of coal samples in anomalous areas were analyzed. We found that desorption characteristics are related to particle size and particle size varies in relation to tectonic stress and magmatic intrusions. It appears that the anomalous readings are related to particle size of the coal, resulting from tectonic/magmatic pulverization. Furthermore, measured particle size of drill cuttings is not actually reflective of coal particle size – larger particles may be comprised of multiple smaller particles. The results show that coal samples with particle size <1 mm accounts for 76.3% of total samples and coal samples with particle size >1 mm only accounts for 23.7% of total samples. The porosity and total pore volume increase as the particle size decreases. The specific surface area increases with decreasing pore diameter. Transitional pores and micropores increase the specific surface area of the coal sample considerably. The desorption capacity increases with decreasing particle size. The additional tectonic stress caused by magmatic intrusion has a crushing effect, and 1–3 mm particles used in test were composed of a large amount of smaller particles, eventually resulting in abnormal gas desorption index phenomena. Therefore, we proposed an improved method for measuring the gas desorption index of pulverized coal.

Keywords

Coal and gas outburst magmatic intrusion pulverized coal adsorption and desorption gas desorption index of drill cuttings

Introduction

Coal and gas outburst is a very complicated gas dynamics phenomenon in coal mines. An outburst is the failure of coal and its ejection by stored potential energy being converted to kinetic form. This failure is associated with the release of seam gas (Fisne and Esen, 2014; Gray, 1983). The world’s major coal producing countries witness differing levels of coal and gas outbursts. Several hypotheses for the mechanism of coal and gas outbursts have been suggested by scholars around the world, and a hypothesis including multiple factors has received broad acceptance, creating a theoretical basis for outburst prediction, as well as selection and examination of outburst prevention measures. According to this hypothesis, the most important factors that influence the occurrence of coal and gas outbursts are geological structures (particularly steeply dipping seams, faults, and mylonite); gas-related properties of coal seams (composition, pressure, content sorption capacity, desorption rate); mining-induced stress state in the coal seam, and properties and structures of coal seams (strength, porosity, and permeability) (Beamish and Crosdale, 1998; Cao et al., 2001; He and Zhou, 1994; Lama and Bodziony, 1998; Wang et al., 2013, 2014). Several studies have shown that almost all coal and gas outburst coal seams contain tectonic coal, and the coal seam where outbursts occur shows low strength, high porosity, low permeability, and complex tectonic environments (Cheng et al., 2013; Jiang et al., 2010; Li et al., 2011, 2013; Shao et al., 2010).

Tectonic coal is coal deformed under the effects of one or more periods of tectonic stress (Hou et al., 2012; Jiang et al., 2010; Ju et al., 2006; Qu et al., 2010; Zhang et al., 2007). In 1958, the Geological Research Institute of the former Soviet Union divided tectonic coal into five categories based on coal luster, joint distribution and fissure spacing. Based on this classification, China classified destruction forms of coal into five stages that are still used as an index for the determination of coal and gas outbursts (Skoczylas and Wierzbicki, 2014). When category III, IV and V damage is involved, the coal strength is extremely low and the pulverization degree is high, resulting in high levels of outburst risk (Yu and Cheng, 2012). The geological structure involved plays a central role in controlling the development and distribution of tectonic coal. Researchers agree that tectonic coal development areas present risks for coal and gas outbursts.

A variety of forecasting methods have been proposed, such as the D (an index relate to the depth, the Protodyakonov coefficient f and the gas pressure of coal seam) and K (an index relate to the Protodyakonov coefficient f and the initial speed of gas diffusion) comprehensive index method, the R index method, and the drill cuttings comprehensive index method (Cheng et al., 2010; SACMS, 2009; Skoczylas et al., 2014). The gas desorption index of drill cuttings has been widely used in China and abroad. French derived V₁ using the direct method to determine the releasable gas content of coal seams, where V₁ represents the gas desorption levels of 10 g of coal cuttings 35–70 s after being exposed, and where the value of V₁ is used to predict outburst hazard levels (Cheng et al., 1997). Winter and Janas (1971) proposed outburst prediction index K_t, which reflects the speed at which the gas desorption rate of drill cuttings decreases with time. He also proposed that the relationship between the gas desorption volume and desorption time of coal samples can be expressed as a power function: $Q_{t} = (\frac{v_{1}}{1 - k_{t}}) t^{1 - k_{t}}$ (where Q_t represents the accumulated gas desorption volume at time t, cm³/g; v₁ represents the desorption rate when t = 1, cm³/(g·min); t represents the desorption time, min; k_t represents the variation index of the desorption rate), and used the attenuation coefficient of desorption as an outburst prediction index. Bolt and Innes (1959) used desorption rate and desorption volume directly as an outburst prediction index. The Coal Research Institute (China) proposed gas desorption index of drill cuttings (e.g. Δh₂ and K₁) as an outburst prediction index that reflects the initial desorption capacities of coalbed methane, which are closely related to the damage degree to the coal structure and adsorption/desorption capacity (Cheng et al., 2010; Gui et al., 2009; SACMS, 2009). Δh₂ is expressed as the total gas desorption amount at the fourth and fifth minutes of the coal samples (10 g) being exposed to the air (Lin and Cui, 2010; Yu and Cheng, 2012). Cheng et al. (2016), Tan et al. (2006), and Tao (2004) conducted experimental studies on the gas desorption index of drill cuttings (Δh₂ and K₁) and applied this knowledge in the field. Δh₂ is one of the most commonly used methods for daily outburst forecasting in mining areas in China (Lin and Cui, 2010; Yu and Cheng, 2012), especially where outburst disasters have been serious (e.g. Huaibei, Huainan, Xuzhou, Pingdingshan, Zhengzhou and Chongqing).

The geologic environment of the Haizi coal mine is very complex, and multiple periods of tectonic movement have resulted in the extensive development of various forms of tectonic coal. Because of high porosity and low permeability characteristics, the tectonic coal has become one of the key factors controlling coal and gas outbursts. Magmatic rock is widespread in the Haizi coal field, and the maximum thickness of the sill is 170 m. Tectonic coal under the magmatic sill in the No. 7 coal seam of the No. 86 mining area has been subjected to extremely high degrees of pulverization, and abnormally high risks of coal and gas outbursts have been detected by in situ stress distribution and outburst indexes measurements (Wang et al., 2017). During mining of coal in this area, gas pressure and content decreased to 0.2 MPa and 3.35 m³/t, respectively. These values are far lower than the stated critical values of 0.74 MPa and 8 m³/t for China (SACMS, 2009; Yang et al., 2012a); however, the measured value of Δh₂ is still much higher than the threshold value for coal and gas outbursts. To ensure the accuracy of outburst predictions of pulverized coal, the relationships among coal pulverization levels, gas desorption characteristics and outburst indexes must be systematically studied. Based on the abnormal phenomena of the gas desorption index of the Haizi Coal Mine, this study analyzes the structural and adsorption/desorption characteristics of pulverized coal in the No.7 coal seam, reveals the mechanism of the formation of this naturally pulverized coal, and proposes a new method to accurately determine the gas desorption index of drill cuttings.

Overview of the abnormal index workface

The Haizi coal mine lies in the middle of the Huaibei coal field and north of the Linhuan mining field. It is surrounded by the Damajia Fault (NE trending), Daliujia Fault (NNE trending) and Wufang Fault (NEE trending) (Wang, 2009; Yang et al., 2011). Thus the mine is structured in the shape of an irregular triangle as shown in Figure 1. Coal-bearing strata in the area are Permian. The Nos.3, 4, 7, 8, 9 and 10 coal seams are mineable coal seams, of which Nos.7, 8, 9, 10 coal seams are well developed and are the primary mineable seams.

The No.86 mining area is located along the western boundary of the first level of the Haizi coal mine. The average length of its strike is 1.4 km and the average length of its trend is 1.2 km. The mining area covers roughly 1.68 km². The No.7 coal seam of the No.86 mining area is one of the coal seams seriously affected by tectonic movement in the Huaibei coal field (Guo et al., 2016), and also presents the highest levels of outburst risk (Wang et al., 2017). The coal is mostly crushed and it is difficult to find large lumps of coal in the working face. Since 1986, five coal and gas outburst accidents occurred in the No.7 coal seam during construction and production. The upper part of the No.7 coal seam is covered with magmatic rock. The distribution and contours of the magmatic rock are shown in Figure 1(a). To better study the intrusion of igneous rock in the area, we draw a profile of the No.22 prospecting line of the No.86 mining area (Figure 1(b)) according to geological data collected from the Haizi coal mine. According to this profile, it is evident that magmatic rock in the mining area is oriented parallel to the coal seam and the thick rock sill lies above the Nos.7, 8, and 9 coal seams. The inclination angle of magmatic rock in the area is 10–25° and the rock reaches depths of −230 m to −390 m.

As the No.7 coal seam of the No.86 mining area has been identified as outburst danger zone on the basis of recorded outbursts, gas pressure, and gas content, measures (such as pre-drainage through penetration boreholes and boreholes along the coal seam) were taken to eliminate outburst risks before mining and tunneling at the 764 working face, as shown in Figure 2. After taking pre-drainage measures, the maximum remaining gas and releasable gas content derived from indirect measurements (Cheng et al., 2010; Wang et al., 2015) was 4.82 m³/t and 2.89 m³/t, respectively. The maximum remaining gas content, releasable gas content and remaining gas pressure levels derived from direct measurements (Wang et al., 2015) were 5.55 m³/t, 3.36 m³/t, and 0.3 MPa, respectively, which are less than the stated critical values of 8 m³/t and 0.74 MPa in China. Therefore, we concluded that coal and gas outburst risk was eliminated at the 764 working face. However, during mining and tunneling, the measured value of the desorption index (Δh₂) was still greater than its critical value. The gas desorption index of drill cuttings (Δh₂) exceeded the critical value (200 Pa) in many areas of conveyor roadways along the 764 working face during mining and tunneling, as shown in Figure 3, and values reached up to 350 Pa, far exceeding the critical value, at some sites.

Coal sampling and experimental methods

Coal sampling

For the abnormal gas desorption index of drilling cuttings (Δh₂) at the 764 working face, we sampled from the 764 working face, and the sampling location is shown in Figure 1. We found that the coal was mostly crushed and it was difficult to find large lumps of coal in the working face. The coal was very soft and weak. Therefore, we screened the coal samples obtained from the field directly without crushing. In order to study the characteristics of the pulverized coal, we obtained the particle size distribution by screening the coal samples in the laboratory and measured the pore distribution and desorption characteristics.

Experimental methods

Particle size distribution test method

In the laboratory, coal samples collected from the field were screened using sifters with pore diameters of 0.074 mm, 0.2 mm, 0.5 mm, 1 mm and 3 mm. We obtained samples with particle sizes of 0–0.074 mm, 0.074–0.2 mm, 0.2–0.5 mm, 0.5–1 mm, and 1–3 mm and calculated the weight percentage for each.

Pore size distribution test method

Pore size distribution characteristics of the coal samples were measured by the mercury intrusion porosimetry (MIP) (Kaufmann, 2010; Schmitt et al., 2013) using an Auto Pore IV 9500 mercury porosimeter with a pressure range of 0–230 MPa. The measureable range of pore diameter was 0.003–360 µm and the volume accuracy of mercury intrusion/ejection was 0.1 µL. The following classification from Hodot (1966) was used for coal pore size: micropores (<10 nm), transitional pores (10–100 nm), mesopores (100–1000 nm), macropores (>1000 nm).

Desorption experiment

We prepared 50 g coal samples with the particle size range of 0–0.074 mm, 0.074–0.2 mm, 0.2–0.5 mm, 0.5–1 mm, and 1–3 mm as the experimental coal samples respectively. Then the prepared coal samples were put into coal sample tanks and the airtightness of the tanks was checked, and then the coal sample tanks were evacuated with a vacuum pump for about 8–10 h in water bath with constant temperature of 333 K; after vacuum pumping, the coal sample tanks were filled with a purity of 99.99% methane until the pressure reached 1.5 MPa. The coal sample tanks were placed in the 303 K water bath, close to the temperature in the workface underground, and the gas pressure was adjusted until the gauge value was stable at 0.5 MPa (usually 2 to 3 days). After the adsorption equilibrium was achieved, the free gas in the coal sample tank was rapidly released and the pressure drastically reduced to atmospheric pressure. Then the coal sample tank was connected to the desorption cylinder as soon as possible. The desorption pressure stayed constant when the desorption process was held. The desorption volume was measured by recording the liquid level. The experimental devices are shown in Figure 4.

Laboratory testing of the gas desorption index of drill cuttings

The applied test method of Δh₂ in laboratory is similar to that of the underground field. Δh₂ was measured using the MD-2 type gas desorption instrument produced by the Coal Research Institute of Fushun. The instrument and a structural sketch are shown in Figure 5. The particle size of coal samples in this test was 1–3 mm. For the MD-2 type gas desorption instrument, the gas desorption volume of per gram coal sample (Q, cm³/g) from the fourth minute to the fifth minute has a relationship with Δh₂, which is described by the equation $Q = 0.0083 Δ h_{2} / 10$ , where 0.0083 is the structure constant of MD-2 type gas desorption instrument and 10 is the measured weight of the coal sample (Cai and Wang, 1992; Cheng et al., 2016; Yang et al., 2012b).

The following steps were performed to measure Δh₂ in the laboratory: (1) putting the coal samples into the airtight coal sample tanks, which were then filled with methane; (2) opening the coal sample tanks when the coal samples reached adsorption equilibrium at 0.5 MPa and timing at the same time; (3) immediately loading the coal samples into the sample bottle up to the scale mark, beginning the test when the exposure time of the coal sample is 180 s (3 minutes), and (4) recording the differential between the water pressure scales (unit in mm) on both sides after 2 min. Δh₂ is equal to the product of the differential and 10 Pa each mm.

Experimental results and analysis

Particle size distribution characteristics of the coal samples

The particle size distribution of the coal samples reflects the pulverization degree of the coal to some extent. To study the pulverization characteristics of the coal samples obtained from the field, we measured the particle size distribution. The test results are shown in Table 1. Three groups of screening results of the pulverized coal samples were found to be similar, so we used the average value of the three groups of screening results as the mass distribution of coal samples with different particle sizes. Samples with particle size <1 mm account for 76.3% while those with particle size >1 mm only account for 23.7%. As we screened the coal samples obtained from the field without crushing artificially, the measured particle size distribution represents the coal particle size distribution of the sampling location. The large proportion of the coal samples with particle size <1 mm indicates that the pulverization degree of coal is high.

Table 1.

The mass distribution of the coal samples with different particle sizes.

Particle size range	The first group		The second group		The third group		Percent (%)
Particle size range	Quality (g)	Percent (%)	Quality (g)	Percent (%)	Quality (g)	Percent (%)	Percent (%)
>3 mm	78.38	6.00	60.50	5.93	67.23	6.29	6.08
1–3 mm	196.90	17.85	155.48	17.60	164.79	17.42	17.62
0.5–1 mm	176.82	16.04	141.36	16.00	151.42	16.08	16.04
0.2–0.5 mm	232.18	21.04	187.71	21.25	198.8	21.01	21.1
0.074–0.2 mm	223.06	20.22	178.78	20.24	193.44	20.44	20.3
<0.074 mm	207.94	18.85	167.66	18.98	177.50	18.76	18.86

Pore distribution characteristics of the coal samples

Coal is a naturally complex porous and fractured organic rock (Clarkson et al., 1999; Green et al., 2011; Meyers, 1986). Coal has a very well developed pore and fracture system with large pore surface area and open spaces, which constitute spaces for methane adsorption and transportation (Clarkson and Bustin, 1999; Guo et al., 2015; Liu et al., 2017). The mercury intrusion method is often used to examine the pore distribution characteristics of coal. The pore distribution test results are shown in Figure 6. The total volumes of intruded mercury in the samples with particle sizes of 0.2–0.5 mm, 0.5–1 mm and 1–3 mm show little difference. The accumulated amount of intruded mercury increases with decreasing particle size during the early stage of mercury intrusion. The total amount of intruded mercury in coal samples with particle sizes of 0.074–0.2 mm is slightly higher than that of coal samples with particle sizes exceeding 0.2 mm. The total amount of intruded mercury for coal samples with particle sizes less than 0.074 mm is much higher than that of the other particle sizes.

For tectonic coal, the pore volume and porosity levels are important indexes for evaluating pore development and fracturing as well as structural damage (Li, 2013). To quantitatively analyze the variations in pore structures of naturally pulverized coal based on particle size, results of tests on the macropores, mesopores, transitional pores, micropores and porosities of different particle sizes were obtained as shown in Table 2. It can be seen that the porosity and total pore volume of the coal samples varies with particle size, with a general increase in these parameters with decreasing particle size. The porosity of coal samples with particle sizes less than 0.074 mm is the largest. The pore volume distribution of micropores and transitional pores decreases with a decrease in particle size more or less, while the pore volume distribution of macropores increases with a decrease in particle size.

Table 2.

The pore volume distribution of the coal samples with different particle sizes.

Particle size (mm)	Porosity (%)	Total amount of mercury (mL/g)	Pore volume (mL/g) Distribution (%)	Pore type
Particle size (mm)	Porosity (%)	Total amount of mercury (mL/g)	Pore volume (mL/g) Distribution (%)	Micropores	Transitional pores	Mesopores	Macropores
1.0–3.0	6.538	0.0456	Pore volume	0.0079	0.0214	0.0105	0.0058
1.0–3.0	6.538	0.0456	Distribution	17.32	46.93	23.03	12.72
0.5–1.0	6.577	0.0457	Pore volume	0.0093	0.0176	0.0099	0.0089
0.5–1.0	6.577	0.0457	Distribution	20.35	38.51	21.66	19.47
0.2–0.5	6.258	0.0443	Pore volume	0.0075	0.0133	0.0104	0.0131
0.2–0.5	6.258	0.0443	Distribution	16.93	30.02	23.48	29.57
0.074–0.2	9.052	0.0665	Pore volume	0.0086	0.0202	0.0161	0.0216
0.074–0.2	9.052	0.0665	Distribution	12.93	30.38	24.21	32.48
<0.074	13.642	0.1305	Pore volume	0.0064	0.0122	0.0144	0.0975
<0.074	13.642	0.1305	Distribution	4.90	9.35	11.03	74.71

The specific surface area of coal shows a close relationship with methane absorption capacities. Generally, the coal with larger specific surface area has stronger gas absorption ability (Guo et al., 2016; Liu and He, 2017; Sun et al., 2015). The distribution of the specific surface area of coal is shown in Table 3. The change in specific surface area with pore diameter was calculated according to variations in mercury content with pressure during mercury intrusion, as shown in Figure 7. The specific surface area of coal samples with different particle sizes increases with decreasing pore diameter, while the variation of the specific surface area with pore diameter shows no obvious differences among coal samples with different particle sizes. The specific surface areas of transitional pores and micropores of coal samples with different particle size account for more than 90% of the total specific surface area, showing that the specific surface area shows an exponential increasing trend as shown in Figure 7, when the pore diameter is reduced to the magnitude of transitional pores and micropores. Therefore, transitional pores and micropores of the coal are well developed, greatly increasing the specific surface area of the coal samples and facilitating gas storage and migration.

Table 3.

The specific surface area distribution of the coal samples with different particle sizes.

Particle size (mm)	Specific surface area (m²/g) Distribution (%)	Pore type
Particle size (mm)	Specific surface area (m²/g) Distribution (%)	Micropores	Transitional pores	Mesopores	Macropores
1.0–3.0	Specific surface area	4.131	3.9986	0.1716	0.0097
1.0–3.0	Distribution	49.71	48.11	2.07	0.12
0.5–1.0	Specific surface area	4.515	3.8310	0.1410	0.0160
0.5–1.0	Distribution	53.1	45.05	1.66	0.19
0.2–0.5	Specific surface area	3.883	2.5613	0.1509	0.0208
0.2–0.5	Distribution	58.69	38.71	2.28	0.31
0.074–0.2	Specific surface area	4.2470	3.8482	0.2258	0.0340
0.074–0.2	Distribution	50.83	46.06	2.70	0.41
<0.074	Specific surface area	3.2640	2.4492	0.1764	0.1184
<0.074	Distribution	54.33	40.77	2.94	1.97

Gas desorption characteristics of the coal samples

Research on the occurrence and development of gas disasters shows that methane desorbs rapidly during coal and gas outbursts, and a large amount of adsorbed gas is emitted from coal over a short period (Guo et al., 2016; Skoczylas et al., 2014; Wierzbicki and Skoczylas, 2014; Xue et al., 2014). As the gas desorption index is abnormal at roughly 0.3 MPa after the elimination of outbursts in underground field cases, we set our experimental equilibrium pressure levels to 0.5 MPa for testing purposes. By studying the gas desorption characteristics of coal samples with different particle sizes, we can provide a theoretical basis for the prediction of coal and gas outbursts.

Variations in accumulated gas desorption levels with desorption time under the same equilibrium pressure level (0.5 MPa) are shown in Figure 8. The desorption curves of coal samples with different particle sizes present the same variation characteristics under the same gas pressure, and the accumulated gas desorption amount and desorption time follow a power function relationship. At the same pressure, the accumulated gas desorption amount increases with decreasing particle size. The average gas desorption rate of each initial desorption stage is shown in Table 4. It indicates that the initial desorption rate is very fast for samples of the same particle size and the desorption rate decreases rapidly. The initial desorption rate of samples with different particle sizes increases with decreasing particle size. With smaller particle sizes, desorption rate declines faster, indicating that with the higher fragmentation degree, the desorption capacity becomes stronger.

Table 4.

The desorption rate of the coal samples with different particle sizes in the first 20 min.

Particle size (mm)	Desorption rate (mL/(g·min))
Particle size (mm)	0.5 min	1 min	2 min	3 min	4 min	5 min	8 min	10 min
<0.074	11.92	6.60	0.54	0.25	0.13	0.10	0.06	0.06
0.074–0.2	8.36	5.08	0.80	0.42	0.30	0.20	0.06	0.06
0.2–0.5	6.25	3.93	0.83	0.45	0.31	0.23	0.16	0.08
0.5–1	4.72	3.20	0.84	0.53	0.35	0.25	0.13	0.09
1–3	2.88	2.21	0.96	0.61	0.55	0.46	0.11	0.10

To quantitatively analyze differences in gas desorption properties between coal samples from the No.7 coal seams of the Haizi coal mine and other coal samples, Guo et al. (2016) compared pulverized coal samples from No.7 coal seams of the Haizi coal mine with two coal samples unaffected by magma intrusion. It was found that the accumulated desorption amount and initial desorption rate of pulverized coal sample are higher than those of the other two coal samples under the equilibrium pressure and particle size. The change in matrix scale and pore structure damage caused by crushing largely determines the gas desorption characteristics of pulverized coal, which exhibits high initial gas desorption levels and rapid initial gas desorption rates. These characteristics contribute to high coal and gas outbursts risks for the No.7 coal seam of the Haizi coal mine.

Analysis of the abnormal gas desorption index

The premise of the abnormal gas desorption index

The gas desorption indexes of drill cuttings (Δh₂) of coal samples with different particle sizes were measured in a laboratory after screening. The results are shown in Table. 5. When the particle size of the coal samples is less than 1 mm, the gas desorption index of drilling cuttings increases with increasing particle size, as shown in Figure 9. The variation characteristic of the desorption index was consistent with our previous conclusions regarding particle size and initial desorption rate and desorption amount: the initial desorption rate and desorption amount increase with decreasing particle size; and the smaller the particle size is, the faster the desorption rate declines. In contrast, the gas desorption index of drilling cuttings reduces with increasing particle size when the particle size increases to 1–3 mm, as shown in Figure 9. Therefore, the measured desorption index is the largest when the particle size is 0.5–1 mm indicating that coal samples with particle sizes of 0.5–1 mm have the highest desorption amount during measurement period of Δh₂ when gas pressure is 0.5 MPa. We deduced that the abnormally high desorption index measured at the 764 working face was caused by the fact that the coal samples with the particle size 1–3 mm used in test were composed of a large amount of smaller particles.

Table 5.

The gas desorption index of the coal samples with different particle sizes.

Equilibrium pressure (MPa)	Mass percent (%)	Particle size (mm)	Δh₂/Pa
Equilibrium pressure (MPa)	Mass percent (%)	Particle size (mm)	1#	2#	Maximum
0.5	18.86	<0.074	130	100	130
	20.3	0.074–0.2	350	390	390
	21.1	0.2–0.5	430	440	440
	16.04	0.5–1	510	540	540
	17.62	“1–3”	432	445	445

To verify this deduction, we conducted a scanning electron microscope test of the coal samples. Upon observing the coal samples with different particle sizes under varying levels of magnification, we found that the coal samples with smaller particle size had simpler structure. Large particles were easy to screen, but their structure is complex. Figure 10 shows scanning electron microscope images of 1–3 mm particles. It is evident that the 1–3 mm particles show a layered structure or agglomerates of smaller particles, affecting the accuracy of the index results.

We further verified the deduction by analyzing the results of desorption experiment. The accumulated desorption amount of coal samples with different particle sizes for the first 20 min is shown in Table 6.

Table 6.

The accumulated desorption amount of the coal samples with different particle sizes in the first 20 min.

Pressure (MPa)	Particle size (mm)	Average particle size (mm)	1 min	2 min	3 min	4 min	5 min	10 min	15 min	20 min
0.5	0.5–1	0.75	3.20	4.02	4.46	4.79	5.04	5.76	6.09	6.29
	0.2–0.5	0.35	3.93	4.75	5.30	5.57	5.85	6.50	6.80	6.95
	0.074–0.2	0.137	5.08	5.89	6.32	6.62	6.80	7.33	7.53	7.62
	<0.074	0.037	6.60	7.13	7.37	7.51	7.60	7.82	7.90	7.93

We used the average particle size (d) as the independent variable and the accumulated desorption amount (Q) for any point in time (t) as the dependent variable to fit the desorption data. The fitting results are shown in Table 7.

Table 7.

The fitting results of accumulated desorption amount and particle size.

Time (t)	Fitting function	Correlation coefficient (R²)
1min	Q = −1.14ln(d) + 2.81	0.998
2min	Q = −1.05ln(d) + 3.71	0.997
3min	Q = −0.97ln(d) + 4.26	0.993
4min	Q = −0.91ln(d) + 4.61	0.987
5min	Q = −0.85ln(d) + 4.91	0.982
10min	Q = −0.69ln(d) + 5.72	0.954
15min	Q = −0.60ln(d) + 6.09	0.935
20min	Q = −0.51ln(d) + 6.30	0.927

From the correlation coefficient, it is evident that the fit is very good. To verify the accuracy of the accumulated desorption amount equations obtained by fitting, we compared the calculated desorption curves derived from the equations with the experimental desorption curves. The corresponding comparison results are shown in Figure 11.

It is evident that the calculated desorption curves by using average particle sizes and experimental desorption curves are basically the same, verifying the accuracy of the accumulated desorption amount equations. So we expected that the experimental desorption curve of 1–3 mm particles would be coincident with the calculated desorption curve (d = 2 mm). However, we used the accumulated desorption amount equations to calculate the desorption curve (d = 2 mm) and compared it with the experimental desorption curve of samples with particle size of 1–3 mm, as shown in Figure 12. The calculated desorption curve falls below the experimental desorption curve, indicating that the accumulated desorption amount of samples with particle size of 1–3 mm is greater than our expectation.

According to the previous conclusion: the accumulated desorption amount increases with decreasing particle size. Using the accumulated desorption amount equations, we recalculated the equivalent average particle size of 1–3 mm screened particles based on the experimental desorption amount at 1 min, and the value is 1.677 mm (<2 mm). Then we obtained the calculated desorption amount at other time by substituting 1.677 into other equations in Table 7 and compared the calculated desorption curve (d = 1.677 mm) with the experimental desorption curve of coal samples with particle sizes of 1–3 mm. As shown in Figure 12. It is clear that the calculated desorption curve (d = 1.677 mm) is more coincident with the experimental desorption curve than the calculated desorption curve (d = 2 mm). Therefore, we concluded that the 1–3 mm coal particles used in test were composed by a large amount of smaller particles. As indicated previously, the gas desorption index of drilling cuttings reduces with increasing particle size when the particle size is >1 mm, and we can explain the abnormally high desorption index measured from the underground field and in the laboratory.

Coal in the No. 7 coal seam of the Haizi coal mine shows obvious layered structure, high pulverization degree, and is easily broken. From the study, we believe that the abnormal desorption index of drill cuttings was caused by the pulverization characteristics of coal samples. These issues are determined by the structural characteristics of the pulverized coal, and cannot be solved by improving screening equipment. Therefore, it is inappropriate to compare the drill cutting desorption index of the coal sample (sieving particle size of 1–3 mm) with the reference critical value of Δh₂ (200 Pa) outlined under China’s Outburst Prevention and Control Regulation.

Causes of pulverized coal formation

Since the formation of the coal-bearing strata in the Haizi coal mine, the coal-accumulating basin has experienced complex tectonic evolution. The coal seam has undergone high levels of deformation under the influence of fracture, fold and layer slip structures, and the coal structure is now broken and pulverized (Guo et al., 2016). However, in addition to the pulverization resulting from tectonic movement to the coal, the magmatic intrusion further crushed coal by changing the geostress, which is another major cause of pulverization in the area. To study the effects of magmatic intrusion on geostress, we carried out geostress measurements and field verifications.

The Haizi coal field used the acoustic emission (AE) method to carry out actual measurements (Wang et al., 2017). The geostress of measured points under magmatic sills with different thicknesses was determined, and the results are shown in Table 8. It is clear that the maximum horizontal principal stress shows regularities with the increasing thickness of the overlying sill. Indeed, the greater the thickness of the overlying sill, the higher the maximum horizontal principal stress level. A linear relationship is also found between the ratio of the average horizontal stress (

σ_{h, av}

) and the vertical gravity stress (

σ_{v}

) and the thickness of the sill (

H_{sill}

). It can be described as

σ_{h, av} / σ_{v} = 0.0071 H_{sill} + 0.7215

(Figure 13). The maximum horizontal tectonic stress (

σ_{T, max}

) increases linearly with the increasing thickness of the overlying sill (

H_{sill}

). This linear relationship can be described as

σ_{T, max} = 0.1369 H_{sill} + 7.8246

(Figure 13). The maximum horizontal tectonic stress is higher than the lateral pressure caused by gravity, showing that the local stress field, created through magma intrusion, is a horizontal tectonic stress field. From the above, it is evident that coal and rock under the sill are not only affected by gravity stress, but also from compressive tectonic stress resulting from magma intrusion in the Haizi coal field. Tectonic stress increases with an increase in sill thickness.

Table 8.

Measurement results of geostress.

Sampling point	Burial depth (m)	Sill Thickness (m)	$σ_{h, max}$ (MPa)	$σ_{h, min}$ (MPa)	$σ_{v}$ (MPa)	$σ_{h, av}$ (MPa)	φ (°)	$\frac{σ_{h, av}}{σ_{v}}$	$σ_{max}$ (MPa)	$σ_{min}$ (MPa)
R559	639.80–647.40	145	29.96	15.44	13.18	22.70	120.1	1.72	25.62	11.09
R558	622.40–627.80	85	26.11	9.87	12.62	17.99	103.2	1.43	21.95	5.70
R557	627.10–630.20	60	21.47	6.29	12.82	13.88	124.8	1.08	17.24	2.06
R556	612.10–618.30	22	15.95	6.80	12.44	11.38	122.4	0.91	11.85	2.69
R555	652.10–659.30	0	9.56	8.29	12.94	8.93	115.5	0.68	5.29	4.02

$σ_{h, max}$ : maximum horizontal principal stress (MPa); $σ_{h, min}$ : minimum horizontal principal stress (MPa); $σ_{v}$ : vertical gravity stress (MPa); $σ_{h, av}$ : average horizontal stress (MPa); φ: maximum principal stress at the horizontal direction (the contrarotated direction is defined as the positive direction); $σ_{T, max}$ : maximum horizontal tectonic stress (MPa); $σ_{T, min}$ : minimum horizontal tectonic stress (MPa) (Brady and Brown, 2004).

The drilling cutting quantity (S) is a predictive index for outburst hazards of working faces and reflects the geostress of coal seams and the pulverization degree of coal. It is measured by drilling a 42 mm diameter borehole along the coal seam, and the quantities of drilling cuttings per meter were defined as drilling cuttings quantity. Generally, with the increase of geostress, the value of drilling cuttings quantity will significantly increase, and the possibility of outburst will increase. The comparative analysis results on drilling cutting quantity for different sill thicknesses are shown in Figure 14. It is evident that that the drilling cuttings quantity increases with the increasing thickness of the overlying sills, consistent with measured results of geostress in the Haizi coal field. Therefore, the magma intrusion has changed the geostress and has aggravated the pulverization of the coal. With a greater thickness of the overlying sills, the transformation becomes more obvious.

Discussion of the desorption index measurement method

As the pulverized coals have been crushed considerably, the physical and mechanical properties and related desorption characteristics of coal with different particle sizes are different. It is not possible to screen coal samples into accurate particle sizes ranges during the few minutes in the underground field. To accurately determine the critical value of the desorption index, we discussed the determination of the critical value of the desorption index under laboratory conditions. Furthermore, a new method for testing the desorption index of pulverized coal drilling cuttings was proposed.

(1) Determination of the desorption index critical value

Many scholars have found that the desorption index of drilling cuttings has a clear power function relationship with equilibrium pressure (An et al., 2013; Cheng et al., 2016). Therefore, the $Δ h_{2, i}$ of coal samples with different particle sizes at different equilibrium pressure levels under laboratory conditions are measured and the power function relationship is determined by fitting data. The function is shown as follows:

Δ h_{2, i} = a_{i} P^{b_{i}}

(1)

where

Δ h_{2, i}

represents the desorption indexes of coal sample drilling cuttings with different particle sizes, Pa; a_i and b_i are fitting coefficients; and P represents equilibrium pressure, MPa.

To determine the critical value of the desorption index of drilling cuttings, the critical outburst pressure of a coal seam must first be analyzed. Critical values of gas pressure and content used to predict outburst risks are 0.74 MPa and 8 m³/t according to China’s Outburst Prevention and Control Regulation, and we can calculate the corresponding gas content and pressure level of critical values via the indirect method (Yu and Cheng, 2012). Combining the relationship between the minimum outburst pressure level and the Protodyakonov coefficient f ( $P_{min} = 2.79 f + 0.39$ ), and the relationship among the minimum outburst pressure, Protodyakonov coefficient (f) and volatile V ( $P_{min} = 0.5 + 0.0085 Vf$ ) (Cheng et al., 1997; Kong et al., 2014), we can determine the minimum critical outburst pressure (P_min) of the examined coal seam. The minimum critical gas desorption indexes of coal sample drilling cuttings with different particle sizes ( $Δ h_{2, i min}$ ) can be obtained by substituting P_min into equation (1)

Δ h_{2, i min} = a_{i} \times P_{min}^{b_{i}}

(2)

where

Δ h_{2, i min}

represents the minimum critical gas desorption indexes of coal sample drilling cuttings with different particle sizes, Pa; P_min is the minimum critical outburst pressure value, MPa.

Using the particle size distribution of the coal samples, the weighted gas desorption index of drilling cuttings ( $Δ h_{2}$ ) can be obtained from equation (3), which is treated as the critical gas desorption index of drilling cuttings in the prediction of outburst risk

Δ h_{2} = \sum_{i = 1}^{N} ω_{i} Δ h_{2, i min}

(3)

where

Δ h_{2}

represents the critical gas desorption index of drilling cuttings, Pa;

ω_{i}

represents the mass percentage of coal samples with different particle sizes.

(2) Site desorption index measurement method

From the critical value of the coal seam gas desorption index of drilling cuttings derived from laboratory experiments and analyses, we can measure the gas desorption index of drilling cuttings of the coal samples directly using the MD-2 type gas desorption instrument in an underground field. The obtained coal samples are not sieved during the measurement process. Comparing test results with the threshold obtained from a laboratory, when test results are larger than the threshold, the measurement field has outburst risks and further measures should be taken to eliminate such risks. Otherwise, the measurement field does not have outburst risks, and mining activities can be carried out.

Taking the No.7 coal seam of the No.86 mining area in the Haizi coal mine as an example, we measured

Δ h_{2, i}

of coal samples with different particle sizes under different equilibrium pressure levels, and obtained the power function relationship between

Δ h_{2, i}

and equilibrium pressure by fitting. The fitting results are shown in Table 9. When the gas pressure reaches 0.74 MPa, the corresponding gas content level reaches 9.89 m³/t, and when the gas content level reaches 8 m³/t, the corresponding gas pressure level reaches 0.5 MPa, which was calculated using an indirect method. This indicates that the coal seam manifests the characteristics of low gas pressure and high gas content. For safety reasons, we used the gas content (8 m³/t) as an index for the division of outburst areas, along with the corresponding gas pressure level of 0.5 MPa. The Protodyakonov coefficient f and volatile V values of coal samples in this region are 0.235 and 10.63%, respectively. We calculated the minimum outburst pressure level of the coal seam as 1.05 MPa and 0.71 MPa, respectively, according to the relationship function of the minimum outburst pressure, Protodyakonov coefficient f and volatile V. We finally selected 0.5 MPa as the minimum outburst pressure of the No.7 coal seam. Substituting 0.5 MPa into equations (2) and (3), we obtained the critical value of the gas desorption index of 355 MPa. After taking pre-drainage measures, the measured values of desorption indexes were found to be lower than the critical value of gas desorption index. Therefore, the drainage measures were effective and outburst risk of the mining area was eliminated. Mining activities can be carried out in this area.

Table 9.

Fitting results of desorption index of drill cuttings and equilibrium gas pressure.

Particle size (mm)	Mass percent (%)	Fitting equation	Correlation coefficient (R²)
<0.074	18.86	Δh₂ = 187.12P^0.55	0.992
0.074–0.2	20.3	Δh₂ = 471.52P^0.26	0.993
0.2–0.5	21.1	Δh₂ = 567.83P^0.37	0.999
0.5–1	16.04	Δh₂ = 677.62P^0.336	0.998
1–3	17.62	Δh₂ = 635.41P^0.51	0.999

Conclusions

Coal shows a high degree of pulverization in the No.7 coal seam of the Haizi coal mine, where the gas desorption index of drill cuttings is abnormal. Coal samples with particle size <1 mm accounts for 76.3% of total samples and coal samples with particle size >1 mm only accounts for 23.7% of total samples. The porosity and total pore volume increase with a decrease in particle size. The specific surface area of coal samples with different particle sizes increases with decreasing pore diameters. The specific surface area of transitional pores and micropores accounts for more than 90% of the total specific surface area, which increases the specific surface area of coal samples considerably and contributes to gas storage and migration.

The desorption curves of coal samples with different particle sizes have similar variation characteristics under the same gas pressure, and the accumulated gas desorption amount and desorption time show a power function relationship. Under the same pressure, the accumulated gas desorption amount increases with decreasing particle size. The initial desorption rate is very fast for coal samples of the same particle size, after which desorption rates rapidly decrease. The initial desorption rate increases with decreasing particle size, and for smaller particles, the desorption rate declines more quickly.

When the particle size of the coal samples is less than 1 mm, the gas desorption index of drilling cuttings increases with increasing particle size. Compared with the desorption index of samples <1 mm, the gas desorption index of drilling cuttings reduces with increasing particle size when the particle size increases to 1–3 mm. We deduced that the abnormally high desorption index measured at the 764 working face was caused by the fact that the coal samples with the particle size 1–3 mm used in test were composed of a large amount of smaller particles. This deduction was verified through scanning electron microscope testing and desorption experiments.

In addition to tectonic movements, magmatic intrusion in the Haizi coal field is another major cause of the high pulverization levels. The effects of the magmatic intrusion on the geostress increase with the thickness of overlying sills. The larger the thickness of overlying sills, the greater the change in geostress, which in turn leads to an increase in coal pulverization.

A new method for determining the critical value of the gas desorption index of drilling cuttings and a new test approach to the gas desorption index are proposed in this study. The critical desorption index (355 MPa) of the No.7 coal seam in the Haizi coal mine was obtained using the new method and was used to guide safe mining activities in the Haizi coal mine.

Footnotes

Acknowledgement

We appreciate the helpful comments from the editors and reviewers.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Fundamental Research Funds for the Central Universities (2017CXNL02) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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