Numerical Simulation of Parameters Optimization for Goaf Gas Boreholes

State Key Laboratory Cultivation Base for Gas Geology and Gas Control, Henan Polytechnic University, Jiaozuo 454003, China School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China Hunan Key Lab of Coal Safety Mining Technology, Hunan University of Science and Technology, Xiangtan 411201, China +e Collaborative Innovation Center of Coal Safety Production of Henan Province, Jiaozuo 454003, China


Introduction
With the increasing coal mining depth in recent years, the gas pressure and content of coal seam increase obviously.Gas disaster has gradually become the primary problem that restricts the safe production of coal mine.e most universal technique to control gas disaster is gas extraction, which has two kinds: extraction by roadway and boreholes [1][2][3].In this paper, we focus on the boreholes drilled from ground.
America was the first country that achieved success in exploiting coalbed methane resources by extracting gas with boreholes drilled from ground.In Australia, vertical borehole was the most common way to extract gas in goaf.Germany exploited coalbed methane resources from Ruhr coal field by boreholes drilled from ground in 1992.In most parts of China, low permeability, low saturation, and low reservoir pressure are the three characteristics of coal seam, which means it is hard to extract gas from the coal.We have drilled 40 ground boreholes in JiaoZuo and YangQuan since 1970 and carried out measures of hydraulic fracturing.en, we conducted experimental studies of gas extracted from ground boreholes and underground drills in KaiLuan and TieFa coal fields with the help of United Nations Development Program.We have made some achievements on gas extraction by the method of ground boreholes in JinCheng and HuaiNan coal fields in China [4][5][6].
In the deep part of goaf, gas concentrating for a long time, the content of methane gas is much more than that in working face.e gas from goaf is one of the main sources of gas to working face.Extracting gas by boreholes drilled to goaf from ground is a main method to handle gas problem.
ere is a large body of work that has been done to investigate ground boreholes.Li et al analyzed the effect of gas extracted from ground boreholes in Jin Cheng field [7].Li et al studied the influential factors on gas extraction by ground boreholes and proposed an approach to enhance the efficiency of gas extraction [8].Many foreign scholars have done a lot of research about gas disaster prevention, gas drainage in coal mine goaf, etc [9][10][11][12][13][14][15][16].Many other scholars have also examined the effects of ground boreholes [17][18][19][20].
However, there are controversies about this question: whether the location of boreholes is a critical factor for gas extraction technique.Some scholars even hold opinion that the effect of the location can be neglected [21,22].Further research on the effect of the location of ground boreholes to the effect of gas extraction needs to be done to reach better effect.Based on the physical parameters of N2206 working face in WangZhuang mine in ShanXi Province, this paper investigates the poor efficiency of ground boreholes 1# and 2# drilled to goaf of N2206 working face by software Fluent to confirm the best location of ground boreholes.

The Theory of Gas Extraction from Ground Drilling to Gob
Gas extraction from goaf by ground boreholes does not have a long history in coal mining industry, whereas mines in America and Australia have made achievements of ground boreholes.e principle of gas extraction from ground drilling to gob is that as the following: the overlying rock above goaf can be divided into three vertical zones, including caved zone, fractured zone, and bended zone due to the influence of coal mining.As the working face advancing, there are many separations and fractures in the bended zone, largely enhancing the permeability of coal seam.Gas migration is much easier in the separations and fractures.en, the ground boreholes are drilled into the bended zone to exact gas, reducing the thread of gas. Figure 1 shows the layout of ground boreholes to goaf.

e General Situation of N2206 Working
Face. e strike length of tape roadway in N2206 is 2252 m; the strike length of intake air roadway is 2260 m; and the strike length of return air roadway is 2257 m. e inclination length of N2206 working face is 285 m. Figure 2 shows the sketch map of the N2206 working face.
e yearly output of N2206 working face is 1.4MT with recovery ratio of 93%.e average thickness of coal seam is 5.99 m; the unit weight of coal is 1.4 t/m 3 .e absolute gas emission rate is 76.5 m / min.Herein air flow counts for 32.2 m /min, and extracted gas counts for 44.3 m /min.
In the design, the ventilation in the N2206 working face is doubled.
e inlet air roadway and tape roadway are filled with fresh air, and in the return air roadway and gas tail roadway is dirty air, i.e., two-intake and two-return.And then it is changed into three-intake and one-return.
e inlet air roadway, tape roadway, and the return air are filled with fresh air, and the gas tail roadway is filled with dirty air [23].

eoretical Calculation on the Height of ree Vertical Zones
(1) e theoretical formula of height to caved zone: where h is the mining height, k is the average bulking coefficient of caved rock, and α is the dip of coal seam.
(2) e theoretical height of fractured zone: where a, b, and c are the undetermined coefficients: they are determined by the "Design code of coal mine" of Wangzhuang mine, shown in Table 1.
(3) e bended zone is stratum that starts from the upper boundary of fractured zone to earth surface, and we do not consider the bended zone in this paper because the sinking variation is small extremely.According to the relating materials of the N2206 working face, the overburden rock is medium hard rock mainly, the roof of N2206 working face is mixed with post stone and mudstone, and the floor is siltstone and mudstone.Figure 3 shows the histogram of coal-rock strata about N2206 working face.e working face follows strike longwall and takes caving coal mining for fully mechanized.e included angle of mining layer and the horizontal direction is 6 °or so.e bulking coefficient of caved rock and roof coal are 1.26 and 1.3.e average theoretical height to caved zone and fractured zone is 14.6 m and 45.5 m based on the calculation formulas (1) and (2).We can calculate the thickness of left coal in gob which is 0.6 m based on the average thickness of coal seam, the recovery ratio, and the bulking coefficient of roof coal [24].

Physical Model of Gob.
e width and height of intake roadway of N2206 working face are 5 m and 4 m; the width e effect of goaf on the gas flowing regulation in gob can be neglected in the reason of the sinking variation which is small extremely in the three zones.is model neglected the bended zone, and the physical model is shown in Figure 4.

Hypothesis
(1) Taking the working zone is turbulent flow zone, and the laminar flow is the main flow state of gas flow in gob.(2) We regard the porous media in gob as an isotropous area, and its permeability is constant in the reason of the randomness of the caved rock and the left coal.4 Advances in Civil Engineering (3) Taking into account the gob gas is incompressible gas; we do not take into account the work of fluid viscosity caused by heat dissipation.We assumed the gas flow is steady and the process remains at a constant temperature.(4) Ignoring the small component gases such as water vapor and nitrogen dioxide and assuming that the diffusion coefficient of the gas remains constant, the gas in the component transport is set to a mixed gas of methane, nitrogen, and oxygen.

Mathematical Model.
e mathematical model of porous media in gob is as the following [25]: (1) e mass conservation equation: (2) e momentum equation (Navier-Stokes): where G k is the generation item of turbulence energy caused by the average velocity of gas; G b is the generation item of turbulence energy caused by buoyancy, which is 0 to the incompressible fluid; Y M is the contribution of pulse expansion caused by the turbulence flow to the compressible fluid; where

Boundary Conditions.
e intake roadway of working face is velocity-inlet, the average velocity of tape roadway, return air roadway and inlet air roadway are, respectively, 2.6 m/s, 1.1 m/s, and 1.6 m/s.e volume fraction of O 2 and CH 4 are, respectively, 21% and 0% in the intake roadway.e gas emissions roadway was set as out-flow; the working face and goaf of solid wall are set as nonslip boundary condition, which is u � v � w � 0; the juncture of working face and goaf is set as the interior boundary.Detailed boundary conditions and parameters are shown in Table 2. e permeability of goaf presents continuous distribution as "O" because of the support effect of surrounding rock in gob.Using C language, the gas emission intensity of goaf and goaf permeability are defined as the continuous distribution of the custom function.

e Numerical Simulation Process.
e modeling tool Gambit is used to construct the physical model in the Cartesian coordinate system and unstructured meshing.Fluent software is used for numerical simulation solution, and finally, the simulation results into Tecplot for postprocessing operation.e control equation is discreted in the method of control volume, among which the format convection term and diffusion term are in the format of Quick.e iteration calculation was accomplished in combination of relaxing factor and tridiagonal-matrix algorithm.
e coupling of pressure-velocity takes simple arithmetic [26].

The Result Analysis of Numerical Simulation Process on Gob
From the documents, it is reasonable to layout gas extraction drilling in the roof of the caved zone and the lower-middle part of the fractured zone.ere would appear series questions about gas extraction difficulties and low quantity of gas extraction if the layout position is too high.If the layout is in the lower level, it is possible to communicate directly with the cave zone, and there would be drainage of large flow, its composition is mainly air leakage problem of goaf etc.If the layout position is near the return air roadway, the return air roadway would be linked up through fractures, and the main component of the gas extraction is air.Since the return air roadway is near the high concentration area of the goaf, if the layout position of the drilling is far from the return air roadway, there will be problems such as lower gas concentration and lower flow rate.
us, the reasonable layout parameters are crucial to the effect of gas extraction.Now, we should combine the existing questions of gas extraction drilling from ground to gob of 1#, 2# on N2206 working face and numerical simulation to optimize the layout parameters.e gas is collected to the circle of "O" through seepage after gas desorption around the coal-rock based on the theory of circle "O" about fracture distribution in overlaying rock.To achieve the long gas extraction time, the big zone of gas extraction, and the high gas drainage efficiency, the gas extraction drilling should be placed to the inner of circle "O".

Optimization of the Vertical Gas Extraction Height from
Ground to Gob.Based on the experience of 1 # and 2 # mining ground drilled holes, the range of the hole location of the borehole drilling is estimated on the basis of grasping the inclination of the coal seam and so on.When the horizontal distance is 45 m, the numerical simulation software is used to simulate the drilling position of the vertical extraction of 10 m, 16 m, and 22 m from the roof of the coal seam.e simulation results are shown in Figures 5 and 6.
From Figures 5 and 6, the distribution trends of gas in gob are approximately same when the vertical height of gas extraction drills is selected as 10 m, 16 m, and 22 m.e deeper the gob, the higher the concentration of the gas in the horizontal direction of gob.In the vertical direction, the concentration of gas upside the gob is bigger than the downside because of the effect of gas buoyancy.For the distributional profile of gas in gob of Y � 50 m and Y � 290 m, the scope of gas concentration of 5% in gob is 129 m and 130 m, which is relevant with the vertical height of gas extraction drilling, when the figure is 10 m. e scope of gas concentration of 5% in gob is 138 m and 140 m, which is relevant with the vertical height of gas extraction drilling, when the figure is 16 m and the scope is the widest with low gas concentration.By contrast, when the scope of gas concentration of 5% in gob is 99 m and 112 m, the figure is 22 m and the scope is the narrowest with low gas concentration.For the distributional planar graph of gas in gob of Z � 1 m and Z � 20 m, the scope of gas concentration of 5% in gob is 121 m and 122 m, which is relevant with the vertical height of gas extraction drilling, when the figure is 10 m near the side of belt crossheading.e scope of gas concentration of 5% in gob is 129 m and 134 m, which is relevant with the vertical height of gas extraction drilling, when the figure is 16 m and the scope was the widest with low gas concentration.e scope of gas concentration of 5% in gob is 108 m and 109 m, which is relevant with the vertical height of gas extraction drilling, when the figure is 22 m and the scope was the narrowest with low gas concentration.Analyzing the distributional profile and planar graph of gas in gob, we found that the scope of low gas concentration in gob is the widest, and the scope of high gas concentration in gob is the narrowest with the vertical height of gas extraction drilling is 16 m.e overall effect of gas extraction is best.

e Optimization of Horizontal Position on Drilling of
Ground Gas Extraction to Gob.We performed numerical simulation by Fluent software with the horizontal distance between the end drilling and the return air roadway of 35 m, 45 m, and 55 m based on the decided vertical height of gas extraction from ground of 16 m.Figures 7 and 8 show the simulation results.
From Figures 7 and 8, the distribution trends of gas in gob are approximately same when the horizontal distance of gas extraction drillings is selected as 35 m, 45 m, and 55 m. e closer to the depth of the gob, the higher the concentration of gas in the strike direction of the gob.In the vertical direction, the concentration of gas upside the gob is bigger than the downside because of the effect of gas buoyancy.For the distributional profile of gas in gob of Y � 50 m and Y � 290 m, the scope of gas concentration of 5% in gob is 112 m and 99 m,

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which is relevant with the horizontal distance of gas extraction drilling, when the figure is 35 m. e scope of gas concentration of 5% in gob is 140 m and 135 m, which is relevant with horizontal distance of gas extraction drilling, when the figure is 55 m and the scope was the widest with low gas concentration.e scope of gas concentration of 5% in gob is 111 m and 98 m, which is relevant with the horizontal distance of gas extraction drilling, when the figure is 55 m and the scope was the narrowest with low gas concentration.For the distributional planar graph of gas in gob of Z � 1 m and Z � 20 m, the scope of gas concentration of 5% in gob is 112 m and 108 m, which is relevant with the horizontal distance of gas extraction drilling, when the figure is 35 m near the side of belt crossheading.e scope of gas concentration of 5% in gob is 120 m and 132 m, which is relevant with the horizontal distance of gas extraction drilling, when the figure is 45 m, and the scope was widest with low gas concentration.e scope of gas concentration of 5% in gob is 110 m and 107 m, which is relevant with the vertical   Advances in Civil Engineering horizontal distance of gas extraction drilling, when the figure is 55 m, and the scope was the narrowest with low gas concentration.Right now, the distance between the end drilling and the return air roadway was too far; thus, it is not in the "O" zone of fracture development and results in the nonideal gas extraction effect.Analyzing the distributional profile and concentration in gob was the widest and the high gas concentration is near the deep of the gob with the horizontal distance of gas extraction drilling, when the figure is 45 m. e overall effect of gas extraction is best.
To verify the best gas extraction location about ground drilling, we monitored the ground drilling with different vertical heights and horizontal distances and simulated and solved the condition of gas extraction with different vertical heights and horizontal distances by combining the in situ data as shown in Table 3.
From Table 3, the gas extraction effect is obviously different with different vertical heights and horizontal distances.When the horizontal distance of ground drilling is 45 m, the vertical height is set as 10 m, 16 m, and 22 m, the e main gas component of gas extraction is air, the mixture quantity of gas extraction is bigger, and the concentration of gas extraction is lower.rough monitoring the outlet flow rate, the mixture quantity of gas extraction is only 18.1 m 3 /min because the distance between the end drilling and the return air roadway was too far; thus, it is not in the "O" zone of fracture development.e pure quantity and concentration of gas extraction are biggest with the horizontal distance of 45 m.In conclusion, the effect of gas extraction is the best with the 16 m vertical height and the 45 m horizontal distance.

In Situ Test
We gain that the best effect of gas extraction with the 16 m vertical height and the 45 m horizontal distance through theoretical analysis and numerical simulation on the low gas concentration question about the ground drilling 1# and 2# to N2206 working face in Wangzhuang coal mine.To give a further validation of the reliability of theoretical analysis and numerical simulation, we performed gas extraction from ground to gob with the optimized parameters.Figures 9 and 10 show the pure quantity and concentration of gas extraction before and after drilling optimization about 1# and 2#.
From Figures 9 and 10, when the pure quantity and concentration of gas extraction are low in 1# and 2#, the concentration of gas extraction from the drilling are both lower than 30%.e effect of gas extraction in 1# is the most nonideal with the concentration of gas extraction only 10% or so.After optimization, the pure quantity and concentration of gas extraction increased obviously.e maximum pure quantity of gas extraction is 12.59 m 3 /min and have an increase of 3.42 m 3 /min than before, the maximum concentration of gas extraction is 63.54% and have an increase about 28.82% than before and the concentration of gas extraction from the drilling is higher and over 30%.e effect of gas extraction is relatively ideal, and it verified the reliability of theoretical analysis and numerical simulation.

Conclusions
(1) eoretical analysis and numerical simulation show that it is obvious to see the range of low gas concentration is widest and the range of high gas concentration is narrowest when vertical height of ground extraction drilling of gob is 16 m and  Advances in Civil Engineering horizontal distance is 45 m.In the deep of gob, the high gas concentration can be shown, and the ground drilling shows the optimum efficiency to the gas extraction.e in situ data and the monitoring simulation data show that it has the best extraction effect with 16 m vertical height and 45 m horizontal distance; under this condition, the average pure quantity and the average concentration of gas extraction are 9.78 m 3 /min and 43.95%, respectively.(2) After optimization, the pure quantity and concentration of gas extraction have an obvious increase.e maximum pure quantity of gas extraction is 12.59 m 3 /min and has an increase of about 3.42 m 3 / min than before; the maximum concentration of gas extraction is 63.54% and has an increase about 28.82%, and the concentration of gas extraction from the drilling is higher over 30%.
e effect of gas extraction is more ideal.e theoretical analysis and numerical simulation have been proved.

Figure 1 : 2
Figure 1: Arrangement plan about ground extraction drilling to gob.

8 XFigure 5 :Figure 6 :
Figure 5: Distributional profile of gas in gob with different vertical heights of ground gas extraction.(a) Horizontal distance is 45 m and vertical distance is 10 m.(b) Horizontal distance is 45 m and vertical distance is 16 m.(c) Horizontal distance is 45 m and vertical distance is 22 m.

Figure 6 : 8 X
Figure 6: Distributional planar graph of gas in gob with different vertical heights of ground gas extraction.(a) Horizontal distance is 45 m and vertical distance is 10 m.(b) Horizontal distance is 45 m and vertical distance is 16 m.(c) Horizontal distance is 45 m and vertical distance is 22 m.

Figure 7 :Figure 8 :
Figure 7: e distributional profile of gas in gob with different horizontal distances of ground gas extraction.(a) Horizontal distance is 35 m and vertical distance is 16 m.(b) Horizontal distance is 45 m and vertical distance is 16 m.(c) Horizontal distance is 55 m and vertical distance is 16 m.

Figure 8 :
Figure 8: Distributional planar graph of gas in gob with different horizontal distances of ground gas extraction.(a) Horizontal distance is 35 m and vertical distance is 16 m.(b) Horizontal distance is 45 m and vertical distance is 16 m.(c) Horizontal distance is 55 m and vertical distance is 16 m.

Figure 9 :Figure 10 :
Figure 9: Correlation curve of pure quantity of gas extraction before and after the optimization of ground extraction drilling to gob.

Table 1 :
Value table of undetermined constants a, b, and c.
s is the diffusion coefficient of the component.eturbulence model of the working face is realizable k where u, v, and w are velocity components of the directions x, y, and z; n is the porosity; S m is the source item of mass; ρ is the density of gas in gob; μ is the dynamic viscosity of air; F x , F y , and F z are the mass force component on microunit in every direction; C s is the volume concentration of component S; S s is the productivity of component; ρC s is the mass concentration of component; and D

Table 2 :
Boundary conditions and parameters.

Table 3 :
Gas extraction effect under different vertical height and horizontal distance.