Advances in Civil Engineering

Volume 2018, Article ID 1912383, 7 pages

https://doi.org/10.1155/2018/1912383

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

^{1}School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China^{2}State Key Laboratory Cultivation Base for Gas Geology and Gas Control, Henan Polytechnic University, Jiaozuo 454000, China^{3}Henan Province Collaborative Innovation Center of Coalbed Methane and Shale Gas for Central Plains Economic Region, Jiaozuo 454000, China

Correspondence should be addressed to Guo Shaoshuai; moc.qq@3487148941

Received 24 March 2018; Accepted 27 May 2018; Published 12 September 2018

Academic Editor: Guang Xu

Copyright © 2018 Li Hui et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

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. The results indicate that the best condition is a negative pressure drainage at 26 kPa and a gas injection pressure at 0.6 MPa 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 12.33 m^{3}/t to 7.12 m^{3}/t, greatly reducing the risk of coal and gas outburst elimination time required.

#### 1. 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. This 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].

The 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 [6–9]. No field tests are carried out, and the effect verification is lacking. Therefore, 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 [10–12], 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.

This 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. The 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.

#### 2. Theory of Gas Injection Technology for Improving Permeability

The 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. The 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)The 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. The 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}. Thus, 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. Therefore, 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.

#### 3. 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.

##### 3.1. 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 iswhere is the density of gas component (kg/m^{3}), is the total seepage velocity of gas (m/s), and is the content of free gas component (kg/m^{3}). , where is the porosity.

##### 3.2. Adsorption Equilibrium Equation of Multicomponent Gas

The content of adsorbed component under the assumed equilibrium pressure can be expressed by the generalized Langmuir isotherm adsorption equation aswhere is the density of coal (kg/m^{3}), is the individual ultimate adsorption capacity of component in coal seams (m^{3}/kg), is the adsorption constant of component (MPa^{−1}), and represent the partial pressure of gas components 1 and 2, respectively.

##### 3.3. Mass Exchange Equation

The mass exchange between adsorbed gas on the surface of coal and free gas in the fracture system can be defined aswhere is the desorption diffusion coefficient, = 1.42.

##### 3.4. 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. The equation of state of ideal gas can be expressed aswhere is the molar mass of the gas component (g/mol), is the universal gas constant, and are the gas pressure and temperature under the standard conditions, respectively. = 0.1 MPa and .

##### 3.5. Seepage Velocity Equation

As the gas component flows in the coal body that conforms to Darcy’s law, the total gas seepage velocity iswhere is the permeability of coal body (m^{2}), is the dynamic viscosity coefficient for gas component (Pa·s), and is the total pressure (gas injection pressure, MPa), .

##### 3.6. Cross-Coupled Equation

Substituting (2)–(5) into (1), we can obtain simultaneous equations with cross coupling

##### 3.7. 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 is the gas component, represents CH_{4}, represents N_{2}, is the mass concentration (kg/m^{3}), is the diffusion coefficient (m^{2}/s), is the gas injection time, is the Hamiltonian operator, and 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.

#### 4. Numerical Simulation and Analysis

##### 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. The geometric model of gas injection and permeability enhancement is shown in Figure 1.