- Email: [email protected]
A new method for determining coal seam permeability redistribution induced by roadway excavation and its applications
Process Safety and Environmental Protection 131 (2019) 1–8
Contents lists available at ScienceDirect
Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
A new method for determining coal seam permeability redistribution induced by roadway excavation and its applications Huihui Liu a,b , Baiquan Lin a,b,∗ , Chenglin Jiang a,b a b
Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China
a r t i c l e
i n f o
Article history: Received 13 June 2019 Received in revised form 23 July 2019 Accepted 20 August 2019 Available online 23 August 2019 Keywords: Gas disaster prevention Coal mine gas extraction Gas utilization Coal seam permeability Coal roadway excavation
a b s t r a c t Changes in coal seam permeability induced by roadway excavation may lead to many engineering problems, such as low gas extraction concentration in in-seam boreholes, coal and gas outburst disasters, and gas emissions in the process of coal roadway excavation. To solve these problems, this paper proposed a new method to directly determine the distribution of coal seam permeability around roadways. For this purpose, method principle was analyzed, equipment was developed, and field experiments were carried out in Baiyangling coal mine. Finally, the engineering applications of the method were analyzed, and a field application effect experiment was conducted to verify the new method. The results showed that the coal seam permeability around roadways was divided into four stages. In stages I and II, permeability increased, in stage III, permeability decreased, and stage IV maintained the initial permeability of the coal seam. These test results will aid in increasing the gas extraction concentration of in-seam boreholes and will guide the design of gas control areas. Using the verification experiment, it was concluded that the new method is able to accurately determine the redistribution of coal seam permeability caused by roadway excavation and provide theoretical guidance in solving engineering problems. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Coal roadway excavation is a primary aspect of mining activities in underground coal mines. Owing to the redistribution of stress and gas pressure around the roadway, and the destruction caused by the roadway tunneling process, coal seam permeability around the roadway tends to change. As the distance from the roadway increases, the permeability of coal seam surrounding the roadway initially increases, then decreases, and finally stabilizes (Hu et al., 2015; Xia et al., 2014; Xiang et al., 2015). This change may induce many engineering problems. First, during the roadway excavation process, gas emissions from the coal seam around roadways are mainly from the highly permeable areas, which is a main factor in inducing gas explosions (Qin et al., 2016; Shi et al., 2017). Second, the in-seam extraction borehole is a widely used method to control gas in China underground mines. However, the gas extraction concentration in 80% of the in-seam boreholes varies from 6% to 20% in a short time owing to the large amount of gas leakage in the extraction process (Lu et al., 2009). This is primarily caused
∗ Corresponding author at: School of Safety Engineering, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China. E-mail address: [email protected] (B. Lin).
by changes in coal seam permeability after roadway excavation. Such low concentrations of gas are not utilizable, resulting in serious environmental pollution and energy waste due to the gas being directly released (Xia et al., 2014; Zheng et al., 2019). In addition, continuous air leakage may also cause spontaneous combustion of coal (Zhou, 2012). Finally, areas with low permeability and high stress caused by roadway excavation are the highest risk of coal and gas outbursts (referred to as outburst), which is one of the major disasters in coal mines (Cao et al., 2019; Wang et al., 2013, 2017; Zhou et al., 2019). The key to solving the aforementioned engineering problems is to accurately determine the redistributions of coal seam permeability after roadway excavation. In recent years, many studies have been carried out in the related fields. Xia et al. (2014) used the COMSOL Multiphysics software to simulate changes in coal seam permeability affected by roadway excavation. Based on stress distributions of the coal mass around roadways, Xiang et al. (2015) qualitatively analyzed the redistributions of coal seam permeability after roadway excavation. Wei et al. (2013) established the distributions of coal seam permeability around roadways by measuring the variation law of the drill cutting desorption index. Hu (2014) used experimental methods to investigate the evolution of coal seam permeability under the condition of original stress field and varying stress field affected by roadway excavation in the labora-
https://doi.org/10.1016/j.psep.2019.08.019 0957-5820/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
2
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
Fig. 1. Schematic diagram of the method (a) principle and (b) equipment.
tory. Hu et al. (2015) first established the relationship between the permeability and the effective stress, and then used FLAC3D software to calculate the effective stress distributions around roadway; based on these results, the evolutions of coal seam permeability around roadway were established. Liu (2015) analyzed stress path variations of coal mass after roadway excavation, and used COMSOL Multiphysics software to establish the evolutions of coal seam permeability around roadway. Zhang (2017) determined the stress distributions of coal mass around roadway by using the drill cuttings method, and then established the evolutions of coal seam permeability around roadway, according to the stress-permeability relationship. Although the above studies established the distributions of coal seam permeability around roadways from various perspectives, they were all indirect methods and only considered a single factor affecting coal seam permeability. However, after roadway excavation, the coal seam permeability around roadways could still be affected by complex factors, such as changes in effective stress, changes in gas pressure, and generation of secondary fractures. Therefore, these indirect methods are only able to determine the evolutions of coal seam permeability to some extent, and there is still a certain gap in guiding engineering applications. In this paper, a new method was proposed to accurately and directly determine the coal seam permeability around roadway. First, the principle of the method was analyzed, and the method equipment was developed. Then, field experiments were carried out in Baiyangling coal mine. Finally, engineering applications of the method were analyzed, and a field application effect experiment was conducted to verify the new method. 2. Method 2.1. Method principle As recognized by many scholars, gas flow in a coal seam obeys Darcy’s law (Liu et al., 2018; Zhao et al., 2018; Zhou, 1990), and coal seam permeability can be expressed as (Hu et al., 2015): K=
2Q0 P0 L A(P12 − P22 )
,
(1)
Where, K is the coal seam permeability, Q0 is the volumetric flow rate at the reference pressure, is the viscosity, L is the length of the coal sample, P0 is the reference pressure, A is the cross-sectional area of the coal sample, P1 is the upstream gas pressure, and P2 is the
downstream gas pressure. From Eq. (1), under the same experimental conditions, the more developed the permeability, the higher is the flow rate. Therefore, the coal seam permeability around roadway can be obtained by measuring the gas flow rate at different positions within the coal seam. On this basis, this paper designed a new method (see Fig. 1a) to accurately and directly determine the coal seam permeability around roadways. 2.2. Equipment and operation procedures Based on the method principle (see Fig. 1a), equipments, including hole packers, water injection pump, roots flowmeter, cylinder, and pressure stabilizer were developed. These devices were connected by pipes (see Fig. 1b). The method used for field testing included the following steps: (1) Borehole of a certain length (exceeding the range of loose band around the roadway) were drilled from the roadway wall to the coal seam, and they had a dimension of 75 mm. Subsequently, the borehole length was recorded, and the drill cuttings were cleaned up using high-pressure air. (2) After the borehole was completed, two hole packers were inserted into the borehole in the connection order (see Fig. 1a). When it was sent to a predetermined location, the hole packers and water pump were connected by water injection pipes, and water was injected into the hole packers by a water pump until the water injection pressure reached 2 MPa, after which the water injection valve was closed. Hence, the sealing device expanded under the action of high-pressure water to form a gas-filled chamber of 1 m length (see Fig. 1a) in the borehole. In addition, the position of the hole packers was recorded to calculate the position of the gas chamber. (3) After the aforementioned steps were completed, flow rate measurements were started. First, the high-pressure gas cylinder, pressure-stabilizing valve, and flowmeter were connected by gas injection pipes. Subsequently, the gas pressure was adjusted to the predetermined value (0.5 MPa) via the pressurestabilizing valve. While the gas injection valve was open, the gas filled the chamber and flowed to the surrounding coal seam. The gas flow rate was measured by a flow meter, and the result was recorded on stabilization. (4) After the completion of step (3), the gas injection valve was closed till the gas pressure in the gas chamber was reduced
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
3
Fig. 2. Experimental area and main mining coal seam characteristics.
to zero, after which, the water injection valve was opened to relieve the high-pressure water. Subsequently, the hole packer was shifted by 1 m to repeat the procedures above. Thus, the flow measurements at different positions in the borehole were completed. With the results of flow rate at different positions in the coal seam, it was easily to determine the distributions of the coal seam permeability around the roadway. 2.3. Field situation The field experiments were performed in the Baiyangling coal mine in Shanxi province, China. The Baiyangling coal mine is located at the eastern edge of the Qinshui coal field, the first coal field for coal bed methane commercial exploitation in China (see Fig. 2). The coal-bearing strata in the coal mine belongs primarily to the carboniferous system of the Taiyuan formation and the second Permian Shanxi formation. Of them, the minable coal seams are stored in the Taiyuan group, while scattered coal seams or coal lines in the Shanxi group no. 9 and no. 15 coal seams are the primary mining coal seams (see Fig. 2). 2.4. Scheme of field experiments The no.15 coal seam of Baiyangling coal mine was selected as the experimental coal seam, where six experimental boreholes were designed. To avoid the influences of anthropic factors, a coal seam area devoid of drilling or other construction disturbance was selected as the experimental site. In addition, to master the flow law of the experimental boreholes, six boreholes were distributed at a distance of 20 m, 30 m, 30 m, 50 m, and 50 m to each other, respectively. The length and diameter of the boreholes were 22 m and 75 mm, respectively. 3. Results and discussion 3.1. Results and analysis The depth of each experimental borehole was measured from 3 m to 20 m, and the data were collected once per meter. A total of 18 sets of data were tested in each borehole. The results are presented in Fig. 3a. Although the flow data of each borehole was different, the overall variation trends were the same and can be
Fig. 3. (a) Test results of experimental boreholes; (b) distribution of coal permeability around roadway.
divided into four stages. In the initial section (stage I), the flow rate was relatively stable, followed by a slight decrease, and the flow rate was the largest. As the borehole depth increased, an area (stage II) with rapidly attenuating flow appeared. With further increase in borehole depth, the flow rate first decreased slightly and then gradually increased to a stable value. In the last stage (IV), the flow rate stabilized. According to Eq. (1), permeability maintained the same variation trend with the flow rate, as shown in Fig. 3b. Compared to the initial permeability (stage IV) of the coal seam, Stage I and stage II had increased permeability, and stage III consisted of low permeability.
4
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
Coal seam
Fractures
A
A—A’
Crack in the borehole Gas drainage pipe
Sealing material
A’
Methane
Air
˄a˅Front view
˄b˅Sectional view
Fig. 4. Analysis of possible leakage factors.
Additionally, Fig. 3a and b revealed that the maximum flow rates Qmax (maximum permeability, Kmax ) was much larger than the minimum flow rates Qmin (minimum permeability, Kmin ), with their ratios ranging from 9.29 at 3# borehole to 14.67 at 4# borehole. The Qmax difference was large between the six experimental boreholes. Third, Qmin (K min ) was slightly less than the stable flow rates Qstab (stable permeability, Kstab ). Finally, there was almost no difference between Qstab (K stab ). After roadway excavation, the stress of coal seam near the excavation area decreased (Lin et al., 1993), and a large number of secondary fractures were generated (Zhai et al., 2012), resulting in the largest flow rate in the initial section, thereby leading to an increase in permeability. As the development of secondary fractures was complicated, they were affected by many factors during roadway excavation, including human factors, excavation process, geological conditions, leading to differences in the development of fractures and permeability in different regions of the roadway. Thus, the difference between Qmax was large. With an increase in the distance from the roadway, stress and secondary fractures decreased. As a result, the flow rates and coal seam permeability reduced rapidly. In stage III, the secondary fractures were negligible, and the coal seam permeability was only affected by the stress. As the volume of the roadway excavation is small, it can be assumed that the stress in the stress concentrated area does not increase significantly compared to the original stress (Kwon et al., 2009; Islam and Shinjo, 2009). Thus, Qmin is slightly less than Qstab . Compared to stage I, II, and III, stage IV is not affected by the roadway excavation, wherein the permeability of the coal seam is at its original value. Therefore, there is almost no difference in the Qstab . 3.2. Evaluation of the method Compared with the current methods, the new method proposed in this paper exhibit several features. First, the new method directly measures the coal seam permeability around roadways in the field according to the permeability test principle, whereas the current methods indirectly determine the coal seam permeability by measuring the parameters affecting it or using numerical simulation. Second, indirect methods required large calculations or laboratory tests to determine the coal seam permeability around roadways, but the new method can determine the coal seam permeability by simple tests conducted on-site. Third, due to roadway excavation, the coal seam permeability around roadways is affected by complex factors, such as changes in effective stress, changes in gas pressure, and generation of secondary fractures. In practice, different geological conditions, coal seam occurrence conditions, coal seam structures, and even different tunneling technology can lead
to varying coal seam permeabilities around roadways. The indirect methods can only consider one or two factors that affects the coal seam permeability, but cannot fully consider all the influencing factors. Therefore, it is almost impossible for the indirect methods to accurately determine the distributions of coal seam permeability around roadways. However, the new method directly measures the coal seam permeability on site, thereby avoiding the consideration of complex factors affecting the coal seam permeability. The new method has the characteristics of a simple operation and a small test workload, and can accurately determine the distributions of coal seam permeability around roadways. Moreover, the results obtained by the new method can provide theoretical guidance for solving engineering problems.
4. Field application and validation 4.1. Application analysis 4.1.1. Increase gas extraction concentration In gas extraction, a difference in pressure is generated between the in-seam borehole and the coal seam roadway. Due to this difference in pressure, air flows into the extraction in-seam borehole through the leakage passage, thereby leading to air leakages. Although, in theory, the concentration of extracted gas should be close to 100%, it is generally not high in the actual extraction practice, especially for in-seam boreholes. In China, the gas extraction concentration of 80% in-seam boreholes varies from 6% to 20% in a short time (Lu et al., 2009) owing to the large amount of air leakage in the extraction process, thereby reducing the gas extraction rate and utilization efficiency. Based on the in-seam borehole extraction technology, there are three possible air leakage factors that can be determined in the process of in-seam borehole extraction: (a) leakage from the sealing material; (b) leakage from the contact area between the sealing material and the borehole wall; (c) leakage from fractures in the coal seam (see Fig. 4) (Xia et al., 2014). To determine the primary factors that lead to air leakage during the extraction, the actual phenomenon in the field is used for corroboration. According to the large amount of on-site gas pressure measurement work that the authors have been engaged in, gas pressure measurement in boreholes share the same sealing process with gas extraction in borehole. Nevertheless, the measured gas pressure can stabilize at several atmospheric pressures without the occurrence of gas leakage. This phenomenon indicates very slight gas leakage originating from the sealing material or the contact area in the extraction process.
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
5
Fig. 5. (a) The primary gas emission area of coal roadway, (b) cross-borehole gas extraction technology, and (c) in-seam borehole gas extraction technology.
In fact, the measurement of gas pressure in the in-seam borehole has strict requirements for the sealing depth (typically beyond 25 m), such that the sealing area avoids the high permeability areas in the coal seam. Hence, gas leakage rarely occurs in the gas pressure measurement borehole. In contrast, the in-seam extraction borehole requires only a sealing depth of 8 m, in general. The inappropriate sealing depth shows that the sealing area probably lies in the high permeability areas, causing serious leakage in the extraction process. To solve the air leakage problem, the appropriate sealing depth of the in-seam borehole needs to avoid the high permeability areas (Stages I and II in Fig. 3). Therefore, the low permeability area (Stage III in Fig. 3) determined by this method is selected as the sealing depth to increase the gas extraction concentration. This way, both the air leakage caused by the shallow borehole sealing, and the considerable cost and the reduction of effective extraction length caused by the deep borehole sealing can be avoided. 4.1.2. Gas disaster prevention In order to prevent gas disasters (such as, coal and gas outburst and gas explosion) during roadway excavation, gas pre-extraction is required. In China, the cross-borehole gas extraction technology (see Fig. 5 b) and in-seam borehole gas extraction technology (see Fig. 5 c) are used to extract gas from the coal seam around roadway (Kong et al., 2016; Lin et al., 2015; Zhang et al., 2017), and gas control area is a key parameter for gas pre-extraction design. 4.1.2.1. No risk outburst coal seams. In the case of no risk outburst coal seams, the key to prevent gas disasters is to control gas emission (Noack, 1998). As stated in Section 3.1, as the distance from the roadway increases, the permeability of coal seam surrounding the roadway first increases, then decreases, and finally stabilizes. In the coal seam, an area with reduced permeability may act as an obstacle, hindering the gas flow from the original coal seam to
the roadway. However, the gas in areas with high permeability may flow greatly to the roadway due to the increase of the coal seam permeability. Therefore, during the coal roadway excavation process, gas emissions from the coal seam around roadway mainly come from the high permeability areas (Stages I and II in Fig. 3 and Fig. 5 a). Areas with high permeability (Stages I and II in Fig. 3) were selected as the gas control area in the case of no risk outburst coal seams. Using this method, high permeability areas were accurately determined (Stages I and II in Fig. 3) and this provided a basis for the gas pre-extraction design of no risk outburst coal seams. 4.1.2.2. Risk outburst coal seams. It is widely recognized that outburst is greatly affected by stress and gas pressure (Beamish and Crosdale, 1998; Xue et al., 2011). In areas with low permeability (Stage III in Fig. 3), gas flowing into the roadway maintains a high gas pressure. Moreover, the stress in this area is higher than other areas (see Fig. 6). Thus, this area can be considered as the high-risk outburst area during the process of roadway excavation. Therefore, for a risk outburst coal seam, the gas control area needs to include not only the areas with high permeability (Stages I and II in Fig. 3), but also low permeability (Stage III in Fig. 3). Thus, based on the permeability evolutions around the roadway using this method, we can easily select the gas control area for risk outburst coal seams. 4.2. Validation of application effect The optimal way to verify the accuracy and reliability of this method is by studying the effects of the field application. According to Section 4.1.1, the air leakage amount can directly reflect the coal permeability in the extraction process, that is greater the leakage amount, greater is the coal permeability. Therefore, studying the gas concentration of in-seam boreholes at different sealing depth conditions is a simple verification way, which can directly reflect the effects of the field application.
6
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
Fig. 6. Stress and permeability distributions around the roadway.
Based on the experimental results, the appropriate sealing depths determined by the six experimental boreholes were found to be 13 m, 12 m, 11 m, 12 m, 12 m, and 13 m. The primary purpose of determining the sealing depth of the in-seam borehole was to solve the problem of air leakage during gas extraction; therefore, the largest value, 13 m, was selected as the appropriate sealing depth of the no. 15 coal seam. According to the leakage factors, when the sealing depth was smaller than 13 m, the leakage flow in the extraction process declined gradually with an increase in seal-
ing depth; on the contrary, when the sealing depth was greater than 13 m, the leakage flow in the extraction process remained unchanged with an increase in the sealing depth. Hence, to better verify the accuracy and reliability of the method, the sealing depths of the four groups of the extraction boreholes were designed to be 6 m, 8 m, 13 m, and 16 m. The experiments were performed in the same area and measured for a period of 30 days. The same sealing technology (cement mortar sealing technology) was applied to all the experimental boreholes. The test data of gas concentrations are shown in Fig. 7. From the overall gas concentration, some results can be observed readily. First, when the sealing depth is smaller than 13 m (see Fig. 7a and b), with time, the gas concentration exhibits an obvious attenuation and finally stabilizes at a relatively low gas concentration level. The primary reasons for this phenomenon are as follows: during the initial stage of extraction, the coal seam gas content, which remains in the original state, is high, and its path of migration is shorter. During this, gas migration is critical; therefore, the initial gas concentration is higher. As time progresses, the gas content in the coal seam reduces and the migration distance increases. In this case, the amount of gas transported to the borehole decreases gradually, while the amount of air leakage remains the same. Therefore, the gas concentration decreases rapidly in the initial stage, while it remains in a stable state in the later stage because of the stable path of gas migration and leakage rate. Next, when the depth of the sealing borehole is over 13 m (see Fig. 7c and d), there are fluctuations in gas extraction concentration, but no obvious attenuation appears as time progresses. This is primarily because when the sealing depth is greater than 13 m, the sealing area of the extrac-
Fig. 7. Test data of gas concentration of (a) sealing depth 6 m, (b) sealing depth 8 m, (c) sealing depth 13 m, and (d) sealing depth 16 m.
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
7
(3) Based on the field test data, 13 m was selected as the sealing depth of in-seam borehole for no. 15 coal seam in the Baiyangling coal mine. A field application effect experiment was conducted to verify the new method; the results showed that there is an obvious air leakage when the sealing depth is smaller than 13 m. On the contrary, the leakage is low when the sealing depth is over 13 m. This indicates that the new method is reasonable and reliable to determine the distributions of coal seam permeability around roadway and is able to provide theoretical guidance for solving the engineering problems. Declaration of Competing Interest None. Fig. 8. Mean gas concentrations of extraction boreholes.
tion borehole avoids the fractures development area. Owing to the relatively small amount of air leakage into the borehole (compared to the amount of gas transported to the borehole), gas migration is always dominant; thus, the gas extraction concentration remains stable at a higher level. With respect to the mean gas extraction concentrations of each group (see Fig. 8), at a sealing depth of 6 m, the gas extraction concentration is 31% on the first day and stabilizes at 10% after 10 days. At a sealing depth of 8 m, the gas concentration is 49% on the first day and stabilizes at 20% after 10 days. The 8 m sealing depth demonstrated a higher gas extraction concentration compared with that of 6 m. At sealing boreholes depth of 13 m and 16 m, the gas concentrations of the first day are 71% and 72%, respectively. As time progresses, although certain fluctuations in gas concentrations exist, they are relatively stable and have similar values. Therefore, gas extraction concentration increases with an increase in sealing depth when the sealing depth is smaller than 13 m, whereas the concentrations of different sealing depths do not change significantly when the sealing depth is over 13 m. To conclude, coal seam permeability around the roadway determined by the new method is accurate and reliable and is able to provide theoretical guidance for solving engineering problems. 5. Conclusions In order to solve the engineering problems induced by the redistribution of coal seam permeability after roadway excavation, a new method to determine the distributions of coal seam permeability around roadway was proposed in this paper. The primary conclusions are as follows: (1) Based on Darcy’s law, a new method to determine the evolution of coal seam permeability after roadway excavation was proposed, and the method and equipments were developed. Through field experiments in the Baiyangling coal mine, the distribution of coal seam permeability around roadway is divided into four stages. Stage I and stage II are high permeability areas, stage III is the low permeability area, and stage IV maintains the initial permeability of the coal seam. (2) The engineering applications of the method were analyzed. First, the position of the low permeability areas was selected as the appropriate sealing depth for the in-seam borehole to increase the gas extraction concentration. Second, high permeability areas were considered as the pre-extracted gas area for no risk outburst coal seams to control the gas emission. Finally, the high permeability and low permeability areas were found to be the least gas control area for risk outburst coal seams to prevent outburst disasters.
Acknowledgment This work was supported by Outstanding Innovation Scholarship for Doctoral Candidate of CUMT (2019YCBS047). References Beamish, B.B., Crosdale, P.J., 1998. Instantaneous outbursts in underground coal mines: an overview and association with coal type. Int. J. Coal Geol. 35, 27–55. Cao, J., Dai, L., Sun, H., Wang, B., Zhao, B., Yang, X., Zhao, X., Guo, P., 2019. Experimental study of the impact of gas adsorption on coal and gas outburst dynamic effects. Process Saf. Environ. 128, 158–166. Hu, S., Doctoral dissertation, 2014. Seepage Characteristics Around Gas Drainage Borehole and Blockage Mechanism of Powder. China University of Mining and Technology, Xuzhou, China. Hu, S., Zhou, F., Liu, Y., Xia, T., 2015. Effective stress and permeability redistributions induced by successive roadway and borehole excavations. Rock Mech. Rock Eng. 48, 319–332. Islam, M.R., Shinjo, R., 2009. Numerical simulation of stress distributions and displacements around an entry roadway with igneous intrusion and potential sources of seam gas emission of the Barapukuria coal mine, NW Bangladesh. Int. J. Coal Geol. 78, 249–262. Kong, X., Wang, E., Liu, X., Li, N., Chen, L., Feng, J., Kong, B., Li, D., Liu, Q., 2016. Coupled analysis about multi-factors to the effective influence radius of hydraulic flushing: application of response surface methodology. J. Nat. Gas Sci. Eng. 32, 538–548. Kwon, S., Lee, C.S., Cho, S.J., Jeon, S.W., Cho, W.J., 2009. An investigation of the excavation damaged zone at the KAERI underground research tunnel. Tunn. Undergr. Sp. Tech. 24, 1–13. Lin, B., Yan, F., Zhu, C., Zhou, Y., Zou, Q., Guo, C., Liu, T., 2015. Cross-borehole hydraulic slotting technique for preventing and controlling coal and gas outbursts during coal roadway excavation. J. Nat. Gas Sci. Eng. 26, 518–525. Lin, B., Zhou, S., Zhang, R., 1993. The stress relaxation zone in coal tunneling and its application to forecast of outburst danger. J. China Univ. Min. Tech. 22, 47–55. Liu, P., Qin, Y., Liu, S., Hao, Y., 2018. Numerical modeling of gas flow in coal using a modified dual-porosity model: a multi-mechanistic approach and finite difference method. Rock Mech. Rock Eng. 51, 2863–2880. Liu, Q., Doctoral dissertation, 2015. Damage and Permeability Evolution Mechanism of Dual-porosity Coal Under Multiple Stress Paths and Its Application. China University of Mining and Technology, Xuzhou, China. Lu, T., Yu, H., Zhou, T., Mao, J., Guo, B., 2009. Improvement of methane drainage in high gassy coal seam using waterjet technique. Int. J. Coal Geol. 79, 40–48. Noack, K., 1998. Control of gas emissions in underground coal mines. Int. J. Coal Geol. 35, 57–82. Qin, B., Li, L., Ma, D., Lu, Y., Zhong, X., Jia, Y., 2016. Control technology for the avoidance of the simultaneous occurrence of a methane explosion and spontaneous coal combustion in a coal mine: a case study. Process Saf. Environ. 103, 203–211. Shi, L., Wang, J., Zhang, G., Cheng, X., Zhao, X., 2017. A risk assessment method to quantitatively investigate the methane explosion in underground coal mine. Process Saf. Environ. 107, 317–333. Wang, L., Cheng, Y., Ge, C., Chen, J., Li, W., Zhou, H., Wang, H., 2013. Safety technologies for the excavation of coal and gas outburst-prone coal seams in deep shafts. Int. J. Rock Mech. Min. 57, 24–33. Wang, L., Liu, S., Cheng, Y., Yin, G., Guo, P., Mou, J., 2017. The effects of magma intrusion on localized stress distribution and its implications for coal mine outburst hazards. Eng. Geol. 218, 12–21. Wei, F., Lian, S., Zhang, C., Liu, S., 2013. Study on reasonable hole-sealing-depth down the seam of Mengjin mine. J. Henan Polytech. Univ. 32, 686–690. Xia, T., Zhou, F., Liu, J., Hu, S., Liu, Y., 2014. A fully coupled coal deformation and compositional flow model for the control of the pre-mining coal seam gas extraction. Int. J. Rock Mech. Min. 72, 138–148. Xiang, X., Zhai, C., Xu, Y., Yu, X., Xu, J., 2015. A flexible gel sealing material and a novel active sealing method for coal-bed methane drainage boreholes. J. Nat. Gas Sci. Eng. 26, 1187–1199.
8
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
Xue, S., Wang, Y., Xie, J., Wang, G., 2011. A coupled approach to simulate initiation of outbursts of coal and gas - Model development. Int. J. Coal Geol. 86, 222–230. Zhang, H., Cheng, Y., Liu, Q., Yuan, L., Dong, J., Wang, L., Qi, Y., Wang, W., 2017. A novel in-seam borehole hydraulic flushing gas extraction technology in the heading face: enhanced permeability mechanism, gas flow characteristics, and application. J. Nat. Gas Sci. Eng. 46, 498–514. Zhang, L., Master dissertation, 2017. Study on the Reasonable Sealing Parameters of Gas Drainage Boreholes Along Coal Seam. North China Institute of Science and Technology, Langfang, China. Zhai, C., Li, Q., Sun, C., Ni, G., Yang, W., 2012. Analysis on borehole instability and control method of pore-forming of hydraulic fracturing in soft coal seam. J. China Coal Soc. 37, 1431–1436. Zhao, Y., Wang, Z., Ye, J., Sun, H., Gu, J., 2018. Lattice Boltzmann simulation of gas flow and permeability prediction in coal fracture networks. J. Nat. Gas Sci. Eng. 53, 153–162.
Zheng, C., Jiang, B., Xue, S., Chen, Z.W., Li, H., 2019. Coalbed methane emissions and drainage methods in underground mining for mining safety and environmental benefits: a review. Process Saf. Environ. 127, 103–124. Zhou, B., Yang, S., Wang, C., Cai, J., Xu, Q., Sang, N., 2019. Experimental study on the influence of coal oxidation on coal and gas outburst during invasion of magmatic rocks into coal seams. Process Saf. Environ. 124, 213–222. Zhou, F., 2012. Study on the coexistence of gas and coal spontaneous combustion (I): disaster mechanism. J. Coal Sci. Eng. 37, 843–849. Zhou, S., 1990. Mechanism of gas flow in coal seams. J. China Coal Soc. 15, 15–24.
Contents lists available at ScienceDirect
Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
A new method for determining coal seam permeability redistribution induced by roadway excavation and its applications Huihui Liu a,b , Baiquan Lin a,b,∗ , Chenglin Jiang a,b a b
Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China
a r t i c l e
i n f o
Article history: Received 13 June 2019 Received in revised form 23 July 2019 Accepted 20 August 2019 Available online 23 August 2019 Keywords: Gas disaster prevention Coal mine gas extraction Gas utilization Coal seam permeability Coal roadway excavation
a b s t r a c t Changes in coal seam permeability induced by roadway excavation may lead to many engineering problems, such as low gas extraction concentration in in-seam boreholes, coal and gas outburst disasters, and gas emissions in the process of coal roadway excavation. To solve these problems, this paper proposed a new method to directly determine the distribution of coal seam permeability around roadways. For this purpose, method principle was analyzed, equipment was developed, and field experiments were carried out in Baiyangling coal mine. Finally, the engineering applications of the method were analyzed, and a field application effect experiment was conducted to verify the new method. The results showed that the coal seam permeability around roadways was divided into four stages. In stages I and II, permeability increased, in stage III, permeability decreased, and stage IV maintained the initial permeability of the coal seam. These test results will aid in increasing the gas extraction concentration of in-seam boreholes and will guide the design of gas control areas. Using the verification experiment, it was concluded that the new method is able to accurately determine the redistribution of coal seam permeability caused by roadway excavation and provide theoretical guidance in solving engineering problems. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Coal roadway excavation is a primary aspect of mining activities in underground coal mines. Owing to the redistribution of stress and gas pressure around the roadway, and the destruction caused by the roadway tunneling process, coal seam permeability around the roadway tends to change. As the distance from the roadway increases, the permeability of coal seam surrounding the roadway initially increases, then decreases, and finally stabilizes (Hu et al., 2015; Xia et al., 2014; Xiang et al., 2015). This change may induce many engineering problems. First, during the roadway excavation process, gas emissions from the coal seam around roadways are mainly from the highly permeable areas, which is a main factor in inducing gas explosions (Qin et al., 2016; Shi et al., 2017). Second, the in-seam extraction borehole is a widely used method to control gas in China underground mines. However, the gas extraction concentration in 80% of the in-seam boreholes varies from 6% to 20% in a short time owing to the large amount of gas leakage in the extraction process (Lu et al., 2009). This is primarily caused
∗ Corresponding author at: School of Safety Engineering, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China. E-mail address: [email protected] (B. Lin).
by changes in coal seam permeability after roadway excavation. Such low concentrations of gas are not utilizable, resulting in serious environmental pollution and energy waste due to the gas being directly released (Xia et al., 2014; Zheng et al., 2019). In addition, continuous air leakage may also cause spontaneous combustion of coal (Zhou, 2012). Finally, areas with low permeability and high stress caused by roadway excavation are the highest risk of coal and gas outbursts (referred to as outburst), which is one of the major disasters in coal mines (Cao et al., 2019; Wang et al., 2013, 2017; Zhou et al., 2019). The key to solving the aforementioned engineering problems is to accurately determine the redistributions of coal seam permeability after roadway excavation. In recent years, many studies have been carried out in the related fields. Xia et al. (2014) used the COMSOL Multiphysics software to simulate changes in coal seam permeability affected by roadway excavation. Based on stress distributions of the coal mass around roadways, Xiang et al. (2015) qualitatively analyzed the redistributions of coal seam permeability after roadway excavation. Wei et al. (2013) established the distributions of coal seam permeability around roadways by measuring the variation law of the drill cutting desorption index. Hu (2014) used experimental methods to investigate the evolution of coal seam permeability under the condition of original stress field and varying stress field affected by roadway excavation in the labora-
https://doi.org/10.1016/j.psep.2019.08.019 0957-5820/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
2
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
Fig. 1. Schematic diagram of the method (a) principle and (b) equipment.
tory. Hu et al. (2015) first established the relationship between the permeability and the effective stress, and then used FLAC3D software to calculate the effective stress distributions around roadway; based on these results, the evolutions of coal seam permeability around roadway were established. Liu (2015) analyzed stress path variations of coal mass after roadway excavation, and used COMSOL Multiphysics software to establish the evolutions of coal seam permeability around roadway. Zhang (2017) determined the stress distributions of coal mass around roadway by using the drill cuttings method, and then established the evolutions of coal seam permeability around roadway, according to the stress-permeability relationship. Although the above studies established the distributions of coal seam permeability around roadways from various perspectives, they were all indirect methods and only considered a single factor affecting coal seam permeability. However, after roadway excavation, the coal seam permeability around roadways could still be affected by complex factors, such as changes in effective stress, changes in gas pressure, and generation of secondary fractures. Therefore, these indirect methods are only able to determine the evolutions of coal seam permeability to some extent, and there is still a certain gap in guiding engineering applications. In this paper, a new method was proposed to accurately and directly determine the coal seam permeability around roadway. First, the principle of the method was analyzed, and the method equipment was developed. Then, field experiments were carried out in Baiyangling coal mine. Finally, engineering applications of the method were analyzed, and a field application effect experiment was conducted to verify the new method. 2. Method 2.1. Method principle As recognized by many scholars, gas flow in a coal seam obeys Darcy’s law (Liu et al., 2018; Zhao et al., 2018; Zhou, 1990), and coal seam permeability can be expressed as (Hu et al., 2015): K=
2Q0 P0 L A(P12 − P22 )
,
(1)
Where, K is the coal seam permeability, Q0 is the volumetric flow rate at the reference pressure, is the viscosity, L is the length of the coal sample, P0 is the reference pressure, A is the cross-sectional area of the coal sample, P1 is the upstream gas pressure, and P2 is the
downstream gas pressure. From Eq. (1), under the same experimental conditions, the more developed the permeability, the higher is the flow rate. Therefore, the coal seam permeability around roadway can be obtained by measuring the gas flow rate at different positions within the coal seam. On this basis, this paper designed a new method (see Fig. 1a) to accurately and directly determine the coal seam permeability around roadways. 2.2. Equipment and operation procedures Based on the method principle (see Fig. 1a), equipments, including hole packers, water injection pump, roots flowmeter, cylinder, and pressure stabilizer were developed. These devices were connected by pipes (see Fig. 1b). The method used for field testing included the following steps: (1) Borehole of a certain length (exceeding the range of loose band around the roadway) were drilled from the roadway wall to the coal seam, and they had a dimension of 75 mm. Subsequently, the borehole length was recorded, and the drill cuttings were cleaned up using high-pressure air. (2) After the borehole was completed, two hole packers were inserted into the borehole in the connection order (see Fig. 1a). When it was sent to a predetermined location, the hole packers and water pump were connected by water injection pipes, and water was injected into the hole packers by a water pump until the water injection pressure reached 2 MPa, after which the water injection valve was closed. Hence, the sealing device expanded under the action of high-pressure water to form a gas-filled chamber of 1 m length (see Fig. 1a) in the borehole. In addition, the position of the hole packers was recorded to calculate the position of the gas chamber. (3) After the aforementioned steps were completed, flow rate measurements were started. First, the high-pressure gas cylinder, pressure-stabilizing valve, and flowmeter were connected by gas injection pipes. Subsequently, the gas pressure was adjusted to the predetermined value (0.5 MPa) via the pressurestabilizing valve. While the gas injection valve was open, the gas filled the chamber and flowed to the surrounding coal seam. The gas flow rate was measured by a flow meter, and the result was recorded on stabilization. (4) After the completion of step (3), the gas injection valve was closed till the gas pressure in the gas chamber was reduced
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
3
Fig. 2. Experimental area and main mining coal seam characteristics.
to zero, after which, the water injection valve was opened to relieve the high-pressure water. Subsequently, the hole packer was shifted by 1 m to repeat the procedures above. Thus, the flow measurements at different positions in the borehole were completed. With the results of flow rate at different positions in the coal seam, it was easily to determine the distributions of the coal seam permeability around the roadway. 2.3. Field situation The field experiments were performed in the Baiyangling coal mine in Shanxi province, China. The Baiyangling coal mine is located at the eastern edge of the Qinshui coal field, the first coal field for coal bed methane commercial exploitation in China (see Fig. 2). The coal-bearing strata in the coal mine belongs primarily to the carboniferous system of the Taiyuan formation and the second Permian Shanxi formation. Of them, the minable coal seams are stored in the Taiyuan group, while scattered coal seams or coal lines in the Shanxi group no. 9 and no. 15 coal seams are the primary mining coal seams (see Fig. 2). 2.4. Scheme of field experiments The no.15 coal seam of Baiyangling coal mine was selected as the experimental coal seam, where six experimental boreholes were designed. To avoid the influences of anthropic factors, a coal seam area devoid of drilling or other construction disturbance was selected as the experimental site. In addition, to master the flow law of the experimental boreholes, six boreholes were distributed at a distance of 20 m, 30 m, 30 m, 50 m, and 50 m to each other, respectively. The length and diameter of the boreholes were 22 m and 75 mm, respectively. 3. Results and discussion 3.1. Results and analysis The depth of each experimental borehole was measured from 3 m to 20 m, and the data were collected once per meter. A total of 18 sets of data were tested in each borehole. The results are presented in Fig. 3a. Although the flow data of each borehole was different, the overall variation trends were the same and can be
Fig. 3. (a) Test results of experimental boreholes; (b) distribution of coal permeability around roadway.
divided into four stages. In the initial section (stage I), the flow rate was relatively stable, followed by a slight decrease, and the flow rate was the largest. As the borehole depth increased, an area (stage II) with rapidly attenuating flow appeared. With further increase in borehole depth, the flow rate first decreased slightly and then gradually increased to a stable value. In the last stage (IV), the flow rate stabilized. According to Eq. (1), permeability maintained the same variation trend with the flow rate, as shown in Fig. 3b. Compared to the initial permeability (stage IV) of the coal seam, Stage I and stage II had increased permeability, and stage III consisted of low permeability.
4
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
Coal seam
Fractures
A
A—A’
Crack in the borehole Gas drainage pipe
Sealing material
A’
Methane
Air
˄a˅Front view
˄b˅Sectional view
Fig. 4. Analysis of possible leakage factors.
Additionally, Fig. 3a and b revealed that the maximum flow rates Qmax (maximum permeability, Kmax ) was much larger than the minimum flow rates Qmin (minimum permeability, Kmin ), with their ratios ranging from 9.29 at 3# borehole to 14.67 at 4# borehole. The Qmax difference was large between the six experimental boreholes. Third, Qmin (K min ) was slightly less than the stable flow rates Qstab (stable permeability, Kstab ). Finally, there was almost no difference between Qstab (K stab ). After roadway excavation, the stress of coal seam near the excavation area decreased (Lin et al., 1993), and a large number of secondary fractures were generated (Zhai et al., 2012), resulting in the largest flow rate in the initial section, thereby leading to an increase in permeability. As the development of secondary fractures was complicated, they were affected by many factors during roadway excavation, including human factors, excavation process, geological conditions, leading to differences in the development of fractures and permeability in different regions of the roadway. Thus, the difference between Qmax was large. With an increase in the distance from the roadway, stress and secondary fractures decreased. As a result, the flow rates and coal seam permeability reduced rapidly. In stage III, the secondary fractures were negligible, and the coal seam permeability was only affected by the stress. As the volume of the roadway excavation is small, it can be assumed that the stress in the stress concentrated area does not increase significantly compared to the original stress (Kwon et al., 2009; Islam and Shinjo, 2009). Thus, Qmin is slightly less than Qstab . Compared to stage I, II, and III, stage IV is not affected by the roadway excavation, wherein the permeability of the coal seam is at its original value. Therefore, there is almost no difference in the Qstab . 3.2. Evaluation of the method Compared with the current methods, the new method proposed in this paper exhibit several features. First, the new method directly measures the coal seam permeability around roadways in the field according to the permeability test principle, whereas the current methods indirectly determine the coal seam permeability by measuring the parameters affecting it or using numerical simulation. Second, indirect methods required large calculations or laboratory tests to determine the coal seam permeability around roadways, but the new method can determine the coal seam permeability by simple tests conducted on-site. Third, due to roadway excavation, the coal seam permeability around roadways is affected by complex factors, such as changes in effective stress, changes in gas pressure, and generation of secondary fractures. In practice, different geological conditions, coal seam occurrence conditions, coal seam structures, and even different tunneling technology can lead
to varying coal seam permeabilities around roadways. The indirect methods can only consider one or two factors that affects the coal seam permeability, but cannot fully consider all the influencing factors. Therefore, it is almost impossible for the indirect methods to accurately determine the distributions of coal seam permeability around roadways. However, the new method directly measures the coal seam permeability on site, thereby avoiding the consideration of complex factors affecting the coal seam permeability. The new method has the characteristics of a simple operation and a small test workload, and can accurately determine the distributions of coal seam permeability around roadways. Moreover, the results obtained by the new method can provide theoretical guidance for solving engineering problems.
4. Field application and validation 4.1. Application analysis 4.1.1. Increase gas extraction concentration In gas extraction, a difference in pressure is generated between the in-seam borehole and the coal seam roadway. Due to this difference in pressure, air flows into the extraction in-seam borehole through the leakage passage, thereby leading to air leakages. Although, in theory, the concentration of extracted gas should be close to 100%, it is generally not high in the actual extraction practice, especially for in-seam boreholes. In China, the gas extraction concentration of 80% in-seam boreholes varies from 6% to 20% in a short time (Lu et al., 2009) owing to the large amount of air leakage in the extraction process, thereby reducing the gas extraction rate and utilization efficiency. Based on the in-seam borehole extraction technology, there are three possible air leakage factors that can be determined in the process of in-seam borehole extraction: (a) leakage from the sealing material; (b) leakage from the contact area between the sealing material and the borehole wall; (c) leakage from fractures in the coal seam (see Fig. 4) (Xia et al., 2014). To determine the primary factors that lead to air leakage during the extraction, the actual phenomenon in the field is used for corroboration. According to the large amount of on-site gas pressure measurement work that the authors have been engaged in, gas pressure measurement in boreholes share the same sealing process with gas extraction in borehole. Nevertheless, the measured gas pressure can stabilize at several atmospheric pressures without the occurrence of gas leakage. This phenomenon indicates very slight gas leakage originating from the sealing material or the contact area in the extraction process.
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
5
Fig. 5. (a) The primary gas emission area of coal roadway, (b) cross-borehole gas extraction technology, and (c) in-seam borehole gas extraction technology.
In fact, the measurement of gas pressure in the in-seam borehole has strict requirements for the sealing depth (typically beyond 25 m), such that the sealing area avoids the high permeability areas in the coal seam. Hence, gas leakage rarely occurs in the gas pressure measurement borehole. In contrast, the in-seam extraction borehole requires only a sealing depth of 8 m, in general. The inappropriate sealing depth shows that the sealing area probably lies in the high permeability areas, causing serious leakage in the extraction process. To solve the air leakage problem, the appropriate sealing depth of the in-seam borehole needs to avoid the high permeability areas (Stages I and II in Fig. 3). Therefore, the low permeability area (Stage III in Fig. 3) determined by this method is selected as the sealing depth to increase the gas extraction concentration. This way, both the air leakage caused by the shallow borehole sealing, and the considerable cost and the reduction of effective extraction length caused by the deep borehole sealing can be avoided. 4.1.2. Gas disaster prevention In order to prevent gas disasters (such as, coal and gas outburst and gas explosion) during roadway excavation, gas pre-extraction is required. In China, the cross-borehole gas extraction technology (see Fig. 5 b) and in-seam borehole gas extraction technology (see Fig. 5 c) are used to extract gas from the coal seam around roadway (Kong et al., 2016; Lin et al., 2015; Zhang et al., 2017), and gas control area is a key parameter for gas pre-extraction design. 4.1.2.1. No risk outburst coal seams. In the case of no risk outburst coal seams, the key to prevent gas disasters is to control gas emission (Noack, 1998). As stated in Section 3.1, as the distance from the roadway increases, the permeability of coal seam surrounding the roadway first increases, then decreases, and finally stabilizes. In the coal seam, an area with reduced permeability may act as an obstacle, hindering the gas flow from the original coal seam to
the roadway. However, the gas in areas with high permeability may flow greatly to the roadway due to the increase of the coal seam permeability. Therefore, during the coal roadway excavation process, gas emissions from the coal seam around roadway mainly come from the high permeability areas (Stages I and II in Fig. 3 and Fig. 5 a). Areas with high permeability (Stages I and II in Fig. 3) were selected as the gas control area in the case of no risk outburst coal seams. Using this method, high permeability areas were accurately determined (Stages I and II in Fig. 3) and this provided a basis for the gas pre-extraction design of no risk outburst coal seams. 4.1.2.2. Risk outburst coal seams. It is widely recognized that outburst is greatly affected by stress and gas pressure (Beamish and Crosdale, 1998; Xue et al., 2011). In areas with low permeability (Stage III in Fig. 3), gas flowing into the roadway maintains a high gas pressure. Moreover, the stress in this area is higher than other areas (see Fig. 6). Thus, this area can be considered as the high-risk outburst area during the process of roadway excavation. Therefore, for a risk outburst coal seam, the gas control area needs to include not only the areas with high permeability (Stages I and II in Fig. 3), but also low permeability (Stage III in Fig. 3). Thus, based on the permeability evolutions around the roadway using this method, we can easily select the gas control area for risk outburst coal seams. 4.2. Validation of application effect The optimal way to verify the accuracy and reliability of this method is by studying the effects of the field application. According to Section 4.1.1, the air leakage amount can directly reflect the coal permeability in the extraction process, that is greater the leakage amount, greater is the coal permeability. Therefore, studying the gas concentration of in-seam boreholes at different sealing depth conditions is a simple verification way, which can directly reflect the effects of the field application.
6
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
Fig. 6. Stress and permeability distributions around the roadway.
Based on the experimental results, the appropriate sealing depths determined by the six experimental boreholes were found to be 13 m, 12 m, 11 m, 12 m, 12 m, and 13 m. The primary purpose of determining the sealing depth of the in-seam borehole was to solve the problem of air leakage during gas extraction; therefore, the largest value, 13 m, was selected as the appropriate sealing depth of the no. 15 coal seam. According to the leakage factors, when the sealing depth was smaller than 13 m, the leakage flow in the extraction process declined gradually with an increase in seal-
ing depth; on the contrary, when the sealing depth was greater than 13 m, the leakage flow in the extraction process remained unchanged with an increase in the sealing depth. Hence, to better verify the accuracy and reliability of the method, the sealing depths of the four groups of the extraction boreholes were designed to be 6 m, 8 m, 13 m, and 16 m. The experiments were performed in the same area and measured for a period of 30 days. The same sealing technology (cement mortar sealing technology) was applied to all the experimental boreholes. The test data of gas concentrations are shown in Fig. 7. From the overall gas concentration, some results can be observed readily. First, when the sealing depth is smaller than 13 m (see Fig. 7a and b), with time, the gas concentration exhibits an obvious attenuation and finally stabilizes at a relatively low gas concentration level. The primary reasons for this phenomenon are as follows: during the initial stage of extraction, the coal seam gas content, which remains in the original state, is high, and its path of migration is shorter. During this, gas migration is critical; therefore, the initial gas concentration is higher. As time progresses, the gas content in the coal seam reduces and the migration distance increases. In this case, the amount of gas transported to the borehole decreases gradually, while the amount of air leakage remains the same. Therefore, the gas concentration decreases rapidly in the initial stage, while it remains in a stable state in the later stage because of the stable path of gas migration and leakage rate. Next, when the depth of the sealing borehole is over 13 m (see Fig. 7c and d), there are fluctuations in gas extraction concentration, but no obvious attenuation appears as time progresses. This is primarily because when the sealing depth is greater than 13 m, the sealing area of the extrac-
Fig. 7. Test data of gas concentration of (a) sealing depth 6 m, (b) sealing depth 8 m, (c) sealing depth 13 m, and (d) sealing depth 16 m.
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
7
(3) Based on the field test data, 13 m was selected as the sealing depth of in-seam borehole for no. 15 coal seam in the Baiyangling coal mine. A field application effect experiment was conducted to verify the new method; the results showed that there is an obvious air leakage when the sealing depth is smaller than 13 m. On the contrary, the leakage is low when the sealing depth is over 13 m. This indicates that the new method is reasonable and reliable to determine the distributions of coal seam permeability around roadway and is able to provide theoretical guidance for solving the engineering problems. Declaration of Competing Interest None. Fig. 8. Mean gas concentrations of extraction boreholes.
tion borehole avoids the fractures development area. Owing to the relatively small amount of air leakage into the borehole (compared to the amount of gas transported to the borehole), gas migration is always dominant; thus, the gas extraction concentration remains stable at a higher level. With respect to the mean gas extraction concentrations of each group (see Fig. 8), at a sealing depth of 6 m, the gas extraction concentration is 31% on the first day and stabilizes at 10% after 10 days. At a sealing depth of 8 m, the gas concentration is 49% on the first day and stabilizes at 20% after 10 days. The 8 m sealing depth demonstrated a higher gas extraction concentration compared with that of 6 m. At sealing boreholes depth of 13 m and 16 m, the gas concentrations of the first day are 71% and 72%, respectively. As time progresses, although certain fluctuations in gas concentrations exist, they are relatively stable and have similar values. Therefore, gas extraction concentration increases with an increase in sealing depth when the sealing depth is smaller than 13 m, whereas the concentrations of different sealing depths do not change significantly when the sealing depth is over 13 m. To conclude, coal seam permeability around the roadway determined by the new method is accurate and reliable and is able to provide theoretical guidance for solving engineering problems. 5. Conclusions In order to solve the engineering problems induced by the redistribution of coal seam permeability after roadway excavation, a new method to determine the distributions of coal seam permeability around roadway was proposed in this paper. The primary conclusions are as follows: (1) Based on Darcy’s law, a new method to determine the evolution of coal seam permeability after roadway excavation was proposed, and the method and equipments were developed. Through field experiments in the Baiyangling coal mine, the distribution of coal seam permeability around roadway is divided into four stages. Stage I and stage II are high permeability areas, stage III is the low permeability area, and stage IV maintains the initial permeability of the coal seam. (2) The engineering applications of the method were analyzed. First, the position of the low permeability areas was selected as the appropriate sealing depth for the in-seam borehole to increase the gas extraction concentration. Second, high permeability areas were considered as the pre-extracted gas area for no risk outburst coal seams to control the gas emission. Finally, the high permeability and low permeability areas were found to be the least gas control area for risk outburst coal seams to prevent outburst disasters.
Acknowledgment This work was supported by Outstanding Innovation Scholarship for Doctoral Candidate of CUMT (2019YCBS047). References Beamish, B.B., Crosdale, P.J., 1998. Instantaneous outbursts in underground coal mines: an overview and association with coal type. Int. J. Coal Geol. 35, 27–55. Cao, J., Dai, L., Sun, H., Wang, B., Zhao, B., Yang, X., Zhao, X., Guo, P., 2019. Experimental study of the impact of gas adsorption on coal and gas outburst dynamic effects. Process Saf. Environ. 128, 158–166. Hu, S., Doctoral dissertation, 2014. Seepage Characteristics Around Gas Drainage Borehole and Blockage Mechanism of Powder. China University of Mining and Technology, Xuzhou, China. Hu, S., Zhou, F., Liu, Y., Xia, T., 2015. Effective stress and permeability redistributions induced by successive roadway and borehole excavations. Rock Mech. Rock Eng. 48, 319–332. Islam, M.R., Shinjo, R., 2009. Numerical simulation of stress distributions and displacements around an entry roadway with igneous intrusion and potential sources of seam gas emission of the Barapukuria coal mine, NW Bangladesh. Int. J. Coal Geol. 78, 249–262. Kong, X., Wang, E., Liu, X., Li, N., Chen, L., Feng, J., Kong, B., Li, D., Liu, Q., 2016. Coupled analysis about multi-factors to the effective influence radius of hydraulic flushing: application of response surface methodology. J. Nat. Gas Sci. Eng. 32, 538–548. Kwon, S., Lee, C.S., Cho, S.J., Jeon, S.W., Cho, W.J., 2009. An investigation of the excavation damaged zone at the KAERI underground research tunnel. Tunn. Undergr. Sp. Tech. 24, 1–13. Lin, B., Yan, F., Zhu, C., Zhou, Y., Zou, Q., Guo, C., Liu, T., 2015. Cross-borehole hydraulic slotting technique for preventing and controlling coal and gas outbursts during coal roadway excavation. J. Nat. Gas Sci. Eng. 26, 518–525. Lin, B., Zhou, S., Zhang, R., 1993. The stress relaxation zone in coal tunneling and its application to forecast of outburst danger. J. China Univ. Min. Tech. 22, 47–55. Liu, P., Qin, Y., Liu, S., Hao, Y., 2018. Numerical modeling of gas flow in coal using a modified dual-porosity model: a multi-mechanistic approach and finite difference method. Rock Mech. Rock Eng. 51, 2863–2880. Liu, Q., Doctoral dissertation, 2015. Damage and Permeability Evolution Mechanism of Dual-porosity Coal Under Multiple Stress Paths and Its Application. China University of Mining and Technology, Xuzhou, China. Lu, T., Yu, H., Zhou, T., Mao, J., Guo, B., 2009. Improvement of methane drainage in high gassy coal seam using waterjet technique. Int. J. Coal Geol. 79, 40–48. Noack, K., 1998. Control of gas emissions in underground coal mines. Int. J. Coal Geol. 35, 57–82. Qin, B., Li, L., Ma, D., Lu, Y., Zhong, X., Jia, Y., 2016. Control technology for the avoidance of the simultaneous occurrence of a methane explosion and spontaneous coal combustion in a coal mine: a case study. Process Saf. Environ. 103, 203–211. Shi, L., Wang, J., Zhang, G., Cheng, X., Zhao, X., 2017. A risk assessment method to quantitatively investigate the methane explosion in underground coal mine. Process Saf. Environ. 107, 317–333. Wang, L., Cheng, Y., Ge, C., Chen, J., Li, W., Zhou, H., Wang, H., 2013. Safety technologies for the excavation of coal and gas outburst-prone coal seams in deep shafts. Int. J. Rock Mech. Min. 57, 24–33. Wang, L., Liu, S., Cheng, Y., Yin, G., Guo, P., Mou, J., 2017. The effects of magma intrusion on localized stress distribution and its implications for coal mine outburst hazards. Eng. Geol. 218, 12–21. Wei, F., Lian, S., Zhang, C., Liu, S., 2013. Study on reasonable hole-sealing-depth down the seam of Mengjin mine. J. Henan Polytech. Univ. 32, 686–690. Xia, T., Zhou, F., Liu, J., Hu, S., Liu, Y., 2014. A fully coupled coal deformation and compositional flow model for the control of the pre-mining coal seam gas extraction. Int. J. Rock Mech. Min. 72, 138–148. Xiang, X., Zhai, C., Xu, Y., Yu, X., Xu, J., 2015. A flexible gel sealing material and a novel active sealing method for coal-bed methane drainage boreholes. J. Nat. Gas Sci. Eng. 26, 1187–1199.
8
H. Liu, B. Lin and C. Jiang / Process Safety and Environmental Protection 131 (2019) 1–8
Xue, S., Wang, Y., Xie, J., Wang, G., 2011. A coupled approach to simulate initiation of outbursts of coal and gas - Model development. Int. J. Coal Geol. 86, 222–230. Zhang, H., Cheng, Y., Liu, Q., Yuan, L., Dong, J., Wang, L., Qi, Y., Wang, W., 2017. A novel in-seam borehole hydraulic flushing gas extraction technology in the heading face: enhanced permeability mechanism, gas flow characteristics, and application. J. Nat. Gas Sci. Eng. 46, 498–514. Zhang, L., Master dissertation, 2017. Study on the Reasonable Sealing Parameters of Gas Drainage Boreholes Along Coal Seam. North China Institute of Science and Technology, Langfang, China. Zhai, C., Li, Q., Sun, C., Ni, G., Yang, W., 2012. Analysis on borehole instability and control method of pore-forming of hydraulic fracturing in soft coal seam. J. China Coal Soc. 37, 1431–1436. Zhao, Y., Wang, Z., Ye, J., Sun, H., Gu, J., 2018. Lattice Boltzmann simulation of gas flow and permeability prediction in coal fracture networks. J. Nat. Gas Sci. Eng. 53, 153–162.
Zheng, C., Jiang, B., Xue, S., Chen, Z.W., Li, H., 2019. Coalbed methane emissions and drainage methods in underground mining for mining safety and environmental benefits: a review. Process Saf. Environ. 127, 103–124. Zhou, B., Yang, S., Wang, C., Cai, J., Xu, Q., Sang, N., 2019. Experimental study on the influence of coal oxidation on coal and gas outburst during invasion of magmatic rocks into coal seams. Process Saf. Environ. 124, 213–222. Zhou, F., 2012. Study on the coexistence of gas and coal spontaneous combustion (I): disaster mechanism. J. Coal Sci. Eng. 37, 843–849. Zhou, S., 1990. Mechanism of gas flow in coal seams. J. China Coal Soc. 15, 15–24.