Investigation of permeability evolution in the lower slice during thick seam slicing mining and gas drainage: A case study from the Dahuangshan coalmine in China

doi:10.1016/j.jngse.2018.01.036

Journal of Natural Gas Science and Engineering

Volume 52, April 2018, Pages 141-154

https://doi.org/10.1016/j.jngse.2018.01.036 Get rights and content

Highlights

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By combining the UDEC numerical simulation with the proposed permeability model, permeability evolution in the lower slice during thick seam slicing mining was studied.
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The lower slice can be divided into six zones in the horizontal direction and two zones in the vertical direction with different permeability characteristics.
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Draining gas from the full pressure relief zone of the lower slice is crucial to gas control in top slicing.

Abstract

During thick seam slicing, the permeability distribution of the lower slice has important significance for guiding the gas drainage. To investigate the characteristics of the permeability evolution in the lower slice during the upper slice mining, a stress-induced permeability evolution model was proposed, and then, based on the top slice mining of the Zhongdacao coal seam in the Dahuangshan coal mine, a series of numerical simulation was performed using the Universal Distinct Element Code (UDEC) software program. The results show that during top slicing, a compacted zone appears in the collapsed overlying strata, which causes more intense compaction in the lower slice. Permeability distribution of the lower slice develops regularly, and after a certain distance of top slicing, the regional distribution characteristic of permeability becomes significant. The lower slice can be divided horizontally into six zones: the original permeability zone with low permeability, the stress concentration zone with permeability decreasing, the initial pressure relief zone with permeability increasing, the full pressure relief zone with high permeability, the pressure recovery zone with permeability decreasing, and the compacted zone with low permeability. The length of each zone is not uniform in the depth direction. Furthermore, the lower slice can be divided vertically into a plastic zone with high permeability and an elastic zone with low permeability. The results are in good agreement with field investigations. Meanwhile, lower slice gas drainage was carried out in the Dahuangshan coal mine, which shows that gas drainage in the full pressure relief zone with high permeability is crucial to gas control during top slicing.

Graphical abstract

Introduction

Thick coal seams are widespread all over the world. Because of the high production efficiency of a thick seam; and the progress of mining technology, more and more countries are expanding the scale of thick seam mining. China is one of the countries with the advanced technology of thick seam mining, and more than 45% of the total coal yields come from the thick seam. However, with a high gas content, the danger of gas disaster during thick seam mining is particularly serious, and gas accidents happen very frequently in China during these years (Table 1).

Downward slicing is a common technique in thick seam mining. According to engineering experience, the worst gas disaster usually occurs during top slicing (Fan et al., 2011) because top slicing causes stress re-distributions, and relief of stress produces new cracks in the lower slice (Fei et al., 2015), which improves the permeability and eventually promotes gas desorption from the coal matrix and gas flowability in the fracture network of the lower slice (Wang et al., 2015), increasing the amount of gas emission during top slicing (Kissell et al., 1981, Saghafi and Pinetown, 2015).

Boreholes, always drilled in the lower slice to drain the gas, are an effective technology that not only control the gas disaster of the top slicing, but also reduce the gas content in the lower slice, thereby ensuring the safety and health of the miners (Yuan, 2015). To ensure the desired application effect, the boreholes should be placed in the area that has high permeability and strong gas flowability.

However, with the change in the mining-induced stress and the non-uniform stress distribution in a goaf, the degree of compaction of a broken rock mass varies with location and time (Fan and Liu, 2017). The permeability distribution certainly will be non-uniform and changing in the lower slice as the floor of the top slice. First, when the upper slice face is mining, the stress re-distributions lead to stress concentration in the lower slice in front of the face. Then, the floor is exposed to the goaf, and stress is relieved (Clifford, 2004). Unloading failure and a mass of fractures occur within a certain depth of the lower slice (Suchowerska et al., 2013). Xu et al. (2016) and Zhu et al. (2014) proposed theoretical formulas to predict the failure depth and the depth of the fractured zone in the floor. These fractures result in a significant increase in the permeability of the floor strata (Aghababaei et al., 2016, Levasseur et al., 2010, Schatzel et al., 2012, Wang et al., 2015). In a previous study, the law of permeability changes was used to analyze the risk of floor water inrush (Lu and Wang, 2015), and as a guide to promote gas desorption from the coal matrix and gas flow in the underlying protected coal seam, thereby reliving the gas pressure and eliminating the risk of coal and gas outburst (Díaz Aguado and González Nicieza, 2007, Noack, 1998, Yang et al., 2011).

Compaction occurs in the floor with overlying strata collapsing, and cracks in the floor close under the compaction stress (Wang et al., 2016), leading to a decrease in permeability (Baghbanan and Jing, 2008, Min et al., 2004). Some researchers found ways to estimate the cover pressure and the distribution of pressure in the goaf of longwall panels. After observations of numerous roadway stability cases behind longwall faces, Wilson (1981) stated that the vertical stress in the goaf increases linearly from zero to the original overburden pressure at some point within the goaf. The breaking angle and the shearing angle of the overburden strata can reflect the compaction characteristics in the floor within the goaf (King and Whittaker, 1970, Smart and Haley, 1987). Some researchers used numerical modeling techniques to investigate the stress distribution in the goaf, and modified the stress–strain relationship of the stone-built pack (Thin et al., 1993, Trueman, 1990, Yavuz, 2004).

Much work has been done on the stress distribution and compaction of the overlying strata in the goaf. Nevertheless, few studies focus on the permeability of the floor (Wang et al., 2015, Wei et al., 2016). Considering the complexity of the permeability of the floor, especially the lower slice during thick seam slicing mining, further effort is required to understand the characteristics of permeability evolution of the lower slice. In this paper, we first propose a stress-induced permeability evolution model, and then, with the background of the Dahuangshan coal mine, China, a method was adopted that combines the Universal Distinct Element Code (UDEC) numerical simulation and the permeability model, the movement of overlying strata and the evolution of permeability of the lower slice underneath the goaf was investigated, and distribution of the deformation and the permeability of lower slicing was studied. The results are supported well by the field test. The outcomes may potentially be used for the gas control during thick seam mining.

Section snippets

Mining and geological conditions

The Dahuangshan coal mine, at longitude 88°41′14″E and latitude 44°01′48″N, is located in the Fukang autonomous prefecture of Xinjiang Province (Fig. 1). Zhongdacao, the primary mineable coalbed, is an extremely thick coal seam where the average normal thickness is 29.47 m (Fig. 2), and the dip angle is 21°, as determined using downward slicing. The No. 2ZW11 face is the working face of the top slice of Zhongdacao. Fully mechanized longwall mining is used on this face. The longwall panel is

Stress-induced permeability evolution model

The coal reservoir contains numerous joints and fractures because of geological tectonism. These fractures are distributed randomly, but there still are some distribution laws. The coal reservoir is always cut by numerous groups of parallel or vertical fractures into a regular block matrix under the static pressure (Hui et al., 2003) (Fig. 3). The flowability of the fluid in the pore is far less than the flowability of the fluid in the fractures (Witherspoon et al., 1980). When the upper slice

Numerical simulation

UDEC is a numerical analysis software based on the theoretical basis of the discrete element method. The software provides a rigorous analysis for the geotechnical problem, and is particularly suitable for simulating the response of jointed rock or discontinuous blocks of a body collection system under conditions of static or dynamic load stress (Gao et al., 2014, Gao, 2013).

In contrast to continuum models, the discrete element method (DEM) can explicitly simulate the fracture and failure

Test drainage boreholes design

To control the gas disaster in the No. 2ZW11 face, various gas extraction technologies were employed such as drainage with buried pipes in the upper-corner, high-level borehole drainage, and drainage with long boreholes down the seam (Fig. 12). Meanwhile, drainage boreholes in the floor should be designed to intercept and drain the gas in the lower slice. To provide the basis for the drainage arrangement in the lower slice, the borehole drainage test was carried out, which can help further

Drainage design

The range within a depth of −14.5 m of the lower slice is the plastic zone where the gas flowability is strong. Therefore, the drainage area should cover the zone. In view of the actual condition of the No. 2ZW11 face, the construction parameters of the drainage boreholes in the lower slice were designed as Fig. 16 and Table 4.

Underground rocks and coals contain much moisture, so there is usually water in the downward drilling boreholes during gas drainage, which increases the extraction

Permeability evolution underneath goaf during top slicing

With the top slicing, the permeability of the lower slice underneath the goaf was changing. In the initial stage of mining, the lower slice was exposed to the goaf, which resulted in the increasing permeability of the coal mass. However, the biggest increment of permeability will not appear because of the inadequate pressure relief.

Further increasing in the length of the goaf resulted in the greater pressure relief, thereby causing the biggest increment of permeability in the coal mass. At the

Conclusions

1)
A stress-induced permeability evolution model was proposed that can reflect the influence of the stress on the permeability of the coal mass. With the top slicing of the Zhongdacao coal seam as a case study, a research method combining the UDEC numerical simulation and the permeability evolution model was used to analyze characteristics of the permeability evolution in the lower slice. The result is consistent with the field investigation and verified the feasibility of the research method.
2)
With

Acknowledgements

The authors are grateful to the National Key Research and Development Program of China, the Project Funded by Ministry of Science and Technology of the People's Republic of China (Grant No. 2016YFC0801800). The 1st author also gratefully acknowledges scholarship provided by the China Scholarship Council.