Study on key grouting blocking parameters of gas drainage boreholes in soft coal seams

Abstract

The construction of gas extraction boreholes in soft coal seams is prone to collapse and deformation, and grouting reinforcement is one of the main methods to solve the problem of efficient sealing. However, the reasonable selection of key grouting parameters still needs further research. In response to the problem of selecting grouting sealing parameters for gas drainage drilling in soft coal seams, based on the “concentric ring” reinforcement sealing technology obtained in previous research, the key parameters and sealing technology of the “concentric ring” reinforcement were studied through theoretical calculation and numerical simulation experiments. The slurry diffusion morphology and range under different grouting pressures and grouting time slurry viscosity were obtained. Finally, in order to explore the application effect of key grouting parameters, on-site industrial tests were conducted in a soft and high gas coal seam. The research results indicate that the optimal grouting pressure for the “retaining wall rock hole ring” should not be less than 3 MPa, the reasonable grouting time should be 10–15 min, and the water material ratio of the grouting material should be greater than 1:1; The use of new reinforcement sealing and grouting technology can ensure long-term good extraction effect. Compared to the testing of drilling sealing effect using ordinary cloth bags with two plugs and one injection, The adoption of new reinforcement sealing technology can effectively prevent the deformation and collapse of the borehole before sealing, and due to two rounds of grouting and pre reinforcement of fractured coal, the sealing effect of the borehole is also relatively good. The research results have important theoretical value for guiding the drilling and sealing grouting engineering of gas extraction in soft coal seams.

1. Introduction

Coal seam gas extraction is a commonly used and effective method in current mines to improve mining efficiency and reduce safety issues caused by gas disasters. Gas pre extraction is an important measure to reduce the amount of gas emitted from mines, prevent gas explosions and coal and gas outburst disasters [[1], [2], [3]]. However, there are still issues with China's mines' gas extraction technology and procedures at the moment; the average mine gas extraction rate is still below 30% [[4], [5], [6]], which also leads to frequent occurrence of mine gas accidents. China's soft coal seam mines are widely distributed, the process of sealing and grouting the drill holes has high requirements for grouting time, grouting pressure and fluidity of grouting materials, etc. The reasonable selection of these key grouting parameters directly affects the sealing performance of the drill holes, and the reasons for the selection of the key parameters of the grouting at the present stage are unclear, which also directly leads to the poor gas extraction effect in soft coal seams [7,8].

Coal seam drilling is a prerequisite for implementing drainage technology engineering, but a considerable number of high gas coal seams are damaged by geological structures during the coalification process, resulting in soft coal quality and low permeability of coal seams. In addition, combined with the effect of gas pressure, after the construction of drainage boreholes, the sealing section is prone to instability and deformation, making it difficult to seal the boreholes and unable to ensure the effectiveness of gas drainage, as shown in Fig. 1. In the field research of soft coal seam mines, it was found that after drilling in the soft coal seam, collapse often occurs during the retreat of the borehole, which caused the end of drilling cannot be timely down the pipe sealing and other issues, and once again thinning and hollowing out the hole, whether from an economic or time perspective, will cause significant waste [9,10].

Fig. 1.

Drilling hole collapse.

Based on the above problems, we proposed a new type of “concentric ring” reinforcement sealing technology, as shown in Fig. 2 [11], the technology has been applied in mines with soft coal seams with good results. However, the degree of hole fragmentation in the sealing section of soft coal seams is high, there are a large number of holes and cracks, in the reinforcement of grouting not only want to have a certain effect of pre-reinforcement of the broken area of the mouth of the hole, and at the same time, more hope that the slurry can penetrate and diffuse around the sealing section hole, and be reinforced after the consolidation of the slurry, the soft coal seam borehole broken area of the coal body to play a role of pre-reinforcement and pre-seal the early holes and cracks. The grouting effect depends on the performance of the slurry material itself, but also needs to consider the key parameters such as grouting pressure, grouting time, etc. [12,13]. Therefore, it is necessary to study the key parameters in the reinforcement and sealing grouting process of the “concentric ring”, which can guide the sealing grouting process after drilling to a certain extent.

Fig. 2.

Schematic diagram of new reinforcement and sealing technology.

The development of sealing technology for coal bed extraction drilling is also accompanied by the updating of sealing materials, and the drilling sealing project requires that the sealing materials should meet the requirements of good sealing performance, easy operation, flame retardant, and so on. Yellow mud clay sealing is the earliest used method of extracting borehole sealing, semi-dry clay, yellow mud or yellow mud cement compounds whose texture is fine and plasticity [14], with the continuous deepening of research on drilling sealing by domestic and foreign scholars, drilling sealing technology and sealing materials have also become diverse. Among them, the inorganic sealing material has a wide range of material sources, low-cost characteristics of long-term as a sealing material used in drilling sealing, the most common that is cement expansion material, expansion cement has a slurry mobility can be controlled, with a certain expansion and other characteristics, the introduction of ultra-fine cement can be effective in enhancing the penetration performance of the expansion cement to improve the effect of drilling sealing. In recent years, extensive research has been conducted on inorganic grouting materials and key grouting parameters. Zhou et al. [15] for the lack of remedial measures after drilling failure and leakage, drawing on the principle of fire zone closure and plugging for the first time proposed a secondary sealing method and supporting devices [16], which are used in the micro-fine expansion powder are common inorganic non-metallic materials composition. Zhang et al. [17] developed a new type of cured expansion sealing material, which showed superior characteristics in terms of fluidity, expansion volume and expansion time, viscosity, etc., and systematically investigated the expansion characteristics and mechanical creep properties of the new material. Mao et al. [18] analyzed the influence of the interaction between three modified materials, fly ash, nano silica, and triethanolamine, on the strength of the sealing materials. Zhang et al. [19] proposed the “strong-weak-strong” pressurized sealing and the reinforced dynamic sealing technologies, which improves the volume fraction of the extraction and the speed of the gas extraction. Zhang et al. [20] proposed a more targeted new step-by-step grouting sealing technique to solve the sealing length of long-distance gas extraction boreholes upward in a bi-directional non-equivalent compressive stress field and derived the boundary equations of the molding zone under a non-equivalent compressive stress field in a circular roadway. Based on the traditional hole sealing technology as well as the cement self-shrinkage effect. Li et al. [21] proposed a two-stage variable diameter pressure sealing technology for drilled holes, and conducted field tests underground in coal mines. Sun et al. [22] studied the effect of sodium hydroxide content in sealing materials on the setting time, and concluded that sodium hydroxide has the effect of accelerating the crystallization of cement-based sealing materials. However, the above studies are based on the traditional theoretical analysis perspective and qualitative analysis of the key parameters of grouting, and the research content for the parameters of grouting in drill holes of soft coal beds is rarely reported, resulting in the sealing and grouting effect of gas extraction drill holes in mines of soft coal beds is still unsatisfactory.

In the research on the “concentric circle” reinforcement and sealing technology in soft coal seams, we combined with the existing material research results, the use of theoretical analysis and experimental simulation means of combining the sealing grouting process of soft coal seam drilling grouting pressure, grouting time and grouting slurry mobility, and further conducted industrial application field experiments on the research results. The research results have significant scientific value in guiding grouting sealing technology and improving gas extraction efficiency.

2. Research on key parameters of seal grouting in drill holes of soft coal seams

High-efficiency sealing of drill holes in soft coal seams is a key link in solving the problem of efficient gas extraction in coal seams, among which, grouting reinforcement for a large number of pore cracks around the sealing section of the borehole is the main way to realize high-efficiency sealing of drill holes. Influence grouting reinforcement effect is good or bad, in addition to depending on the performance of the grouting material itself, but also need to consider the grouting pressure, grouting time and other technical parameters. Therefore, it is necessary to study the key parameters of the grouting process for strengthening and sealing the “concentric ring”, in order to provide scientific guidance for gas extraction and sealing technology.

2.1. Theoretical analysis of slurry diffusion control

For the slurry flow problem in the gas-containing coal body belongs to the multiphase flow problem. The coal body belongs to the continuous medium, choose any shape of the control body, which is fixed in time and space, and utilize the law of mass conservation and the momentum theorem to derive the continuity equation and the equation of motion of the slurry diffusion, respectively.

• (1)Slurry diffusion continuity equation

The mass conservation law was used to derive the slurry diffusion continuity equation. To this end, take a control object Ω in the flow field, and the outer surface of the control object is s. Take a surface element ds on the outer surface s, and the outer normal direction is n. The diffusion velocity through the surface element ds is v, so that the mass per unit of time through the surface element ds is ρv-nds, and by the principle of mass conservation, the increment of the mass of the slurry fluid within the control object Ω shall be equal to the mass of the slurry fluid exiting through the surface area s, Equation (1) is the slurry diffusion continuity equation in integral form.

$$\underset{\mathrm{\Omega }}{\int }\frac{\partial \rho }{\partial t}d\mathrm{\Omega }={∯}_{\mathrm{\Omega }}\rho v·nds$$ (1)

Using the Gaussian formula, we obtain Equation (2),

$${∯}_{\mathrm{\Omega }}\rho v·nds=\underset{\mathrm{\Omega }}{\int }\nabla ·\left(\rho v\right)d\mathrm{\Omega }$$ (2)

Substituting the above equation into (1) to obtain Equation (3),

$$\underset{\mathrm{\Omega }}{\int }\left[\frac{\partial \rho }{\partial t}+\nabla ·\left(\rho v\right)\right]d\mathrm{\Omega }=0$$ (3)

An arbitrarily chosen control body Ω must necessarily result in its product function being equal to zero as long as the product function is continuous throughout the integral is equal to zero. The continuity equation for slurry diffusion in differential form is obtained as Equation (4),

$$\frac{\partial \rho }{\partial t}+\nabla ·\left(\rho v\right)=0$$ (4)

The slurry fluid studied in the paper is an incompressible fluid, so the continuity equation in the orthogonal linear coordinate system is Equation (5)

$$\frac{\partial v_i}{\partial x_i}=0$$ (5)

• (2)Equation of motion for slurry diffusion

Based on the arbitrariness of the control volume, the equation of motion for slurry diffusion is:

$$\frac{\partial v}{\partial x}+\left(v·\nabla \right)v=\frac{1}{\rho }\nabla ·P+f$$ (6)

where P is the stress tensor and ▽ is the dispersion operator, writing Equation (6) in component form as:

$$\{\begin{array}{c}\frac{\partial v_x}{\partial t}+v_x\frac{\partial v_x}{\partial x}+v_y\frac{\partial v_x}{\partial y}+v_z\frac{\partial v_x}{\partial z}=\frac{1}{\rho }\left[\frac{\partial P_{xx}}{\partial x}+\frac{\partial P_{yx}}{\partial y}+\frac{\partial P_{zx}}{\partial z}\right]+f_x\\ \frac{\partial v_y}{\partial t}+v_x\frac{\partial v_y}{\partial x}+v_y\frac{\partial v_y}{\partial y}+v_z\frac{\partial v_y}{\partial z}=\frac{1}{\rho }\left[\frac{\partial P_{xy}}{\partial x}+\frac{\partial P_{yy}}{\partial y}+\frac{\partial P_{yz}}{\partial z}\right]+f_y\\ \frac{\partial v_z}{\partial t}+v_x\frac{\partial v_z}{\partial x}+v_y\frac{\partial v_z}{\partial y}+v_z\frac{\partial v_z}{\partial z}=\frac{1}{\rho }\left[\frac{\partial P_{xz}}{\partial x}+\frac{\partial P_{yz}}{\partial y}+\frac{\partial P_{zz}}{\partial z}\right]+f_z\end{array}$$ (7)

Equation (7) is the control equation of slurry fluid motion, which is based on the conservation of momentum, namely Navier-Stokes equation. In this control equation, v_i represents the velocity component of the slurry fluid motion in each direction, and f_i represents the volumetric force per unit mass in each direction, which mainly includes the roles of grouting pressure, gravity and gas pressure.

Using the average value as the pressure-p of the slurry fluid, the stress tensor is split into two parts, the stress ball tensor and the stress bias tensor, as shown in Equations (8), (9):

$$\{\begin{array}{c}{P}_{ij}=-p+{\tau }_{ij}i=j=1,2,3\\ P_{ij}={\tau }_{ij}i\ne j\end{array}$$ (8)

$$p=-\frac{1}{3}\left(P_{xx}+P_{yy}+P_{zz}\right),{\tau }_{xx}+{\tau }_{yy}+{\tau }_{zz}=0$$ (9)

Therefore, Equation (7) can be rewritten as:

$$\{\begin{array}{c}\frac{\partial v_x}{\partial t}+v_x\frac{\partial v_x}{\partial x}+v_y\frac{\partial v_x}{\partial y}+v_z\frac{\partial v_x}{\partial z}=-\frac{1}{\rho }\frac{\partial p}{\partial x}+\frac{1}{\rho }\left[\frac{\partial {\tau }_{xx}}{\partial x}+\frac{\partial {\tau }_{yx}}{\partial y}+\frac{\partial {\tau }_{zx}}{\partial z}\right]+f_x\\ \frac{\partial v_y}{\partial t}+v_x\frac{\partial v_y}{\partial x}+v_y\frac{\partial v_y}{\partial y}+v_z\frac{\partial v_y}{\partial z}=-\frac{1}{\rho }\frac{\partial p}{\partial y}+\frac{1}{\rho }\left[\frac{\partial {\tau }_{xy}}{\partial x}+\frac{\partial {\tau }_{yy}}{\partial y}+\frac{\partial {\tau }_{yz}}{\partial z}\right]+f_y\\ \frac{\partial v_z}{\partial t}+v_x\frac{\partial v_z}{\partial x}+v_y\frac{\partial v_z}{\partial y}+v_z\frac{\partial v_z}{\partial z}=-\frac{1}{\rho }\frac{\partial p}{\partial z}+\frac{1}{\rho }\left[\frac{\partial {\tau }_{xz}}{\partial x}+\frac{\partial {\tau }_{yz}}{\partial y}+\frac{\partial {\tau }_{zz}}{\partial z}\right]+f_z\end{array}$$ (10)

In Equations (1), (10)) is the time derivative of velocity, which reacts to the time variation of slurry diffusion velocity; 2 is the variable of slurry pressure in each direction, which reacts to the spatial variation of slurry pressure; 3 is the level of slurry stress, which reacts to the characteristics of slurry stress response; 4 is the derivative of the diffusion velocity of the slurry, which reflects the variation of diffusion velocity in the slurry.

2.2. Experimental modelling and computational methods

COMSOL Multiphysics numerical simulation software is used to simulate the “concentric ring” grouting reinforcement process, the establishment of 5 m × 5 m porous media model, the reinforcement section of the grouting hole is arranged in the center of the model, the diameter of the grouting hole is set to 160 mm. Numerical computation of the model and the boundary conditions, as shown in Fig. 3.

Fig. 3.

Grouting numerical model of “Concentric ring”.

The slurry diffusion pattern in the model calculation process adopts the volume fraction representation method, and in order to investigate the influence of grouting pressure on diffusion law, the working conditions with different reinforcement grouting pressures are analyzed, respectively. According to the test results of loose coal samples in the literature [23], the permeability of the model coal body is defined as 6.29 × 10⁻¹⁵ m², the grouting pressure p₁ is taken as 1 MPa, 2 MPa, 3 MPa, 4 MPa, and 5 MPa in the model according to the actual situation on site, respectively, and the initial value p₂ of coal gas pressure is taken as 0.2 MPa. The numerical calculation working conditions and parameters are shown in Table 1, Table 2, which are determined based on on-site grouting operations in soft coal seams and existing commonly used equipment.

Table 1.

Calculation parameters.

water to material ratio	Slurry density/kg·m−3	Slurry viscosity/Pa·s	Water density/kg·m−3	Water viscosity/Pa·s	Rock porosity/%
1:1	1450	0.0962	1000	0.001	20

Table 2.

Calculation working condition parameters.

Water-cement ratio of reinforced cement slurry	Grouting pressure/MPa	Gas pressures/MPa	Medium permeability/m2
1:1	1, 2, 3, 4, 5	0.2	6.29 × 10−15m2

The calculation principle is that the mass conservation of gas gas and reinforcement slurry in the model area is obtained from the analysis of the slurry control equations above, and the mass conservation equations of reinforcement slurry and gas are shown in Equations (11), (12).

$$-\left[\frac{\partial \left({\rho }_s{v}_{sx}\right)}{\partial x}+\frac{\partial \left({\rho }_s{v}_{sy}\right)}{\partial y}+\frac{\partial \left({\rho }_s{v}_{sz}\right)}{\partial z}\right]=\frac{\partial \left(\phi {\rho }_s{s}_s\right)}{\partial \tau }$$ (11)

$$-\left[\frac{\partial \left({\rho }_w{v}_{wx}\right)}{\partial x}+\frac{\partial \left({\rho }_w{v}_{wy}\right)}{\partial y}+\frac{\partial \left({\rho }_w{v}_{wz}\right)}{\partial z}\right]=\frac{\partial \left(\phi {\rho }_w{s}_w\right)}{\partial \tau }$$ (12)

According to the generalized Darcy's law and its supplementary equation (13):

$$-\left[\frac{\partial \left({\rho }_w{v}_{wx}\right)}{\partial x}+\frac{\partial \left({\rho }_w{v}_{wy}\right)}{\partial y}+\frac{\partial \left({\rho }_w{v}_{wz}\right)}{\partial z}\right]=\frac{\partial \left(\phi {\rho }_w{s}_w\right)}{\partial \tau }$$ (13)

where ρ_s is the density of the slurry, ρ_w is the gas density, φ is the porosity of the media medium, s_w is the volume fraction of the reinforcing slurry in the media, s_s is the gas volume fraction in the media, v is the velocity of the seepage field, v_s is the flow rate of the reinforcing slurry in the media, v_w is the flow rate of the gas in the media, μ is the dynamic viscosity of the fluid, and k is the absolute permeability of the media.

The movement of each of the solidified slurries and gas gases is expressed in terms of Darcy's law, and when the volume fraction of either is 1, the permeability inherent in the medium itself depends on its permeability to the two fluids, and we will take the permeability of each of the two fluids to be less than all of the permeability of the medium itself as the effective permeability, and the ratio of that to that of the medium itself is called the relative permeability. Usually in use, it is customary to adopt their ratio to the absolute permeability k, as shown in Equation (14).

$$k_{wr}=\frac{k_w}{k},k_{sr}=\frac{k_s}{k}$$ (14)

where k_wr and k_sr are the relative permeability of the gas gas and the reinforcement slurry, respectively, as a function of volume fraction, k_w and k_s are the effective or phase permeability of the gas and the reinforcement slurry, and k is the absolute permeability of the medium.

Control equations for two-phase flow are shown in Equation (15).

$$\frac{\partial \left(\phi \rho \right)}{\partial t}+\nabla •\left(-\rho \frac{k}{\mu }\nabla p\right)=0\rho =s_w{\rho }_w+s_s{\rho }_s$$ (15)

$$\frac{1}{\mu }=s_w\frac{k_{wr}}{{\mu }_w}+s_s\frac{k_{sr}}{{\mu }_s}s=s_w+s_s=1$$

where φ is the porosity; s_w and s_s are the volume fractions of gas and reinforcing slurry, which completely fill the whole pore fracture, so their sum is 1; ρ_w and ρ_s are the densities of gas and reinforcing slurry; and μ_w and μ_s are the kinetic viscosities of gas and reinforcing slurry. Ultimately, the distribution of the volume fraction at any moment is calculated in the numerical model, and it is possible to characterize the diffusion pattern and range of the grouting slurry under specific conditions.

3. Key parameter modelling results and analysis

3.1. Calculation results and analysis of grouting pressure and grouting time

According to the relevant parameters of the on-site grouting project, the pressure setting for reinforcement grouting is 1 MPã5 MPa and the reinforcement grouting time is 0∼30 min during the model calculation process, as shown in Fig. 4, Fig. 5, Fig. 6 for the cloud diagram of slurry diffusion distribution of the calculation results of the grouting pressure of 1 MPa (Fig. 3, Fig. 4 MPa (Fig. 5(a–f) and 5 MPa (Fig. 6(a–f) under the different pre-reinforcement grouting time.

Fig. 4.

Diffusion radius at different grouting times with a grouting pressure of 1 MPa: (a) 5 min; (b) 10 min; (c) 15 min; (d) 20 min; (e) 25 min; (f) 30 min.

Fig. 5.

Diffusion radius at different grouting times with a grouting pressure of 3 MPa: (a) 5 min; (b) 10 min; (c) 15 min; (d) 20 min; (e) 25 min; (f) 30 min.

Fig. 6.

Diffusion radius at different grouting times with a grouting pressure of 5 MPa: (a) 5 min; (b) 10 min; (c) 15 min; (d) 20 min; (e) 25 min; (f) 30 min.

According to the calculation results, in order to determinate of appropriate reinforcement grouting pressure and time, the graphs of grouting time versus slurry diffusion radius under different reinforcement grouting pressures and grouting pressure versus slurry diffusion radius under different reinforcement grouting times were plotted, as shown in Fig. 7, Fig. 8, respectively.

Fig. 7.

Curve of grouting time and slurry diffusion radius.

Fig. 8.

Curve of grouting pressure and slurry diffusion radius.

It can be observed from Fig. 7 that the spreading range of reinforcement slurry expands with the increase of reinforcement grouting time and grouting pressure, and when the grouting pressure reaches 5 MPa, the slurry can diffuse to a range of 2.4 m around the hole under the maximum grouting time. Comparison of different reinforcement grouting time can be seen, when the reinforcement grouting time more than 15min after the slurry diffusion range of the trend of increasing gradually reduced, due to the reinforcement of the slurry material's own gelatinization characteristics of the considerations as well as the site sealing of the actual situation, the grouting process of “protective wall rock hole ring” is more appropriate in 10–15 min.

From Fig. 8, it can be observed that as the pre reinforcement grouting pressure increases, the grouting diffusion range changes significantly at each moment of grouting. Comparison of grouting pressure changes and slurry diffusion range at 6 time points, when the grouting pressure is greater than 3 MPa, the slurry diffusion range gradually decreases with the increase of grouting pressure. For practical application of “concentric ring” reinforcement and sealing, a higher grouting pressure not only increases the requirements for the grouting pump, but also increases the sealing requirements for each connecting part. Based on the simulation calculation results, a grouting pressure of 3 MPa for the “protective wall rock hole ring” was finally determined.

Through the above analysis and on-site grouting sealing process, it can be determined that the grouting time for the “grouting sealing ring” should be 10–15 min for the drill holes after reinforcement and the grouting pressure should not be less than 3 MPa.

In the “concentric ring” reinforcement sealing method, the size of the sealing section of the “protective wall rock hole ring” grouting diffusion range and grouting the viscosity of the reinforcing slurry used is also directly related to the reinforcing slurry, reinforcing the sealing section of the “grouting sealing ring” grouting slurry infiltration effect is directly related to the viscosity of the slurry.

3.2. Calculation results and analysis of the spreading range of grouting slurry

In order to study the variation of slurry viscosity and the law of grouting diffusion range, this paper conducted research on five slurry viscosity levels: 0.1725 Pa s, 0.1339 Pa s, 0.0962 Pa s, 0.0624 Pa s, and 0.0285 Pa s, with a reinforcement grouting time of 15min and a grouting pressure of 3 MPa. The calculation results are shown in Fig. 9.

Fig. 9.

Calculation results of grouting reinforcement slurry diffusion under different viscosity levels: (a) 0.1725 Pa s; (b) 0.1339 Pa s; (c) 0.0962 Pa s; (d) 0.0624 Pa s; (e) 0.0285 Pa s.

From Fig. 9(a–e), it can be observed that the diffusion range of reinforcement slurry is different under different viscosity levels, and the calculation results show that with the decrease of viscosity, the diffusion radius of reinforcement grouting when other conditions are the same gradually increases, and according to the calculation results, a curve of the change rule of viscosity and reinforcement slurry diffusion is plotted, as shown in Fig. 10.

Fig. 10.

Curve of slurry viscosity and reinforcement slurry diffusion variation.

From Fig. 10, it can be observed that with the gradual reduction of the viscosity level of the reinforcement seal grouting slurry, the radius of the slurry increases sequentially, and it can be concluded from the observation of the change rule of the curve that the diffusion radius of the slurry rapidly increases after the viscosity of the slurry is reduced to within 0.1 Pa s. This shows that the viscosity of the slurry largely affects the penetration effect of “concentric ring” reinforcement sealing grouting, which is directly related to the coal body reinforcement and air leakage channel sealing in advance. Accordingly, combining with the research basis of the fluidity performance of sealing materials in the early stage, we can get the inorganic expanded cement material water ratio should be greater than 1:1, at this time, the fluidity performance of the material can better ensure that the diffusion radius of the slurry meets the needs of sealing holes.

4. Industrial test

The final on-site effect detection test was carried out in a soft coal seam mine in three provinces of central China to investigate the application effect of the key parameters of grouting, on-site coal seam drill holes are downgradient gas extraction drill holes, the coal seam is drilled from the working face of the N2106 belt channel, and the test site and the test drill holes are arranged as shown in Fig. 11.

Fig. 11.

Layout of drilling holes on the working face N2106.

The testing boreholes in N2106 transport roadway are divided into three groups, each group consists of ten drill holes. Group A (boreholes A1-A10) adopts the conventional method of “two blocking and one injection”, the sealing materials and grouting process are the original process of the mine, after the grouting is completed, the grouting pumps are turned off, and the grouting pressure is 2.0 MPa, and the ratio of water to material is 1:1 for the deployment of the slurry. Group B (boreholes B1–B10) adopts the new “concentric ring” reinforcement sealing technology, and its sealing material and grouting process are also the original process of the mine, and its sealing material adopts the ordinary expansion cement. Group C (boreholes C1–C10) employs new reinforcement and sealing technology, that is, “grouting sealing ring”, with a grouting time of 10∼15 min, a grouting pressure of 4.5 MPa, and water to material ratio of 5:3 blending slurry to ensure that its good mobility performance.

Gas monitoring was conducted on three groups of boreholes on site for three months. The frequency of gas volume fraction testing in boreholes was once every two days, and the average value was taken after testing three shifts each day, and the gas concentration statistics are shown in Fig. 12, Fig. 13, Fig. 14.

Fig. 12.

Test results of group A.

Fig. 13.

Test results of group B.

Fig. 14.

Test results of group C.

In addition, Fig. 15 shows the statistical comparison of gas extraction concentrations in three sets of test boreholes. Combined with Fig. 12, Fig. 13, Fig. 14, it can be seen that after 30 days of extraction, the gas concentration in Group A was generally less than 30%, with an average concentration of 21.2%, 10.1%, and 2.0% on the 30th, 60th, and 90th day, respectively. The overall gas concentration decreases faster in the early stage. The gas concentration in Group B was basically above 30% within 30 days before extraction, and then the gas concentration in most boreholes began to decline to within 30%. The average gas concentration on the 60th and 90th day of extraction was 18.4% and 10.9%, respectively. The test holes in Group C were able to ensure a gas extraction concentration of 30% or more for the first 60 days of extraction, with higher stability and continuity of gas concentration, and the gas concentration was 53.9% after 30 days of extraction. Additionally, the average gas concentration was still more than 30% at 60 days of extraction, and it reached 21.2% on the 90th day.

Fig. 15.

Average gas extraction concentration and trend curve of three test boreholes.

According to the above analysis, the test holes in Group C adopt the new reinforced sealing technology and sealing grouting process, which can ensure the good extraction effect in a longer period of time. Compared with the sealing effect of the test borehole in Group A, which adopts ordinary bag with two plugs and one injection, the sealing effect of the test boreholes in Group B is better than that of Group A due to the adoption of the new reinforcing sealing technology to avoid collapsing of the borehole before sealing, and also due to the two injections of grouting and pre-consolidation of the crushed coal body. However, due to the inherent defects of the sealing material, it cannot resist the instability of the drilling sealing section, so the testing holes of Group B cannot guarantee a high extraction level for a long time.

5. Conclusions

Based on the theoretical analysis of gas extraction drilling sealing grouting slurry diffusion control, a numerical calculation model for drilling grouting reinforcement was established. The volume fraction was used to characterize the sealing grouting slurry diffusion process at any time, and the slurry diffusion morphology and range under different grouting pressures and grouting times were obtained. The trend of increasing the diffusion range of slurry gradually decreases when the reinforcement grouting time exceeds 15 min. Considering the cementitious properties of the reinforcement slurry material itself and the actual situation of on-site sealing, it is determined that the grouting time for the “wall protection rock hole ring” is 10–15 min, which is more suitable. Based on the analysis of grouting pressure and the selection of grouting equipment, it is preliminarily determined that during the sealing grouting process of the “grouting sealing ring”, the grouting pressure should not be less than 3 MPa. When the viscosity of the drilling sealing grouting slurry decreases to within 0.1 Pa s, the diffusion radius of the slurry rapidly increases. The water material ratio of inorganic expansive cement materials should be greater than 1:1 based on the research on the flow performance of existing sealing materials. At this time, the material's flow performance is good, which can ensure that the diffusion radius of the slurry meets the sealing requirements. Compared to the testing of drilling sealing effect using ordinary cloth bags with two plugs and one injection, the use of new reinforcement sealing technology can avoid collapse before drilling sealing, and the sealing effect of drilling is also relatively good due to two injections and pre reinforcement of broken coal bodies, and ensure good extraction effect for a long time.

Data availability statement

The data that support the findings of this study are available upon reasonable request from the corresponding author.

CRediT authorship contribution statement

Ruoyu Bao: Writing – original draft, Conceptualization. Fubao Zhou: Resources, Conceptualization. Hongbo Shang: Writing – review & editing.

Declaration of competing interest

The authors declare no competing interests.