Abstract
To solve the problem of a rapid attenuation of gas concentration along with drainage borehole collapse in the tectonic coalbeds of South China, a constant pressure grouting technology with inorganic, noncondensable material was proposed. Firstly, the slurry fluidity, water separation rate, and sealing performance of the inorganic sealing materials were tested under different water-cement ratios. The seepage model of slurry in a layer-through borehole was built with COMSOL Multiphysics simulation software, to explore the scopes of loose circles around drainage roadway and borehole, and to analyze the seepage capacity of the slurry under different grouting pressures. Eventually, the sealing performance of the slurry was investigated in the field. The results showed that the inorganic, noncondensable material with the water-cement ratio of 5 : 1 has a strong fluidity, low water retention, high permeability, and good sealing performance. After the excavations of No.2164 drainage roadway and layer-through borehole, there are obvious stress concentrations both at the shoulder corners of the roadway and at the borehole bottom, and the scope of the loose circle around the roadway is about 6.2 m. The effective seepage radius of the inorganic slurry gradually increases with a rising grouting pressure, and the slurry seepage range in the sandstone section is broader than that in the mudstone section. Adopting the constant pressure grouting technology with the slurry, the average drainage concentration of boreholes in Puxi coal mine is 51.5%, and the average gas flow rate is 0.005 m3/min, which are 1.35 times and 1.67 times than those with the cement grouting method.
1. Introduction
The efficient and low-carbon development of the coal industry is the key to achieving the goal of carbon neutrality, and is also the only way to achieve the “bottom-up guarantee” of energy security [1, 2]. Coal seam gas is associated with coal, which is a clean energy with high calorific value and no pollution. Currently, it has entered the stage of commercial exploitation worldwide. The recoverable reserves of coal seam gas in China are about below the buried depth of 2000 meters, with a huge development potential [3, 4]. However, many coal seams have experienced numerous large-scale tectonic movements, and thus the structural soft coal is well developed with poor permeability, resulting in a low efficiency of underground gas drainage [5, 6]. It also brings great hidden dangers to the mine safety. Therefore, promoting the efficiency of coal seam gas recovery has multiple meanings of making up for the shortage of conventional oil and gas resources, reducing greenhouse gas emissions, and promoting coal mine production safety [7–9].
Besides the influence of factors such as coalbed permeability, drilling layout, drainage method, and drainage system capacity, the performance of borehole sealing directly determines the quality of the drainage effect [10, 11]. In recent years, many scholars have carried out the theoretical and experimental studies on the characteristics of borehole sealing under different geological conditions. The early sealing materials of clay, rubber rings, or cement have been eliminated on account of the shortcomings of a short sealing distance and a bad sealing performance, resulting in air leakage from the fracture zone around the borehole [12–14]. Although the polyurethane material has the advantages of rapid expansion and convenience, it displays a low bonding strength [15]. Currently, the pressurized grouting method of “two ends blocking and injection” is widely popularized and applied in many coal mines. Its principle is to use polyurethane or sealing capsule bags to block both ends of the borehole and then inject cement mortar into the space between the two ends, so as to effectively block the fissures around the borehole [16, 17]. However, there would be a gap (“water line”) on the upper part of borehole after cement solidification in the sealing of horizontal boreholes, due to the self-shrinking effect of cement, which connects with the secondary fissures to form a gas leakage channel [18]. Consequently, Wang et al. [19] suggested to drill a reducing-nipple borehole to prevent the sealing issue of the “water line”, and successfully measured the original gas pressure of the low-permeability coalbed in Guizhou Province. Based on the perspective of solid sealing liquid and liquid sealing gas, Cui et al. [20] proposed the sealing technology of injecting viscous liquid with an expandable capsule, significantly improving the hole-sealing effect. However, the expandable capsule is hardly pulled out for recycling, once borehole collapses. Wang et al. [21, 22] considered that the pressurized sealing technology just nominally exists in coal mines, owing to the lack of the further research on the selection of a reasonable grouting pressure and the initial sealing depth of the borehole.
In addition, scholars also developed a variety of sealing materials and studied the hydration properties. Zhou and Li [23, 24] put forward a secondary hole sealing method by using ultrafine expansive powder to overcome the problem of rapid decline of gas drainage flow rate along with the borehole deformation. Zhang et al. [25] found that the modified sulphoaluminate-based material can diffuse and fill the microcracks around the borehole, leading to a good sealing effect. Sun [26, 27] studied the influences of successively adding water-soluble polymer powder, redispersed polymer, and fly ash on the water separation rate, viscosity, and compressive strength of cement slurry. Peng et al. [28, 29] analyzed the adhesive force between polyurethane rigid foam material and fly ash concrete through the changes of thickness and height of polyurethane rigid foam. By using cement/sodium bentonite as the base sealing material, new cracks often occur, and the aperture and width of primary cracks continuously change with the further deformation of surrounding rock of the borehole, which seriously affects the sealing performance [30]. The curing reaction polymerization temperature of polyurethane is so high that coal spontaneous combustion may appear in a closed underground space [31]. Therefore, it is urgent to develop a sealing material with excellent performances to ensure the long-term and efficient production of gas drainage boreholes.
The coal resources in Hunan Province are mainly stored in the Lower Carboniferous Ceshui Formation and the Upper Permian Longtan Formation. The nappe structure and the strata overlap are well developed, and the coalbed gas content is large and the diffusion rate is high [32, 33]. The conventional layer-through borehole often fails to achieve the expected drainage effects, which are mainly manifested in the borehole collapse, the rapid attenuation of gas concentration and flow rate, etc. To solve the problem of borehole sealing in the soft tectonic coalbeds, we proposed a constant pressure grouting technology with an inorganic noncondensable slurry. The fluidity, water separation rate, and sealing performance of the inorganic sealing materials were tested. By establishing the coupling model of the slurry seepage in a layer-through borehole of No.2164 drainage roadway, the scopes of the loose circles of the roadway and the borehole were explored, and the slurry diffusion ability under different grouting pressures was also analyzed. Eventually, the sealing performance of the inorganic slurry was investigated in the field.
2. Properties of Inorganic Noncondensable Materials
The sealing materials used in the constant pressure grouting technology are inorganic composites, mainly composed of sodium bentonite and kaolin, supplemented by a certain amount of Portland cement, fly ash, etc. When the inorganic materials are mixed with water (shown in Figure 1), the viscosity coefficient of the slurry is greater than , the pH value is between 6.5 and 8.5, and the loss on ignition is less than 10%. Furthermore, the inorganic slurry can maintain a fluid state for a long time and has low water separation, high permeability, good suspension stability, and good sealing performance.
The principle of borehole sealing by constant pressure grouting is shown in Figure 2. In the process of sealing drainage boreholes, we first injected the prepared inorganic slurry into the sealing capsule bags at both ends of the borehole with a grouting pipe. Once the capsule bags are filled with the slurry, it expands rapidly and contacts closely with the borehole wall, which blocks the initial air leakage channels formed during the drilling. When the threshold pressure of the bag blasting valve is reached, it can be automatically opened, and then the pressurized inorganic slurry is injected into the space between the two bags through a grouting nozzle. After the sealing section of the borehole is filled with slurry, we continue grouting for a while until the slurry pressure reaches the threshold of the routing pump. Thus, the slurry in the borehole can always keep a relatively constant pressure. If there are secondary fissures generated around the borehole disturbed by tectonic and mining stresses, the noncondensable slurry can actively seep to seal the fissures and isolate the inflow of external air, reducing the channels of gas leakage. Therefore, the constant pressure grouting technology with the inorganic slurry has opened up a new borehole sealing way of “liquid sealing gas”, so as to improve the gas drainage concentration of the borehole in the soft coalbed.
2.1. Slurry Fluidity
Slurry fluidity is an intuitive parameter to characterize the flow performance of slurry. At the initial stage of grouting, the slurry with a low fluidity cannot seep into the microfissures around the drainage borehole, which has a great influence on the sealing quality of boreholes. Here, we measured the fluidity of inorganic noncondensable slurry according to the test method for fluidity of cement mortar (GB/T2419-2005) [34]. The instruments used in the test process mainly include a mortar round mold, a glass plate, a steel ruler, and tape measure. During the test, we successively poured the prepared inorganic slurry with different water-cement ratios (3 : 1~12 : 1) into the round mold for tamping and wiped off the excess slurry higher than the round mold. Then we gently lifted the mold vertically and measured the diffusion diameter of the slurry three times after the slurry stopped flowing. The average value of diffusion diameter was taken as the slurry fluidity under the test conditions, as shown in Figure 3.
Figure 3 shows that the fluidity of the inorganic slurry gradually increases with the rise of the water-cement ratio, which is divided into two stages: rapid growth and stable growth. When the water-cement ratios of the slurry range from 3 : 1 to 4 : 1, the slurry presents a paste and its diffusion ranges are only 56 and 104 mm, respectively. While the water-cement ratio is greater than 5 : 1, the slurry presents a semipaste, and its fluidity increases linearly with an increasing water-cement ratio. Has a certain fluidity and enters a stable growth period. When the water cement ratio is greater than 10 : 1, the slurry is no longer pasty, and the fluidity exceeds 280 mm. From the perspective of the slurry morphology, the grouting slurry should have a strong fluidity during borehole sealing, but it should not be a nonviscous fluid. Consequently, the suitable water-cement ratio is between 5 : 1 and 8 : 1, and the slurry is a semipaste accordingly.
2.2. Water Retention of Slurry
An outstanding advantage of the inorganic, noncondensable sealing materials is that it can always keep gel state and promptly plugs the new cracks around borehole. Therefore, the water retention property of the material is very vital. Only when the slurry of the inorganic materials locks water for a long time, it can maintain a good fluidity and seal the new cracks at any time. The water retention rate is an important indicator of the water retention performance of slurry. According to GB/T1346-2011, test methods for water requirement of normal consistency, setting time, and soundness of the Portland cement [35]. We poured the prepared slurry into a 100 mL measuring cylinder and then sealed off the top of the cylinder with adhesive tapes to prevent moisture loss. After standing for 2 h, we measured the percentage of the upper clear water in the total volume of the slurry, and the results of water retention rate tests of the slurry with different water-cement ratios (3 : 1 ~ 12 : 1) are shown in Figure 4.
Figure 4 shows that the water retention rate of slurry increases with the rise of water-cement ratio. When the water cement ratio of the slurry is less than 6 : 1, the water retention rates are always less than 3%, and there is almost no water retention. While the water cement ratio of the slurry is 6 : 1 to 12 : 1, the water retention rate gradually increases, showing a linear increase trend. Finally, the water retention rate of the slurry reaches 16.3%, when the water cement ratio is 12 : 1. It can be seen that when the water cement ratio of the inorganic sealing slurry is less than 6 : 1, the slurry has an excellent water retention performance.
2.3. Sealing Performance
In order to test the sealing performance of the inorganic noncondensable slurry for small fissures, we injected the stirred slurry of 500 mL into the sieve of 200 mesh with a syringe, and let it stand for 15 min. Then, we poured the slurry passing through the sieve into a measuring cylinder, and the percentage in the total volume of the slurry is recorded. During the borehole sealing process, it is required that most of the grouted slurry be retained on the sieve to block macrofissures, and a small part of the slurry can penetrate the sieve to continue to block smaller fissures. The sealing test results of the inorganic slurry were shown in Figure 5.
Figure 5 shows that with the increase of the water-cement ratio of the inorganic slurry, the sealing performance for the sieve gradually decreases, which is divided into two stages: valid sealing and invalid sealing. When the water-cement ratio of the slurry is 3 : 1, the slurry is very viscous, so that there is almost no leakage on the sieve. When the water-cement ratio is 5 : 1, there is a small part of the slurry passing through the sieve, but the sealing rate is still above 80%, while the sealing ratio of the slurry drops rapidly from 60.5% to 24.3% with the water-cement ratio gradually rising from 6 : 1 to 8 : 1, showing a bad borehole sealing performance.. When the water-cement ratio is greater than 10 : 1, the sealing ratios for the sieve of 200 mesh are basically below 10%, which means that the borehole sealing performance is completely lost.
Through the analysis of the fluidity, the water retention and the sealing performance for 200 mesh sieve of the inorganic slurry under different water-cement ratios, it can be found that the valid water-cement ratio of the slurry should be 5 : 1 for grouting to seal drainage boreholes. In this case, the slurry has a good fluidity, low water retention, and fine sealing performance (see Table 1).
3. Simulation of Borehole Sealing Parameters
When grouting the pressurized slurry for borehole sealing, there are two main reasons for gas leakage in drainage borehole as follows. On the one hand, whether the initial sealing depth exceeds the range of the loose circle of surrounding rock formed by roadway excavation. If the borehole sealing depth is too shallow, the air in roadway may enter the borehole along the loose circle around the roadway. On the other hand, the mixed slurry can only penetrate and then seal the secondary fissures in the depth around the borehole under a sufficient pumping pressure. If the grouting pressure is not enough, these deep fissures around the borehole cannot be blocked, further affecting the gas drainage performance of borehole. Therefore, the key of the constant pressure grouting technology with the inorganic materials depends on the determination of a reasonable grouting pressure and initial sealing position.
Based on the geological conditions of the No.2164 floor drainage roadway and the gas occurrence properties of the V coalbed in the Puxi coal mine, a fluid-structure coupling model of grouting diffusion in the layer-through borehole was established by using Comsol Multiphysics simulation software. We hope to obtain a reasonable position for the initial borehole sealing depth by analyzing the characteristics of stress distribution and plastic zone distribution in the surrounding rock after the No.2164 drainage roadway excavation and the borehole drilling. And a suitable pumping pressure should be also selected by investigating the diffusion range of the inorganic slurry under different pumping pressures. The basic assumptions of the coupling model of grouting diffusion are as follows: (1)Both coal and rock strata are regarded as continuous porous mediums, and they are isotropic and homogeneous(2)The deformation and failure of rock strata meet the Mohr-Coulomb yield criterion(3)The inorganic noncondensable slurry is regarded as incompressible fluid, which flows only in the rock fissures around the sealing section of borehole(4)The slurry seepage in fissures follows Darcy’s law(5)There is no pressure loss of slurry during grouting, and the pumping pressure on the borehole wall is equivalent to the working pressure of the pump
3.1. Governing Equation
3.1.1. Slurry Seepage
The flow of the inorganic noncondensable slurry in borehole follows Darcy’s seepage equation [16]. where is the seepage velocity, m/s; is the permeability of rock strata, m2; is the slurry viscosity, Pa · s; is the grouting pressure, MPa; is the slurry density, kg/m3; is the gravity acceleration, m/s2; and is the vertical coordinate.
While, the continuity equation of the slurry flow is expressed as follows: where is the porosity rock strata around borehole, %; is the fluid volume of the slurry, m3; and is time, s.
By combining the above two equations, the relationship between the grouting pressure and the slurry density can be obtained as follow [16].
3.1.2. Rock Deformation
Based on the assumption of rock experiencing the elastic deformation, the equilibrium equation of stress is expressed as follow: where is stress, MPa; the subscripts of and are the main directions, and is the bulk stress of rock, MPa. Besides, the geometric equation for the plastic deformation of roadway is described as follow: where is the strain component of surrounding rock, and is the displacement in the -direction.
Assuming that the elastic-plastic deformation occurs within the surrounding rock of No.2164, and the layer-through borehole, according to Hooke’s law, the constitutive equation of coal and rock deformation is expressed as follow [36]. where is the shear modulus of rock strata, MPa; is the rock Young’s modulus, MPa; is the Possion’s ratio of rock strata; is the rock volumetric strain; is the Kronecker variable; and are the gas pressures on fissure and coal matrix, respectively, MPa; and are the Biot coefficients of fissure and coal matrix, respectively.
By combining Equation (4)–(6), the Navier equation for the deformation of surrounding rock is obtained as follow.
3.2. Coupling Model
To avoid the influence of the boundary effect, the total size of the grouting diffusion model in a layer-through borehole is . No.2164 floor drainage roadway (the height of 3 m and the width of 3.2 m) is located in the sandstone layer below the main floor of V coalbed, and a layer-through borehole (the length of 20 m and the diameter of 94 mm) is drilled from the roof of the drainage roadway to uncover V coalbed, as shown in Figure 6. The average buried depth of the V coalbed is about 400 m, so the overburden load on the upper boundary of the model is set at 10 MPa. The bottom boundary of the model is a fixed constraint, and the initial velocity field and displacement field are 0. The left and right boundaries of the model are roll boundary constraints. The grouting and sealing sections of borehole are ranging from the main floor section (sandstone) to the immediate floor section (mudstone). The grouting pressure on the borehole boundary is set at the initial pump pressure, which is taken as 1~4 MPa, respectively. There is no slurry flow on other boundaries. The relevant physical parameters of each rock stratum in the coupling model are shown in Table 2.
3.3. Simulated Results
3.3.1. Range of Loose Circle
The fissure zones around the layer-through borehole are formed under the double stress disturbance of the roadway excavation and the borehole drilling. The stress distribution of surrounding rock after the excavation of No.2164 drainage roadway and layer-through borehole are shown in Figure 7.
(a) Vertical stress
(b) Horizontal stress
Figure 7(a) shows that after the excavation of the drainage roadway and borehole, there are obvious concentration areas of vertical stress both at the two ribs and at the shoulder corners of the roadway, and the peak vertical stress reaches 32.5 MPa, resulting in a large displacement of the both roadway ribs. While the vertical stress reduction areas occurs not only at the roof and floor of the roadway but also along the axial direction of the borehole, which indicates that the surrounding rock in these areas has undergone severe plastic failures. It can be seen from Figure 7(b) that the horizontal concentration stress occur both at the shoulder corners of the roadway, and at the bottom of the borehole (coal uncovering point). The peak value of the horizontal stress reaches 28.0 MPa, causing the plastic damage to extend along the roadway shoulders, and the borehole bottom is more prone to collapse.
The relations between the vertical stress of the surrounding rock and the left rib of No.2164 drainage roadway are shown in Figure 8. Figure 8 shows that the range of 0–1.5 m from the left rib of the roadway is in the stress-relax area, where the surrounding rock is already subjected to many tensile failures, causing the stress concentration transferred to the deep. While the surrounding rock within the range of 1.5–6.5 m from the left rib of the roadway is just right in the support stress concentration area, the loaded state of the surrounding rock changed to biaxial or even triaxial loading in the plastic deformation area. With the bearing strength increasing, the peak support stress of 32.5 MPa emerges in the position of 2 m from the left rib. When the distance from the left rib of the roadway exceeds 6.5 m, the surrounding rock is in an in-situ stress area, not disturbed by the roadway excavation any longer.
Figure 9 shows that the plastic zone of the surrounding rock of the No.2164 roadway is symmetrically distributed, and the range of plastic zone of 6.2 m is the largest at the shoulder corners, which is consistent with the stress distribution results. In addition, there is an obvious plastic deformation, with a range of 0.3 m, generated in the V coal and the immediate floor section along the borehole axis, which easily causes numerous secondary fissures around the borehole connected with the loose circle of the roadway. Therefore, the initial position of borehole sealing should exceed the scope of the loose circle of the drainage roadway, at least 6 m.
3.3.2. Seepage Range of Slurry
The seepage ranges of the inorganic noncondensable slurry under different grouting pressures in the layer-through borehole are shown in Figure 10. Figure 10 shows that when the grouting pressure is kept constant, the seepage range of the inorganic slurry varies with the permeability and the storage coefficient of the rock stratum. Since the porosity and permeability of the main floor section (sandstone) are both larger than those of the immediate floor section (mudstone) and the fissures are well developed, the seepage range of the slurry in the sandstone section is broader than that in the mudstone section. Additionally, the slurry diffusion pressure gradually decreases as the distance from the borehole wall increases. While the seepage ranges of the slurry both in the immediate floor and in the main floor section continue to increase with the gradual increase of grouting pressure, to further analyze the relationship between the effective seepage radius of the inorganic slurry in different sealing sections of the borehole and the grouting pressure, the grout pressure data in the borehole radial direction was plotted as shown in Figure 11.
(a) Sandstone section
(b) Mudstone section
Only under a certain grouting pressure can the inorganic slurry overcome the material viscosity, friction resistance, and pore pressure of the matrix to enter into the deeper fissures of the surrounding rock, which is the effective seepage pressure [19, 21]. The smaller the fissure aperture in rock stratum, the greater the effective seepage pressure required; the diameter of the fracture, the higher the effective seepage pressure required. The effective seepage radius is defined as the radial distance which the slurry flows when it maintains the effective seepage pressure. Here, the effective seepage pressure in the grouting model is set as 0.5 MPa. Therefore, Figure 11(a) indicates that with a rising grouting pressure, the effective seepage radius of the inorganic slurry gradually increases. When the grouting pressure varies from 1 MPa to 4 MPa, the effective seepage radii of the slurry in the sandstone section are 0.21 m, 0.43 m, 0.64 m, and 0.75 m, respectively. Figure 11(b) shows that the seepage range of the inorganic slurry in the immediate section (mudstone) is obviously smaller than that in the sandstone section. When the grouting pressure varies from 1 MPa to 4 MPa, the effective seepage radii in the mudstone section are 0.12 m, 0.25 m, 0.37 m, and 0.43 m, respectively.
Considering that the plastic zone range of the layer-through borehole in the immediate section is 0.3 m (Figure 9), only when the grouting pressure is not less than 3 MPa can the deeper secondary fissures around the borehole be effectively blocked. However, the growth rate of the effective seepage radius of the slurry slows down when the grouting pressure exceeds 3 MPa. Moreover, a higher grouting pressure not only requires a stronger pump power but also may break through the sealing of the capsule bags at the two sides of the borehole. Therefore, a reasonable grouting pressure of 3 MPa is determined for the layer-through borehole sealing in No.2164 drainage roadway.
4. Field Test of Borehole Sealing
4.1. Drilling Design
To examine the borehole sealing performance of the new noncondensing constant pressure grouting technology, a total of 6 groups of drainage boreholes from A-13 to A-18 were selected as the objects of the borehole sealing test in No.2164 drainage roadway of Puxi coal mine. The drilling layout of each group of layer-through boreholes is basically the same, as shown in Figure 12. The drainage holes of the test group and the control group were alternately arranged so as to eliminate the impact of geological changes in the V coalbed on the sealing performance. The new inorganic, noncondensable slurry was grouted into the layer-through boreholes of Group A-13. While the PO 42.5 cement mortar was used in the boreholes of Group A-14, in the next two months, we mainly measured and observed the drainage parameters of 7 boreholes (4#, 5#, 6#, 7#, 13#, 14#, and 15#) of each group.
It is noteworthy that the length of a single section of the sealing capsule bag is 500 mm, and the pressure threshold of the squib valve is 1.0 MPa. The capsule bag has the functions of air exhaust and water return. According to the range of the plastic zone of No.2164 roadway, the initial depth of the outer section of the bag is 6 m, and the length of the whole sealing section is 10 m. After the total 6 groups of boreholes from A-13 to A-18 were completely sealed with the inorganic slurry or cement mortar, the drainage pipelines were connected for continuous gas drainage. The implementations of borehole grouting are shown in Figure 13.
4.2. Drainage Performance
The average gas drainage concentrations of boreholes in group A-13 and group A-14 in two months are plotted as shown in Figure 14, and the average pure gas drainage flow rates of boreholes are shown in Figure 15.
(a) Inorganic noncondensable slurry
(b) PO 42.5 cement mortar
(a) Inorganic noncondensable slurry
(b) PO 42.5 cement mortar
Figures 14 and 15 show that within the next 2 months after borehole sealing, the average gas concentration of the test boreholes in Group A-13 sealed with the inorganic noncondensable slurry reaches 51.5%, and the average drainage flow rate of pure gas is 0.005 m3/min. While the average gas concentration of the control boreholes in Group A-14 sealed with the traditional cement mortar is 38.3%, the average flow rate of pure gas is only 0.003 m3/min. After adopting the constant pressure grouting technology with the new inorganic slurry, the gas concentrations of boreholes in the No.2164 drainage roadway increased to 1.35 times of the previous, and the gas flow rates increase to 1.67 times of the previous. Therefore, the borehole sealing performance was significantly improved in the Puxi coal mine.
5. Conclusions
The slurry fluidity, water separation rate, and sealing performance of the inorganic sealing materials were tested under different water-cement ratios, and a coupling model of the slurry seepage was established to explore the loose zone ranges around the roadway, and to analyze the seepage capacity. Finally, the borehole sealing test with the inorganic slurry was investigated. The following conclusions can be drawn: (1)The inorganic sealing material has a strong fluidity, low water retention, high permeability, and good sealing performance, and the appropriate water-cement ratio of the inorganic slurry is 5 : 1(2)After the excavations of No.2164 drainage roadway and layer-through borehole, stress concentrations are prone to occur at the shoulder corners of the roadway and the borehole bottom, and the range of the loose circle around the roadway is 6.2 m(3)The effective seepage radius of the inorganic slurry gradually increases with a rising grouting pressure, and the slurry seepage range in the sandstone section is broader than that in the mudstone section(4)Adopting the constant pressure grouting technology with the inorganic slurry, the borehole sealing performance is significantly improved in Puxi coal mine; the average gas concentration of boreholes reaches 51.5%, and the average drainage flow rate is 0.005 m3/min
Data Availability
The data used to support the findings are included within the article.
Consent
Verbal (recorded) informed consent was obtained from all subjects involved in the study.
Conflicts of Interest
The authors declare no conflict of interest.
Authors’ Contributions
Z.P. and W.H. conducted tests and wrote the paper. W.L. proposed the conceptualization, methodology, and validation. W.X. and M.H. analyzed data and prepared graphics and tables. All authors have read and agreed to the published version of the manuscript.
Acknowledgments
This work is supported and funded by the National Natural Science Foundation of China (Grant No. 52104224).