Characterizing the Permeability and Methane Release with the Treatment of Pressure-relief Mining: Intensive Field Measurements

: Characterizing the permeability evolution and methane release is of great significance for the safe mining of 11 the high gas outburst seams, as well as coal and gas simultaneous extraction. Theoretical analysis, laboratory testing, 12 and numerical simulation are widely used methods to characterize the permeability and methane release with the 13 treatment of pressure - relief mining. However, these methods cannot fully reflect the complexity of filed practice. In this 14 study, we report the effectiveness of protective coal seam (PCS) mining and the pressure - relief area in the protected coal 15 seam (PDCS) based on detailed and integrated field measurements in a Chinese coal mine. To the best of our knowledge, 16 it is the first time to measure the permeability coefficient and gas pressure evolution in the PDCS during the PCS 17 longwall mining. The evolution of the permeability coefficient in the pressure - relief area during PCS mining can be 18 divided into four stages: slowly decreasing, sharply increasing, gradually decreasing, and basically stable. The 19 maximum permeability coefficient is 322 times of the initial value and finally stabilized at 100 times after the goaf 20 compacted. The gas pressure evolution in the PDCS indicates that the strike pressure relief angle is 52.2° at the active 21 longwall face zone, and 59.3° at the installation roadway side. The tendency pressure relief angles at the lower and 22 upper sides of the longwall panel are 75° and 78.9°, respectively. The residual gas content and gas pressure of the PDCS 23 in the pressure - relief area are reduced to less than 6 m 3 /t and within 0.4 MPa, respectively. The field measurements 24 show that pressure - relief mining can prevent coal and gas outbursts in PDCSs. The field observations in this paper can 25 serve as benchmark evidence for theoretical analysis and numerical simulations, and also provide insights into realizing 26 safety mining in similar conditions.


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At present, numerical simulations and field observations are the main approaches to characterize the evolution of 46 permeability and gas migration paths in surrounding rock mass.

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The slug method is commonly used to assess the permeability of surrounding rocks and the characteristics of fracture 48 development, especially in saturated coal seams (Koenig and Schraufnagel, 1987;Shu et al., 1995). Moreover, pumping 49 and flow meter tests are also widely employed to evaluate the permeability of overlying rocks (Baptiste and Chapuis, 50 2015; Barrash et al., 2006). Three geophysical approaches (the flow meter test, color television observation, and the 51 transient electromagnetic method) have been employed to detect fracture development in overlying strata (Sun et al., 52 2013), in which color television is considered the best one. Based on numerical simulations, Yuan et al. (2013) found 53 that an annular fracture system is created in the roof after longwall mining, which is the primary gathering space for 54 pressure-relief gas. He also proposed a scheme for gob gas drainage. Using physical modeling and the slug method, 3 permeability calculation results can be imported into seepage calculation software for seepage calculation. This method 65 provide a promising way to evaluate the effectiveness of pressure relief and permeability enhancement in the PDCS 66 ( Rutqvist, 2011;Whittles et al., 2006;Zhang et al., 2018;Guo et al., 2020). Si et al. (2015) 67 employed FLAC3D to obtain the stress distributions during thick coal seams mining, and the stress data is then used in 68 ECLIPSE300 to study the gas migration. However, the numerical results are poorly verified by field applications, 69 doubting the reliability of the simulations.

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In this study, we performed intensive field measurements to characterize the permeability enhancement and the 71 associated pressure relief regionsin the PDCS during longwall retreating in the above PCS. Different from previous 72 studies, which usually test these parameters after the PCS mining, we continually monitor the parameters mentioned 73 above, enabling us to summarize the whole evolution of the permeability and the associated pressure-relief effectiveness 74 during the PCS minging. Five groups of testing boreholes are arranged in PDCS to monitor the gas pressure, gas content, 75 and permeability coefficient. The gas pressure was measured by the active pressure measurement (APM) method; the 76 gas content was measured by the high-pressure adsorption (HPA) method; and the permeability coefficient was 77 measured by the radial flow permeability (RFP) method. To the best our knowledge, it is the first time to investigate the 78 permeability and gas pressure evolution of the PDCS during the PCS mining process. Moreover, regional measurements, 79 instead of point measurements, were carried out in this work; thus, spatio-temporal evolutions of these parameters can 80 be characterized. The outputs in this study provide fundamental insights into how permeability and gas pressure 81 evolution of PDCS during the PCS mining process. The results can also serve as benchmarking evidence for field-scale  This study adopts the APM method to measure the coal seam gas pressure. All the measurement holes are downward to 96 the PDCS (Fig.3). The cement slurry seals the hole and prevents the leakage of gas during the tests, thus ensuring the 97 accuracy of the measurements. The arrangement of the test boreholes and sealing method are shown in Fig.3. The

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process for the measurement are as follows: pipe laying → grouting and sealing → sealing the top orifice → checking 99 the gas flow path → installing the pressure gauge → observing the gas pressure. The specific operation for each step 100 includes: (1) Using galvanized iron pipe with pressure measuring kit (measure gas pressure), and choosing a pressure

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The gas content is mainly measured by underground coring, underground desorption, coal sample weighing and 110 crushing, moisture measurement, and other methods. After drilling into the coal seam, the drill bit is withdrawn, and the 111 coring bit is replaced. The obtained coal sample is loaded a tank immediately and sealed. Sample desorption is carried 112 out in the site, and the data is recorded. Finally, the sealed coal sample tank is brought to the laboratory for further 113 desorption to obtain the accurate gas content.
where r is the radius to the borehole, m; P is the square of the gas pressure in the coal seam, MPa 2 ; α is the gas content 129 coefficient, m 3 /(m 3 ·MPa 1/2 ); t is the time of gas flow, d; λ is the permeability coefficient of the coal seam, m 2 /(MPa 2 ·d),

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it can also be calculated where pn is the atmospheric pressure in the site, MPa; μ is the dynamic viscosity of methane, Pa s; k is the permeability, 133 m 2 . After Laplace transform and combined with the similarity theory, λ can be determined using the following to where Y is dimensionless flow criterion; F0 is dimensionless time criterion; a1 and b1 are regression coefficient; p0 is the 139 initial gas pressure in the coal seam, MPa; p1 is the gas pressure in the borehole, MPa; r1 is the radius of the borehole, m; The formulas for calculating the permeability coefficient by the RFP method are listed in Table 3. When calculating the 147 permeability coefficient, any formula in Table 2 can be used to obtain an assessed value first, then the results are 148 checked by F0= Bλ. If the value of F0 is in the range of the selected formula, the result is correct. Otherwise, another 149 formula can be used, until the value of F0 is in the range of the selected formula.
6 the atmospheric pressure.
(3) Measure the natural gas emission from the borehole two hours at a time, twice a day, and 153 then calculate the permeability coefficient according to formulas in Table 2. Permeability coefficient tests are performed 154 after the gas pressure measurement at the same test borehole, as shown in Fig. 3. Because of the damage near the 155 borehole, the gas flow rate usually indicates a high value with strong fluctuations during the first few hours. However, it 156 rapidly reduces and remains constant after one day. Field operations in this study suggest that the gas flow rate is more 157 reliable two days after the drilling.

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(1) Test holes at the PCS2 tailgate-1 (Group A) 165 Holes were drilled into the PDCS longwall face in the tailgate-1 of the PCS2 longwall face before the PCS longwall 166 face retreated. The PDCS longwall face will be mined after the PCS longwall face. The gas pressure of the PDCS 167 longwall face was measured as the PCS longwall face advances. There were four test points along the strike and 168 tendency of the PDCS longwall face, respectively, as seen in Fig. 6A. By testing the residual gas pressure in the 1 # , 5 # , 169 6 # , 7 # , and 8 # holes, it is possible to examine the pressure-relief effectiveness of the 8 # coal seam before and after mining 170 of the PCS longwall face. The test holes in the tailgate-1 of the PCS2 longwall face were failed, so measurements are 171 not available in these holes.

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(2) Test holes at the PCS2 tailgate-2 (Group B) 173 Due to the ventilation requirements, the tailgate-2 was constructed on the PCS2 longwall face, as shown in Fig. 5. 174 There are four test holes in the tailgate-2 to observe the change in gas pressure and permeability coefficient in the 175 PDCS panel during the PCS longwall retreating. The layout of the drilled holes is shown in Fig. 6B. As can be seen in 176 Figs. 5 and 6B, the 2 # drill hole is located outside the pressure-relief scope at a distance of 12 m from the TPRBL; the 3 # drill hole is near the TPRBL at a distance of 2 m to the boundary; the 4 # drilling hole is located within the TPRBL at a 178 distance of 8 m to the boundary. 179 (3) Test holes at the PDCS main roadway (Group C) 180 After finishing the PCS panel, three test holes were drilled in the main roadway in the PDCS mining area. As seen in 181 Fig. 5, the 3 # drilling hole was located within 10 m of the TPRBL, the 2 # drilling hole was located within 1 m of the 182 TPRBL, and the 1 # drilling hole was located 14 m outside the TPRBL. Figure 6C shows the sectional view of the 7 drilling holes. Because of the faults in this area, coal samples were not obtained during the construction of the 3 # 184 drilling hole. The other two drilling holes were constructed according to the design angle. The measured gas content of 185 the 1# and 2# holes is 8.71 and 5.39 m 3 /t, respectively. The gas content of the 2 # drilling holes decreased by 62.96%.

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However, there is no pressure-relief in the 1# holes.

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(4) Test holes at the PDCS headgate cross-cut (Group D) 188 Six test holes were designed at the headgate cross-cuts to measure the residual gas pressure in the PDCS and inspect the 189 strike and tendency pressure-relief angles of the PCS longwall face. In Fig. 5, the 1#~3# drilling holes were used to test 190 the strike pressure-relief angle and Fig. 6D shows the profile of the arrangement. The 1# hole was located within 10 m 191 to the TPRBL; the 2# drilling hole was located on the TPRBL; and the 3# drilling hole was located 10 m outside the 192 TPRBL. The 4#~6# holes were used to determine the tendency pressure-relief angle. The 4# hole was located within 10 193 m to the TPRBL; the 5# hole was located on the TPRBL; and the 6# drilling hole was located 10 m outside the TPRBL.

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As the 4#~6# holes were close to the PCS headgate, fractures around the holes developed. After the holes were sealed, a 195 leakage occurred in the roadway; thus, the gas pressure and permeability coefficient measurements were not available.

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Therefore, only the gas pressure measurements in the 1#~3# drilling holes were implemented. The coal sample 197 collection was executed in the 1#~6# holes, and the samples were used to test the coal seam gas content. The

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corresponding results are shown in Table 3. The gas pressure of the 1#~3# drilling holes is shown in Fig. 7. It clearly 199 shows the gas pressure gradually increases but is still lower than the original gas pressure in the coal seams, indicating 200 that there is also a certain pressure-relief beyond the TPRBL. Moreover, the further inside the TPRBL, the higher the 201 pressure-relief.

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(5) Test holes at the PDCS tailgate cross-cut (Group E) 203 Three test holes were arranged in the PDCS tailgate cross-cut. The section and planar graphs of the arrangement are 204 shown in Fig. 5. The 1# hole was located 10 m beyond the TPRBL; the 2# hole was located on the TPRBL; and the 3# 205 hole was located within 20 m of the TPRBL. The gas content and permeability coefficient are listed in   face. The gas pressure gradually decreases when the PCS longwall face passes the monitoring point. The gas pressure at 211 the 4# hole within the pressure-relief range has the greatest reduction; the gas pressure at the 3# hole located on the 212 pressure-relief line also drops, indicating that the PDCS located around the 3# monitoring point also experienced 8 scope is larger than that of the theoretical one, but the 3# hole only underwent partial pressure-relief comparing with the 215 4# monitoring hole. The gas pressure at the 2# monitoring point is unchanged, indicating that the pressure-relief effect 216 is limited. Gas pressure measurement in the 1# hole failed, so the data is not available.

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The gas pressure evolution showing in Fig. 8 indicates the strike pressure relief angle is smaller than the theoretical 218 value. The gas pressure decreased rapidly 2.5 m behind the theoretical pressure relief line, and then remained constant 219 until 7.5 m behind the theoretical pressure relief line. Therefore, it concludes the measured pressure relief boundary 220 should be 2.5m smaller than the theoretical one, and the corresponding strike pressure relief angle is 52.2°. Most of the 221 pressure-relief gas will flow into the PCS longwall face through the floor fractures, which affects the safety of PCS   which was about 322 times the original permeability (0.008 m 2 /(MPa 2 ·d)); therefore, it is the best choice for gas 246 extraction within this area. In the compacted stable zone, due to the created fractures (Zhang et al., 2018), the 247 permeability coefficient still stays at a high level even if the stress recoveries to the in-situ state (Bai et al., 2017). It 248 clearly shows the permeability coefficient usually increased by 100 times due to the PCS mining.

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The evolution of gas extraction rate and concentration in the PDCS during PCS advancing are shown in Fig.11. When

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To study the pressure relief scope, the evolution of permeability coefficient in 3# and 2# boreholes located outside of 4# 257 borehole is also given in Fig. 9. The permeability coefficient at the 3# monitoring point (located on the pressure-relief

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In the process of drilling, some boreholes only took out the coal samples, but the gas pressure was not measured.

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Besides, since the coal sample needs to be taken out during the gas content measurement, the same borehole can only be 271 measured once. For continuous monitoring, the borehole in group B has been drilled before PCS mining; thus, the 272 measurement of residual gas content cannot be implemented. According to the results of the gas pressure or gas content

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where WCY is the residual gas content (m 3 /t); PCY is the residual gas pressure (MPa); a and b are adsorption constants 277 (mL/g and MPa -1 , respectively); π is the coal porosity. The calculated gas content in group E is 8.27, 5.60, and 4.82 m 3 /t,

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(9) could provide convincing calculations. The calculated gas pressure and gas content are shown in Table 3. The strike pressure-relief angle is generally divided into two sides, as shown in Fig. 12b. One side is at the active 283 longwall face, and the other is at the installation roadway. Since the longwall face is advancing along the strike(i.e., no 284 inclination angle), the two angles on both sides are the same. According to the evolution of gas pressure in group B 285 boreholes, the measured strike mining pressure-relief angle during the longwall face advancing is 52.2°, which is 286 smaller than the theoretical one (56°). Gas extraction is carried out on the PDCS, which can accelerate the reduction of 287 gas pressure and content, and expand the pressure-relief scope near the installation roadway. Therefore, the permeability 288 coefficient is used to determine the pressure-relief angle. According to the changes in the permeability coefficient pressure relief range extends 2 m from the theoretical line (Fig.12a), and the corresponding pressure-relief angle is 293 59.3°, a little larger than the theoretical one (56°), as shown in Fig. 12b. The pressure-relief angle at the installation 294 roadway is higher than that at the active longwall face side, which may result from the following two reasons: (1) the 295 pressure-relief angle at the longwall face is monitored in real-time by borehole group B, which is a dynamic parameter,

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and lag behind the stress transfer process. However, at the installation roadway side, the angle was measured after 297 sufficient pressure relief occurred.

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which caused further expansion of the pressure relief range. Therefore, the strike pressure relief angle at the installation 299 roadway is larger than the dynamic one at the longwall face side.

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The tendency pressure-relief angles at each side of the longwall panel may differ due to the tendency angle (15°) of the 302 coal seam. The boreholes in group B and group E are used to determine the tendency pressure-relief angles on each side 303 of the panel, as shown in Fig. 13. 1# and 2# boreholes in group E are used to calculate the pressure-relief angle at the relief line, and the corresponding pressure-relief angle is 75°. In contrast, the pressure relief line on the tailgate side 306 expands outward by 2.2 m, and the pressure relief angle is 78.9°, as shown in Fig. 13b Table 3).

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The gas content of the PDCS in the pressure-relief area is reduced to less than 6 m 3 /t (minima of 4 m 3 /t). The

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(2) The permeability coefficient evolution in the pressure-relief range of the PDCS can be divided into four stages:

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(3) The PDCS has the best permeability enhancement in the pressure-relief zone and the gradual compaction zone, with 337 the length of 20 m and 40 m, respectively, and the maximum permeability is 322 times of the initial permeability. The

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final permeability coefficient in the compacted stable zone can reach 100 times of the initial permeability. The 339 permeability in the regions located outside the pressure -relief boundary, can also be improved to a certain extent when 340 entering the pressure-relief zone but returns to the initial value after it compacted.

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(4) The tendency pressure relief angle at the lower end of the inclined longwall face is 75° consistent with the 342 theoretical pressure relief angle; the measured pressure-relief angle at the upper end of the air roadway is extended to 343 78.9°.

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(5) The residual gas content and residual gas pressure of the PDCS in the pressure-relief area are reduced to less than 6 345 m 3 /t and within 0.4 MPa, respectively. These two parameters also decreased outside the pressure-relief zone, albeit not 346 significantly. The field measurements show that pressure-relief mining can prevent coal and gas outbursts in PDCS.

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Because of the high cost, delicate operation, and danger of field measurements, it is usually challenging to monitor and 348 evaluate the effectiveness of the pressure-relief of PCS mining. Besides, field measurements are greatly affected by 349 mining activities and always have a low success rate. Therefore, most field measurements focused on monitoring the 350 residual gas content or gas pressure of the PDCS after the PCS mining, which cannot fully characterize the evolution of 351 these parameters. In this paper, by using intensive boreholes, detailed and whole mining process field measurement is 352 carried out. For the first time, the evolution law of permeability and gas pressure in the mining process is obtained. Thus,

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the field observations in this paper can serve as benchmark evidence for theoretical analysis and numerical simulations.

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Authors' contributions 355 Cun Zhang and Jianhang Chen performed the parameter design, participated in the field test, and drafted the manuscript.

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Qingsheng Bai, Ziyu Song and Yang Luo conducted the field measurement and conceived of, designed, and coordinated 357 the study. All authors gave their final approval for publication.

Data availability statement
359 Some or all data, models, or codes that support the findings of this study are available from the corresponding author Diagram of eld measurement of the gas pressure and the permeability coe cient Layout of the test drilling holes in the protected coal seam Figure 6 Design of the test drilling holes Gas pressure after protective coal seam mining Permeability coe cient evolution during protective coal seam mining. A is In-situ stress zone, B is abutment stress in uence zone, C is signi cant pressure-relief zone, D is gradually compacted zone, and E is compacted stable zone. In the gure, negative and positive of the x-axis mean the monitoring holes are located in front of and behind the longwall face, respectively. Gas extraction ow rate and gas concentration in the high roof roadway drainage in PDCS Figure 12 Strike pressure relief angle and its calculation Figure 13 Inclined pressure relief angle and its calculation Figure 14 14 Distribution of the gas content (a) and residual gas pressure (b)