Study on Permeability Improvement Technology by Injecting Air into the Gas Drainage Borehole in Low-Permeability Coal Seam

Gas extraction cycle is too long in low-permeability coal seam. In order to solve the problem, the basic principle about gas drainage drilling for gas injection technology is studied to increase permeability. And the mathematical model is established. Gas is injected into the low-permeability coal seam by numerical simulation. ,e results indicate that the best condition is a negative pressure drainage at 26 kPa and a gas injection pressure at 0.6MPa in the vertical direction and in the horizontal direction of the injection hole. In Shanxi Daping Coal Mine 3113 working face, the field test is implemented. As a result, the test is successful. During the 14 d gas injection constantly, gas content of coal seam is reduced from 12.33m/t to 7.12m/t, greatly reducing the risk of coal and gas outburst elimination time required.


Introduction
At present, the overall situation of gas extraction in China is bad, mainly because of the low-permeability of coal seam [1].China coal seam permeability is less than 1 mD accounted for 82%, 2-3 orders of magnitude lower than the United States [2,3].Low-permeability coal seam has the characteristics of low pressure, low permeability, and low saturation, and improving the permeability of coal seam is a big problem.
is has a direct impact on the process of improving gas extraction rate and preventing and controlling gas disaster.Gas injection technology for permeability enhancement is a new type of permeability increasing technology, which could increase the permeability of coal seam effectively [4].
e technology of gas injection and permeability enhancement has been developed from the field of coal bed methane development.In 1970s, Americans injected CO 2 into the coal seam in the San Juan basin to improve CBM recovery and achieved good results [5].America, Japan, Canada, and China have conducted different degrees of research, but the research on displacement mechanism of gas injection mostly adopts the method of theoretical analysis and numerical simulation [6][7][8][9].No field tests are carried out, and the effect verification is lacking.erefore, further improvements are still needed.Since 2007, Yang Hong-min of Henan Polytechnic University had applied gas injection to replace coal seam gas technology in the field of coal mine gas control.Field tests were successfully carried out [10][11][12], such as gas injection to eliminate outburst danger and gas injection to promote gas drainage, and certain effect was achieved.But, they were not ideal.
is paper aims at solving the low-permeability coal seam existing drainage problems in large quantities in China, through the study of gas injection to increase permeability mechanism and model of gas drainage drilling holes.
e mathematical model of gas drainage drilling through gas injection is constructed for low-permeability coal seam, and the numerical simulation analysis is carried out.According to the actual situation of mine, this paper adopts the test method of injecting compressed air into the gas drainage borehole and carries out field test in the 3113 Lane driving face of Daping Mine in Shanxi Province.

Theory of Gas Injection Technology for
Improving Permeability e mechanism of gas injection technology to improve permeability is mainly based on the different adsorption capacities of CH 4 , CO 2 , N 2 , and other gases.And the competitive adsorption, displacement desorption, and mutual displacement are analyzed.e adsorption capacity of coal to CH 4 , CO 2 , and N 2 was CO 2 > CH 4 > N 2 in turn.A large number of studies have shown that CO 2 and N 2 have great differences in the displacement and displacement of CH 4 in coal seams [13].
(1) e adsorption constant of CO 2 is larger than that of CH 4 , and CO 2 has obvious advantage in gas competition absorption in coal seam.e mechanism of enhancing the permeability by CO 2 injection into coal seam mainly is that, on one hand, CO 2 can displace the free CH 4 in coal seam by pressure displacement; on the other hand, it can displace CH 4 adsorbed on coal body through CO 2 .us, CO 2 is retained in the coal seam, and CH 4 is displaced out of the coal seam, thus increasing the permeability of the coal seam.
(2) N 2 has a disadvantage in competing with CH 4 .First of all, the displacement of CH 4 by N 2 is driven and carried by pressure gradient.Secondly, after the N 2 was injected into the coal seam, the concentration difference was formed, and N 2 and CH 4 were diffused and "displaced" inwards and outwards under the action of concentration difference.Finally, after N 2 injection into the coal seam, partial pressure of CH 4 is reduced to produce the competitive adsorption of multiple gases.In these three aspects, N 2 completed the displacement of CH 4 to have increased permeability of coal seam.
Because the proportion of N 2 in air accounts for 79.8% and it is very convenient to obtain compressed air under the mine, the air injection is selected to enhance the permeability.erefore, the numerical simulation process uses air as the simulation condition, and the compressed air in the compressed air system is taken as the gas source to conduct the field gas injection permeability enhancement test.

Mathematical Model of Gas Injection for Improving Permeability
According to the theory of seepage mechanics, seepage Darcy law, Fick's law of diffusion and multicomponent gas adsorption equilibrium theory, the law of conservation of mass and the ideal gas equation, the paper constructs the continuity equations of gas injection and permeability enhancement model and establishes the model of gas injection and permeability enhancement in low-permeability coal seam.

Seepage Equation of Gas in Fracture.
It is assumed that the migration of free gas in the fracture can be regarded as a fluid percolation process, and the mass conservation equation of gas flowing through coal is where ρ i is the density of gas component i (kg/m 3 ), v is the total seepage velocity of gas (m/s), and m i is the content of free gas component i (kg/m 3 ).m i � φρ i , where φ is the porosity.

Adsorption Equilibrium Equation of Multicomponent
Gas. e content m pi of adsorbed component under the assumed equilibrium pressure p i can be expressed by the generalized Langmuir isotherm adsorption equation as where ρ i is the density of coal (kg/m 3 ), a i is the individual ultimate adsorption capacity of component i in coal seams (m 3 /kg), b i is the adsorption constant of component i (MPa −1 ), p 1 and p 2 represent the partial pressure of gas components 1 and 2, respectively.

Mass Exchange Equation.
e mass exchange between adsorbed gas on the surface of coal and free gas in the fracture system can be defined as where τ is the desorption diffusion coefficient, τ � 1.42.

Equation of State for Ideal
Gas.Because the gas injection pressure is small, the gas compression process can be ignored, and gas can be regarded as ideal gas.e equation of state of ideal gas can be expressed as where M i is the molar mass of the gas component i (g/mol), R is the universal gas constant, p a and T a are the gas pressure and temperature under the standard conditions, respectively.p a � 0.1 MPa and T a � 273 K.

Seepage Velocity Equation.
As the gas component flows in the coal body that conforms to Darcy's law, the total gas seepage velocity v is where k is the permeability of coal body (m 2 ), μ i is the dynamic viscosity coefficient for gas component i (Pa•s), and p is the total pressure (gas injection pressure, MPa), 2 Advances in Civil Engineering 3.6.Cross-Coupled Equation.Substituting (2)-( 5) into (1), we can obtain simultaneous equations with cross coupling

Diffusion Equation of Gas in Porous Media.
Assuming that the motion of the adsorbed gas agrees with Fick's diffusion law, we can see that the diffusion equation of CH 4 and N 2 in the pore system is the equation of diffusion: where i is the gas component, i � 1 represents CH 4 , i � 2 represents N 2 , c i is the mass concentration (kg/m 3 ), D i is the diffusion coefficient (m 2 /s), t is the gas injection time, ∇ is the Hamiltonian operator, and Q i is the Huiyuan item.
According to the reasoning analysis, ( 6) and ( 7) constitute the continuity control equation of gas permeability enhancement in low-permeability coal seam.e geometric model of gas injection and permeability enhancement is shown in Figure 1.

Numerical Simulation and Analysis
In order to facilitate the comparison between numerical simulation and field test and to better solve practical problems, the parameters in the simulation are all used in Shanxi Daping Coal Mine.
e other parameters are obtained by referring to the data, and the specific parameters are shown in Table 1:

Initial Conditions and Boundary Conditions.
e geometric model selected in the simulation experiment is 2 m high and 3.6 m wide.e borehole layout is revealed on the coal face in the heading face, as shown in Figure 1.We set two gas injection holes, and the surrounding boreholes are all empty.e distance between the injection hole and the horizontal drainage hole is 0.6 m. e initial adsorption gas content is 12.33 m 3 /t.e drilling depth was 100 m.

Initial Condition.
e original methane pressure of coal seam is p 0 .e pressure injected into compressed air is 0 kPa.

Boundary Condition.
e numerical model has a zero flow boundary.e atmospheric pressure in the mine is 0.1 MPa.
e boundary conditions of the drainage holes, respectively, are 18 kPa, 22 kPa, 26 kPa, and 30 kPa. e boundary conditions of the gas injection holes, respectively, are 0.2 MPa, 0.4 MPa, 0.6 MPa, and 0.8 MPa.

Results of Numerical Simulation.
Using the established mathematical model, the numerical simulation software is used for the simulation analysis.In the horizontal direction and vertical direction of gas injection, we simulate the variation of gas content in coal seam under different pumping negative pressure and different gas injection pressure and get the time needed to eliminate outburst danger.In order to facilitate detection, in the horizontal direction, we choose distance gas injection hole 0.8 m as the gas content detection position and choose distance gas injection hole 0.4 m as gas content detection position in the vertical direction.

4.3.1.
In the Horizontal Direction.In the negative pressure of 26 kPa and gas injection pressure of 0.8 MPa, we simulate the gas injection pressure of 0.2 MPa, 0.4 MPa, and 0.6 MPa.And in the gas injection pressure of 0.6 MPa, the negative pressure of 18 kPa, 22 kPa, and 26 kPa are respectively simulated.e changes is 30 kPa vacuum extraction in the horizontal 0.8 m of gas injection hole.e simulation results are shown in Figure 2.

4.3.2.
In the Vertical Direction.We set up the same test conditions as the suction pressure and gas injection pressure in the horizontal direction and had analyzed the change of  Advances in Civil Engineering gas at 0.4 m in the vertical direction of the gas injection hole.e simulation results are shown in Figure 3.
From Figure 2 we can obtain that, in the horizontal direction, rstly, negative pressure is negatively correlated with coal seam gas content.When the suction pressure increases from 18 kPa to 30 kPa, the gas content at 0.8 m in the horizontal direction decreases by 0.13 m 3 /t.e outburst cycle is shortened for 6.5 h after 12 days of gas injection.Secondly, the gas injection pressure is negatively correlated with the gas content of the coal seam.When the injection pressure increases from 0.2 MPa to 0.8 MPa, the gas content in the horizontal direction 0.8 m decreases by 0.28 m 3 /t.e outburst prevention cycle 12.7 h is shortened.When the injection pressure is about 0.6 MPa and the suction pressure is about 26 kPa, the e ect of eliminating outburst danger is most obvious.erefore, it can be concluded that this scheme is the best scheme for gas permeability enhancement in low-permeability coal seam.From Figure 3, we can obtain that the vertical direction is similar to the horizontal direction and the gas content of coal seam is negatively correlated with the negative pressure of drainage.When the suction pressure increases from 18 kPa to 30 kPa, the gas content in the vertical direction 0.4 m decreases by 0.12 m 3 /t and the outburst prevention cycle 5.9 h is shortened after 12 days gas injection.Gas content is also negatively correlated with gas injection pressure.However, the in uence coe cient of gas injection pressure is smaller in the horizontal direction due to gravity.When the injection pressure increases from 0.2 MPa to 0.8 MPa, the gas content in the vertical direction 0.4 m decreases by 0.19 m 3 /t and the outburst prevention cycle 14 h is shortened.When the injection pressure is about 0.6 MPa, the suction pressure is about 26 kPa and the e ect of eliminating outburst danger is most obvious.erefore, it can be concluded that this scheme is the best scheme for gas permeability enhancement in low-permeability coal seam.
From simulation we can nd that, in both horizontal and vertical negative pressure and gas injection pressure play an important role in reducing the gas content and reducing the time needed for outburst elimination.e in uence coe cient of suction pressure is 0.076, and the in uence coe cient of injection pressure is 0.178.Under the condition of ensuring safety and high e ciency, it is concluded that the suction pressure of 26 kPa and the injection pressure of 0.6 MPa are the best scheme.

General Situation of Test Site.
At present, the length of No.3113 roadway openings of Daping Coal Mine in Shanxi province is 1436 m. e elevation located in the southeast of the South slope is +390 m ∼ +483 m. e coal seam belongs to low-permeability coal seam, whose average thickness of coal seam is 6.25 m. e actual measured gas content of coal seam is 12.33 m 3 /t.e current mine drainage parameters are as follows: extraction negative pressure 18 kPa, gas extraction concentration 5.5%-6.5%,standard condition mixed ow 120 m 3 /min-130 m 3 /min, gas extraction pure 7-8 m 3 /min, and 3113 Lane gas extraction 0.1 m 3 /min.

Test Scheme.
According to the actual production situation of Daping Coal Mine in Shanxi, the driving face of 3113 Lane is selected in the test site.According to research and design, the construction of drilling is three rows, 21 holes totally, which is divided by pumping holes and gas injection holes.All the drainage holes are used to test the gas concentration detection hole.e drilling depth is 100 m.We use 26 kPa suction negative pressure and 0.6 MPa injection pressure to enhance permeability test.During the test, the roadway should be reinforced to ensure the normal excavation, the gas prediction work should be done well, and emergency warning and related safety measures should be done.e speci c drilling arrangement is shown in Figure 4.

Test Result.
e e ect of the permeability enhancement test of the gas drainage borehole in low-permeability coal seam is mainly achieved by measuring the gas extraction volume fraction of the drainage hole and taking the average value.Because of the limitation of the length, we choose to analyze the drainage e ect of 2#, 4#, and 6# drainage holes and 9#, 11#, and 13# drainage holes in the vertical direction.
e measurement results are shown in Figures 5 and 6.
From Figure 5, we analyze the extraction e ect of 2#, 4#, and 6# drainage holes: when the hole diameter is 113 mm, extraction pressure is 26 kPa, gas injection pressure is 0.6 MPa, injected gas is compressed air (mainly nitrogen), and mining hole gas volume fraction in the horizontal direction rstly rises smoke adjacent, reaches a maximum value, decreases slightly, and nally tends to a steady value.In general, there is a signi cant increase in the total growth rate of 4 h within 73.6%, which is consistent with the theoretical analysis and numerical simulation results.From Figure 6, we analyze the extraction e ect of 9#, 11#, and 13# drainage holes: in the vertical direction, the gas volume fraction in the drainage hole rstly increases continuously, reaches a larger value, decreases slightly, then rises steadily, and nally tends to a steady value.On the whole, there is a signi cant increase, and the average growth rate in 4 h is 69.7%.Advances in Civil Engineering en, in order to further verify the model of the drainage borehole gas injection scheme of the antire ective e ect test in 3113 roadway heading face of Daping Mine in Shanxi Province, when the conditions are the gas injection pressure 0.6 MPa and the vacuum extraction 26 kPa, we continue to gas drainage drilling for gas injection side drainage boundary test for 14 days.As a result, the gas injection permeability enhancement test has achieved remarkable results.e coal seam gas content of the 3113 Lane driving face decreased from 12.33 m 3 /t to 7.12 m 3 /t, which greatly shortened the time needed to eliminate the risk of coal seam gas outburst.Although the required theoretical time is longer than the simulation, the speci c conditions of the test are not completely consistent.In the error range allowed by the test, the eld test of No. 3113 roadway heading face of Daping mine in Shanxi province has succeeded.

Conclusions
(1) According to the basic principle of borehole gas injection pump-type coal gas permeability of low permeability, and on the basis of gas seepage mechanics, gas adsorption theory, gas di usion theory, and law of energy conservation and mass conservation theory, we establish the mathematical model through the coal seam gas injection in low permeability increase.(2) e simulation software is used to simulate the inuence law of gas on the gas content in coal seam under the condition of gas injection in the side of the gas drainage borehole.e in uence law of di erent pumping negative pressure and gas injection pressure on eliminating outburst danger is simulated.It is concluded that the negative pressure and gas injection pressure have a signi cant e ect on shortening the outburst cycle, and the negative pressure of 26 kPa extraction and the injection pressure of 0.6 MPa are the reasonable parameters for gas permeability enhancement in low-permeability coal seams.
(3) rough the eld test in 3113 roadway heading face of Daping mine in Shanxi Province, it is concluded that gas injection and permeability increase in the gas extraction borehole of low-permeability coal seam can obviously improve the gas volume fraction in the drainage borehole in a short time.And after continuous gas injection test, the gas content of the coal seam decreased from 12.33 m 3 /t to 7.12 m 3 /t, which greatly shortened the time needed for outburst elimination.
Data Availability e data used to support the ndings of this study have been deposited in the Scienti c Data's List of Recommended Repositories repository (DOI https:// gshare.com/account/home#/collections).Advances in Civil Engineering

4. 1 .
Geometric Model and Related Parameters.Because the gas permeability enhancement model is a three-dimensional structure, in order to facilitate the simulation, we simplify the model to a two-dimensional geometric model and select the radial study of drilling.

Figure 1 :
Figure 1: Plots of gas injection to improve the permeability model.

Figure 2 :
Figure 2: Plots of content variation of gas at 0.8 m in the horizontal direction of gas injection.(a) In uence of di erent negative pressure on eliminating outburst risk.(b) In uence of di erent gas injection pressure on eliminating outburst risk.

Figure 3 :
Figure 3: Plots of content variation of gas at 0.4 m in vertical direction of gas injection.(a) In uence of di erent negative pressure on eliminating outburst risk.(b) In uence of di erent gas injection pressure on eliminating outburst risk.

Figure 6 :
Figure 6: Plots of gas concentration variation in the gas hole during gas injection.

Table 1 :
Parameters for coal and gas in need.