Determination of roof horizontal long drilling hole layout layer by dynamic porosity evolution law of coal and rock
Graphical abstract
Introduction
Coal mine methane (CMM), also known as coalbed methane, is mainly composed of methane, which is not only a major disaster source in coal mines, but also a clean energy source, can effectively alleviate the problem of natural gas shortage in China [1]. With the continuous extension of China's coal mines to the deep, the content and emission of coalbed methane gradually increase, and the possibility of gas disasters gradually increases, thus posing a huge threat to coal mine production [2]. At the same time, the ability of coalbed methane to absorb infrared rays is 26 times that of carbon dioxide, and it is also a greenhouse gas with harmful effects on the environment, which makes the global effect more serious [3]. However, when methane is burned as an efficient and clean energy fuel, the calorific value is as high as 35.9 MJ/m3, and only water and CO2 are produced without other products [4].
When the coal seam is affected by mining work, the overlying strata lose their original balance and gradually collapse to form goaf, which makes the gas existing in the coal seam desorb to form free gas, which gradually flows and converges to the return air roadway and the working face [5], which makes the phenomenon of gas accumulation appear, and it is easy to cause the disaster of a gas explosion, which seriously threatens the safety production of the coal mine. In order to ensure the safety of coal seam mining in deep mines and prevent greenhouse gas emissions, it is urgent to extract gas from goaf. At present, the most effective way to extract gas is to utilize the gas-guiding fissure zone formed by mining factors, realize high-efficiency methane extraction, and control in goaf [6]. The principle is that the working face is affected by mining factors, and the overlying strata is unstable and collapsed, which is vertically divided into “caving zone, mining fracture zone, and bending subsidence belt”, in which the caving zone and mining fracture zone constitute the gas-guiding fissure zone [7]. The mining fissure system is formed by original voids and fissures, secondary fissures, and separation fissures in the interior [8]. To provide a channel for gas migration, so that a gas-rich area is formed in the gas-guiding fissure zone [9].
Due to the complexity of the goaf, it is impossible to observe the development of cracks in the rock layer directly by humans. Therefore, relevant experts and scholars at home and abroad have adopted different research methods to study the development of fissures in rock formations. For example, Wu et al. [10,11] used DEM simulation to analyze the mechanical characteristics of rock formation lithology from a meso-level perspective. Ning et al. [12] found that the separation distance of coal seams and the final subsidence value, and put forward a method to determine the height of fracture zone in the mining process of short-distance coal seams. Lu et al. [13] established a large-scale three-dimensional physical similarity model, and it is found that the increasing mining distance of the working face indicates that the fractal dimension of fractures tends to be stable. He et al. [14] combined with theoretical analysis and engineering verification, the crack penetration coefficient (FPC) value and compression-tension ratio (i.e., the ratio of compressive stress to tensile stress) were used to express crack development. Li et al. [15] used onsite scanning with GPR technology to demonstrate that the periodic triangular depression features in overlying strata caving.
Secondly, in order to accurately locate the gas-rich area inside the gas-guiding fissure zone, it is necessary to master the gas flow law inside the goaf. Some scholars used the CFD simulation method to compare the gas flow in goaf to the seepage in a porous medium [1]. The influence of ventilation volume, working face width and coal seam dip angle on gas diffusion in goaf was analyzed [16]. To determine the distribution law of gas in goaf during normal mining and roof weighting, and the gas distribution in goaf of the thin coal seam [17]. Jie et al. [18] and Li et al. [19] used COMSOL Multiphysics software to quantitatively determine the gas concentration field in goaf and the law of gas migration in overlying strata.
As a new technical means of “replacing roadway with a hole”, the technology of gas drainage by a roof horizontal long drilling hole has the advantages of obvious pre-drainage effect, short construction period, and low drilling cost [20]. It offsets the shortcomings of the common measures of gas drainage by drilling holes and relieves the necessity of re-drilling holes when replacing the working face [21,22]. This protocol has been widely used in China, the United States, Australia, and other countries [[23], [24], [25]]. However, the key to achieving high-efficiency gas extraction is to determine the horizon of borehole location, that is, to accurately locate the range of gas-guiding fissure zone, when the coal mine is arranged with the roof horizontal long drilling hole [26].
At present, there are relatively little researches on determining the roof horizontal long drilling hole, but accurate positioning of the range area of the gas-guiding fissure zone is the key to realize high-efficiency drainage. Therefore, this paper selects 6306 working faces of Tangkou Coal Mine, a high gas mine, as the research object. Firstly, the three-dimension porous media model is constructed by DEM simulation software. Analyze the formation process of the gas-guiding fissure zone from a macro perspective. Furthermore, from a mesoscopic view perspective, porosity is used as a quantitative analysis standard to analyze the development of internal fissures in the gas-guiding fissure zone, and then the height range and height of the gas-guiding fissure zone are determined, which is compared with the results of previous research [27]. Therefore, the range, area, and height of the gas-guiding fissure zone are obtained. Secondly, the three-dimensional porosity dynamic distribution data set is extracted and converted into a two-dimensional array form based on a digital elevation model, and then imported into CFD simulation software to simulate the gas distribution law in goaf before and after drilling in the gas-guiding fissure zone, and verify the rationality and accuracy of drilling layout. Finally, taking the determined range and height of the gas-guiding fissure zone as a reference, the horizontal long drilling holes of the roof are arranged on site. It provides a new way to determine the location of drilling holes for similar high-gas coal mines, and it is also of great significance to reduce gas disasters and improve the safety of coal mine production.
Section snippets
Geological condition of Tangkou coal mine
According to the relevant geological data of the coal mine, the distribution of overburden on the 6306 working face is shown in Fig. 1. The design strike length of the working face is 817 m on average, the inclination length is 120 m on average, and the thickness of the coal seam is 8.1– 10.4 m with an average thickness of 9.44 m. The dip angle of the coal seam on the working face is between 0– 7° with an average angle of 2°. The immediate roof is mudstone with a thickness of 2.46– 7.56 m and
Theoretical analysis
The migration and accumulation of gas in goaf is a complicated and tedious process. In the process of coal seam mining, the overlying strata collapse and gradually form a gas-guiding fissure zone composed of a caving zone and mining fracture zone [2]. With the progress of mining work, the separated layer fissures and cross-layer fissures in the gas-guiding fissure zone cross each other to form a longitudinal through the fissure, and a dynamic mining fissure field is formed. The gas adsorbed in
Principle of CFD calculation process
Computational Fluid Dynamics (CFD) is a branch of fluid mechanics. Numerical methods and algorithms are mainly used for research and analysis of fluid flow problems. The essence of gas flow under the borehole in the goaf by CFD is the solution of the Navier-Stokes equation, and the finite element is used as the basis of the Fluent 18.0 simulation software to obtain the regular characteristics. At the same time, when simulating the gas flow inside the goaf, it is necessary to follow the
Onsite layout of horizontal long drilling hole
According to the mathematical model of the height of the gas-guiding fissure zone and the results of PFC simulation, the height of the gas-guiding fissure zone is finally determined to be 49.88 m, and the law of gas flow in goaf after drilling arrangement is simulated by CFD, which shows that this height can be used as a reference for the field layout of horizontal long drilling holes in the roof, but because the coal seam and overlying strata mined in 6306 working face of Tangkou Coal Mine are
Summary and conclusion
In this paper, the key issues of the arrangement of roof horizontal long drilling hole were investigated based on the engineering geology of the 6306 working face of Tangkou Coal Mine, and the scope of the gas-guiding fissure zone was determined.
The main conclusions are as follows:
- (1)
The DEM simulation software PFC3D was used to analyze the overburden collapse law, and the dynamic spatial and temporal distribution of porosity was quantitatively described from the perspective of mesoscopic view.
Declaration of Competing Interest
All the authors do not have any possible conflicts of interest.
We certify that we have participated sufficiently in the work to take public responsibility for the appropriateness of the experimental design and method, and the collection, analysis, and interpretation of the data.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgment
The authors thank the support of the National Natural Science Foundation of China (Project No. 51974176, 51934004, 51704187), the key projects supported by Natural Science Foundation of Shandong Province (Project No. ZR2020JQ22, ZR2017BEE054), the key research and development plan of Shandong Province (Project No. GG201809190180), the key projects supported by Youth Science and Technology Innovation of Shandong Province (Project No. 2019KJH006), the key projects supported by the Special Funds
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