The Influence of Geological Formation on the Pore Size Distribution And Compressibility of Coal Fei Wang Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou, China National Engineering Research Center for Coal Gas Control, China University of Mining & Technology, Xuzhou, China School of Safety Engineering, China University of Mining & Technology, Xuzhou, China e-mail: [email protected]

Yuanping Cheng* Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou, China National Engineering Research Center for Coal Gas Control, China University of Mining & Technology, Xuzhou, China School of Safety Engineering, China University of Mining & Technology, Xuzhou, China e-mail: [email protected]

ABSTRACT In order to study the influence of geological formation on pore characteristics in coal, two tectonic coal samples and two original coal samples were collected respectively near the Mafangquan Fault in Jiulishan Mine. Pore size distribution, pore compressibility, proximate analysis, sorption capability, and soundness coefficient of four coal samples were analyzed and compared in this paper. The results of my study are as follows: 1) Geological formation increases moisture and ash content of coal and reduces the fixed carbon content, sorption ability and soundness coefficient. 2) Geological formation compresses not only coal matrix but also pores in coal, and prevents the promotion of coalification on micropore development, which results in the fact that the pores in tectonic coal is lower than that in original coal. 3) Pore compressibility of coal decreases with pressure increasing, while pore compressibility of tectonic coal is lower than that of original coal, which shows that the geological formation reduces the pore compressibility of coal.

KEYWORDS: tectonic coal

size distribution

original coal

pore compressibility

pore

INTRODUCTION Geological formation is a common phenomenon in the process of coal formation, which usually undermines the integrity of coal seam and changes the structure and properties of coal[1]. As a kind of complex porous organic rock, coal is typically a naturally fractured system containing pores and cleats[2-7]. While pores and cleats in coal are the primary places for gas adsorbing and storing, and they are the primary channels for gas diffusing and seepage, which controls the storage and transport of gas in coal seam[8-15]. Geological formation leads to the result that original coal turns into tectonic coal and tends to change the pore characteristics of coal, which has an important impact on coal-bed methane (CBM) recovery, safe coal mining, and CO2 geo-sequestration[16-20]. Presently, there are so many - 1651 -

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studies on the pore characteristics of original coal but few on tectonic coal[2, 21-23]. Therefore, we studied pore characteristics of tectonic and original coal and analysed the difference between them with two tectonic coal samples and two original coal samples from Jiulishan Mine in this paper.

EXPERIMENTAL WORK Jiulishan Mine, being located in Jiaozuo City, Henan Province, China, has many faults and the Mafangquan Fault is the largest among them. Mafangquan Fault, being situated in the center of the mine, is a normal fault with high angle and its gap ranges from 45 m to 165 m. There are a large number of tectonic coal near the Mafangquan Fault. A coal and gas outburst accident killing 18 people happened near the Mafangquan Fault on October 27, 2011. The possible reason is that fault formation destroyed the structure of coal, causing methane accumulation here, and it increased the risk of coal and gas outburst. Therefore, two tectonic coal samples and two original coal samples were collected directly from the working faces near the accident location in Jiulishan Mine. Moreover, four coal samples were carefully sealed in the canister in order to prevent samples from oxidization and then instantly carried to the laboratory for experiments. Subsequently, the collected coal samples were dried at 60 °C for 24 h in a vacuumed oven, sieved to three size fractions of 0.074−0.2, 0.2−0.25, and 1−3 mm, and dried again in the same way. The proximate analysis, initial methane diffusion rate, mercury intrusion porosimetry (MIP), and adsorption constant were carried out with four coal samples and specific experimental methods and instruments are shown in Table 1.

Table 1: Experimental methods and instruments

Experiment name Proximate analysis

standard GB/T 212-2008

instrument

size/mm

A 5E-MAG6600 automatic industrial analyzer A HCA high pressure adsorption device with volume method by the Chongqing Coal Science Research Institute

0.074-0.2

Adsorption constant

GB/T 19560-2008

Initial methane diffusion rate

AQ 1080-2009

A WT-I rate tester produced by the Fushun Coal Science Research Institute

0.2-0.25

MIP

GB/T-21650.1-2008

A Quantachrome PoreMaster 33 GT porosimeter produced in the United States

1-3

0.2-0.25

RESULTS AND DISCUSSIONS

Proximate analysis, sorption capability and soundness coefficient Table 2: Results of proximate analysis, sorption capability and soundness coefficient Coal samples Tectonic coal Original coal

#1 #2 #3 #4

Proximate analysis Mad% Aad% Vad% Fcad% 3.76 21.98 6.79 67.48 4.60 16.54 6.48 72.39 2.27 20.26 7.16 70.32 2.69 14.11 6.24 76.97

Sorption capability VL PL ΔP 44.25 1.45 34.65 40.09 0.78 22.25 43.70 0.72 32.05 46.92 0.91 43.35

Soundness coefficient 0.51 0.57 0.83 0.92

In the long period of geological age, a variety of geological formations tend to damage the structure of coal, and then coal would come into a different mature path and form different

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characteristics under the influence of the surrounding rock, groundwater, geothermal and other factors. As shown in Table 1, moisture and ash content of tectonic coal are greater than that of original coal, while fixed carbon content of tectonic coal is lower. It shows that tectonic formation undermines the integrity of tectonic coal and leads to their destruction and chalking, so it is easy for water and minerals to penetrate into coal. At the same time, the increasing of moisture and ash content decreases fixed carbon content. Besides, VL and ΔP of tectonic coal is lower than that of original coal, which shows tectonic formation not only destroys the structure of coal but also decreases the adsorption and desorption capability of coal, because tectonic formation compressed pores in coal. It is disadvantageous for CBM recovery and CO2 geo-sequestration .What’s more, the soundness coefficient of tectonic coal is lower than that of original coal, which is resulted from the fact that tectonic formation destroyed the structure of coal and made coal softer. Therefore, it increases the risk of coal and gas outburst.

Pore size distribution Table 3: Result from MIP Coal samples Tectonic coal Original coal

#1 #2 #3 #4

Total Intruded Volume cm3/g

Total Surface Area m2/g

0.0203 0.0181 0.0363 0.0293

3.4204 3.1806 6.9274 5.9592

Apparent Density g/cm3 1.923 2.3117 1.4838 1.3813

Skeletal Density g/cm3 2.0016 2.4128 1.5683 1.4377

Total Porosity % 3.9020 4.1928 5.3826 4.0365

As shown in Table 3, the bulk and apparent density of tectonic coal is much greater than that of original coal, which is due to that tectonic stress compresses coal. In this process, tectonic stress also compresses pores in coal and the compression degree of pores is greater than the matrix of coal, so the porosity of tectonic coal is lower than that of original coal in Table 3. Because of compression, total volume and surface area of tectonic coal is lower than that of original coal in Table 3. Therefore, the sorption ability of tectonic coal is weaker than that of original coal. In consideration of pore characteristics of coal, Hotot’s classification on coal pore size is adopted in this paper[24]. The four pore size ranges can be used based on the critical pore sizes of 10, 100, and 1000 nm in diameter. So pores in coal can be divided into macropores (≥1000 nm), mesopores (100−1000 nm), transition pores (10−100 nm), and micropores (≤10 nm), respectively. The pore size distribution of four coal samples from MIP is illustrated in Figure 1.

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Figure 1: Pore size distribution of four coal samples from MIP As shown in Figure 1, the volume and surface area of macpores, mesopores, transition pores and micropores of tectonic coal are greater than that of original coal, which shows tectonic stress compresses not only bigger pores but also small ones[23, 25, 26]. As shown in Figure 1-c, the proportion of macpores and mesopores in tectonic coal is greater than that in original coal, while the proportion of transition pores and micropores in tectonic coal is lower than that in original coal, which shows the influence of tectonic formation on bigger pores is lower than that on smaller pores. The reason is that smaller pores in coal gradually grow by coalification under normal circumstances, while tectonic formation not only compresses pores in coal but also hinders the development of smaller pores. As shown in Figure 1-b and d, the surface area of macpores and mesopores can be ignored, which shows transition pores and micropores control the surface area of coal.

Pore compressibility According to the definition of effective pore compressibility of the porous medium[27, 28], the pore compressibility of coal can be expressed as eq (1). β𝑃𝑃 =

1

𝑉𝑉𝑃𝑃

∗

𝑑𝑑𝑉𝑉𝑃𝑃 𝑑𝑑𝑑𝑑

(1)

Where βp denotes pore compressibility (MPa-1 ), Vp represents pore volume (ml ), and P stands for pressure (MPa). When the intrusion mercury pressure exceeds 10 MPa, the volumetric increment contains the volume directly resulting from pore compression[29, 30], so we can compute the pore compressibility from the MIP data.

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Figure 2: Relationships between pressure and accumulative volume of Hg-injection of coal samples Table 4: Pore compressibility of four coal samples at different pressures Coal samples #1 #2 #3 Original coal #4 Coal samples #1 Tectonic coal #2 #3 Original coal #4 Tectonic coal

10 MPa 0.00611 0.009055 60 MPa 0.003972 0.00468 0.005304 0.006233

Pore compressibility (MPa-1) 20 MPa 30 MPa 40 MPa 0.004722 0.004509 0.004314 0.005758 0.005445 0.005164 0.006733 0.006308 0.005934 0.008303 0.007667 0.007121 80 MPa 100 MPa 150 MPa 0.003679 0.003427 0.002926 0.00428 0.003942 0.003293 0.004795 0.004376 0.00359 0.005542 0.004989 0.003993

50 MPa 0.004136 0.00491 0.005601 0.006647 200 MPa 0.002552 0.002828 0.003044 0.003329

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Figure 3: Pore compressibility of four coal samples As shown in Figure 2, the mercury intrusion volume in four coal samples sharply increases in the low pressure stage and then slowly increases in the high pressure stage. What’s more, the MIP data in the high stage can be fitted well with a linear relationship and the slope of these fitted lines varying from 4.89088 E-5 to 1.04645 E-4. Therefore, the pore compressibility of coal can be calculated by eq 1. The results are summarized in Table 4 and Figure 3, which give the pore compressibility of four coal samples at several representative pressures. Figure 3 shows that the pore compressibility of original coal is greater than that of tectonic coal, while pore compressibility of four coal samples decreases with the pressure increasing. It shows that tectonic coal has relatively low pore compressibility. The reason is that tectonic formation compresses tectonic coal and leads to the decreasing of pore compressibility. Besides, pore and matrix in coal are gradually compressed with pressure increasing, so they gradually become more compact and difficult to be compressed.

CONCLUSIONS Geological formation usually undermines the integrity of coal seam and changes the structure and properties of coal, which has an important impact on coal-bed methane (CBM) recovery, safe coal mining, and CO2 geo-sequestration. Therefore, the pore size distribution, pore compressibility, proximate analysis, sorption capability, and soundness coefficient of two tectonic coal samples and two original coal samples from Jiulishan Mine in China have been investigated and analyzed in this paper. This paper provides a lot of experimental information about the properties of tectonic and original coal, which is very important for safe coal mining, CBM recovery, and CO2 geo-sequestration. The results of my study are as follows: (1) Geological formation increases moisture and ash content of coal and reduces the fixed carbon content, sorption ability and soundness coefficient. So it decreases the ability of CBM recovery, and CO2 geo-sequestration and increases the risk of coal and gas outburst. (2) Geological formation compresses not only coal matrix but also pores in coal, and prevents the promotion of coalification on micropore development, which results in the fact that the pores in tectonic coal is lower than that in original coal. (3) Pore compressibility of coal decreases with pressure increasing, while pore compressibility of tectonic coal is lower than that of original coal.

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ACKNOWLEDGEMENTS The authors are grateful to for the financial support from the National Natural Science Foundation of China (No. 51574229) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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