TY - JOUR
AU - Kong,, Xiangguo
AB - Abstract Hydraulic flushing in gas predrainage is widely used, but the hydraulic-flushing effect is evaluated in a traditional way, by determining the desorption volume, moisture content, gas drainage rate and other conventional indices. To verify the rationality and feasibility of the multielectrode resistivity method in the evaluation of coal-seam hydraulic flushing and to research the spatio-temporal evolution of apparent resistivity during hydraulic flushing, a field test was conducted in 17# coal seam at Nuodong Mine, Guizhou. During hydraulic flushing, four stages were defined according to the variation in coal rock resistivity with time, namely, the preparation stage, the sharply decreasing stage, the rapidly increasing stage and the steady stage. The apparent resistivity of the coal rock mass is affected mainly by its own degree of fragmentation and flushing volume. A more serious rupture and a greater flushing volume yield a smaller apparent resistivity during the sharply decreasing stage and a higher resistivity during the stable stage. After three months of gas predrainage, the residual gas content and the gas pressure at different points in the expected affected area decrease below the critical value. Changes in the residual gas content and gas pressure at these points are consistent with the apparent resistivity, which validates the rationality and feasibility of the multielectrode resistivity method in evaluating coal-seam hydraulic flushing. multielectrode resistivity method, hydraulic flushing, effect valuation, spatio-temporal evolution 1. Introduction Coal and gas outbursts are some of the most serious disasters in coal mining (An et al2013, Yang et al2014b, Chen and Cheng 2015), and these have become more prevalent with increases in coal-mine depth (Díaz Aguado and Gonzalez Nicieza 2007, Islam and Shinjo 2009). Increases in mining depths, higher stresses, lower coal-seam permeabilities and longer gas-migration distances to the surface make conditions more favourable for gas accumulation and storage. Gas-storage conditions, high gas contents and deep mining activities make coal and rock more prone to failure under high stresses and result in dynamic mechanical phenomena in mines that have no initial outburst danger (Zhang et al1999, Hu et al2008, Yao et al2016, Zhang et al2016), which increases their risk of outburst significantly. The main measures to mitigate coal and gas outburst risks in China include protection-layer mining, predrainage of coal-seam gas, artificial pressure relief and hydraulic fracturing (State Administration of Work Safety and State Administration of Coal Mine Safety 2009). The crustal stress and potential gas energy in the coal seam can be released by protection-layer mining and gas predrainage, which can change the coal-body properties and decrease outburst dangers of the coal seam (Liu et al2005, Wang and Liu 2005, Liu et al2011). However, most of China’s high-gas mines are of a low permeability and high fluidity, with permeability coefficients of 0.004–0.04 m2 MPa-2 d-1, and numerous mines have no protective layer to be mined or have areas that cannot be protected (Liu et al2011). Because of difficulties in drilling pump holes, boreholes through coal seams need to be drilled for gas drainage before coal-roadway excavation to eliminate the outburst danger in advanced regions of coal seams that do not have protective layers. Such preparation is disadvantageous because of the long drainage time, low utilization rate and large quantity of drilling that is required. Artificial pressure relief to enhance gas drainage is an important means of eliminating regional outburst danger in coal seams with a low permeability and with no protective layer. Artificial pressure-relief measures can reduce coal-mine pressure, and improve coal-seam permeability, which can eliminate crustal stresses and gas dynamics that contribute to coal and gas outbursts. Driving forces of artificial pressure relief include hydraulic, mechanical and blasting forces (Wang and Li 2004, Kong et al2005, Li and Xu 2016), but mechanical and blasting forces are difficult to produce, yield limited pressure relief and generate sparks. Hydraulic flushing is a technique that uses rock roadway as a safety barrier to wash out part of the coal and gas using hydraulic pressure (Douglas et al1991, Jeffrey et al2013, Ge et al2014), decreases surrounding coal and rock stress and gas pressure and improves permeability (Kukkonen 2011, Kukkonen et al2012, Song et al2015b). Liu et al (2005) proposed the mechanism of hydraulic flushing and elaborated on its technological process. They applied the technology in severe outburst coal. Stress that surrounded the strata and gas in the coal seam was released efficiently after hydraulic flushing, which reduced the risk of coal and gas outburst significantly. Yang and Wang (2010), and Wang et al (2011) researched the role of hydraulic flushing on the prevention of high-risk outburst coal seams. These studies indicate that hydraulic flushing could improve the gas-drainage efficiency and reduce coal and gas outburst hazards in mining. The wide use of hydraulic flushing in outburst coal seams has resulted in many researchers investigating different indices to evaluate the effect of the technology. Liu et al (2011) used gas content as an index to evaluate the effect of hydraulic flushing as applied in No. 14211 transportation gateway in Xin’an Mine, and found that the gas content dropped from 10 to 11 m3 t-1 before hydraulic flushing to ∼6 m3 t-1 after application of the technology, which was below the critical value (8 m3 t-1). Wang et al (2013) believe that the change in permeability around the borehole is consistent with that of the stress, and verified the assumption by using RFPA 2D-flow. They proposed that the stress change around the flushing area could indicate the hydraulic-flushing effect to a certain degree. Kong et al (2016) analysed factors that influence the effective radius of hydraulic flushing using the response-surface method and concluded that the gas-pressure distribution is an important factor that influences the effective influence radius, so it could be an index to evaluate the hydraulic-flushing effect. These parameters are ‘point evaluations’; they are obtained by coring coal and rock samples in the hydraulic-flushing area and by measuring related parameters to evaluate whether and how significantly the test point is influenced by the flushing. These point evaluations of the area may be influenced by hydraulic flushing; they cannot evaluate comprehensively and continuously in time and space the coal and rock structure evolution within the area, which does not allow for an evaluation of the effect and later gas drainage. This blindness affects coal mine safety, and leads to a substantial increase in construction cost. Therefore, a simple, low-cost, high-accuracy method is needed to evaluate the effects of the hydraulic-flushing method. The multielectrode resistivity method is an exploration technique that is based on the conventional direct-current (DC) resistivity method, and for which the laying of electrodes needs only be done at a time before data acquisition, and then the apparent resistivity at the profile can be monitored (Yang et al2014a, Yang et al2016). The resistivity can reflect the lithologies along the horizontal direction at a certain depth and along the vertical direction at different depths (Yang et al2012, Loke et al2013). Compared with the conventional DC method, the multielectrode resistivity method has merits of a high efficiency, a low labour intensity and a high detection precision. The electrode layout and observation format are upgraded, which improves the information content for electrical prospecting and makes it reflect the shape and occurrence of anomalies in the profile intuitively (Dong and Wang 2003, Liu et al2014). Yang et al (2014a) analysed the application of the multielectrode resistivity method and the transient electromagnetic method in detecting coal-mine goaf based on an area in Xinjiang. By using the multielectrode resistivity method, Li (2009) detected bauxite in a karst basin that was based on a large difference in electrical properties between bauxite and surrounding rock. Lv et al (2005) discussed the application of the multielectrode resistivity method in mapping surface collapse according to an actual situation of goaf in the He Yetang mining area in Wuyi County. Ma and Wang (2009) determined the mining conditions of a coal seam by changes of the apparent resistivity, and judged the water condition in goaf from the distribution of electrical resistivity. Liu et al (2002) applied the multielectrode resistivity method in the karst disaster investigation, and divided the karst area, explored bedrock faults and analysed the tectonic development of bedrock karst. Guo et al (2004) established a standard geoelectric model profile of a homogeneous soil-slope surface, soft interlayer surface, accumulation layer surface and structural fracture surface by two-dimensional finite-element forward modelling, and conducted research to determine landslides by the multielectrode resistivity method. Song et al (2015a) carried out a hydraulic-fracturing test of a coal and rock mass. They believe that high-pressure water and internal stresses can reduce the apparent resistivity of coal and rock, and that high-pressure water affects the apparent resistivity. In recent years, the multielectrode resistivity method has been applied widely in karst exploration, mine-water disaster prevention, mineral-resources exploration, ground subsidence and goaf detection. However, research into the evaluation of hydraulic flushing in coal seams by the multielectrode resistivity method and a spatio-temporal evolution of the apparent resistivity of coal seam during hydraulic flushing has not been reported. To study the feasibility of the multielectrode resistivity method in the evaluation of hydraulic flushing of a coal seam and the spatio-temporal evolution of apparent resistivity, a field test at Nuodong coal mine in Guizhou was carried out, and electrodes were laid in 11703 transportation bedding-plane tunnels to detect the apparent resistivity of 17# coal seam. An apparent-resistivity nephogram was obtained by an inversion method after data acquisition. The results were analysed and conclusions were obtained. This research will have important guiding significance for applying the multielectrode resistivity method to evaluate hydraulic-flushing effects in coal mines. 2. Multielectrode resistivity method 2.1. Basic theory The multielectrode resistivity method is an electric exploration system that has been developed to meet the needs of hydrological engineering and environmental geological surveying (Chen et al1993). It includes two parts: data acquisition and data processing. During field measurements, all electrodes are to be laid at the measurement points at certain intervals, and they are connected to a programmable multichannel electrode switch with multicore cable. The multichannel electrode switch is an automatic switching device that is controlled by a micromachine, which can transfer automatically the electrode-device form and the distance between electrodes and measurement points. Measurement signals are sent to the microcomputer resistivity meter through the electrode switch, and the measurement results are stored in random-access memory. These original data can be processed according to the given program after being transmitted to the computer. Because of the rapid data acquisition and microcomputer processing of the multielectrode resistivity method, it has changed the traditional working mode of electrical prospecting, increased the work efficiency, reduced the labour intensity and improved the information content for electrical prospecting. The system structure of the multielectrode resistivity method is shown in figure 1 (Xu and Wang 1993). Figure 1. View largeDownload slide Structure schematic diagram of multielectrode resistivity method system. Figure 1. View largeDownload slide Structure schematic diagram of multielectrode resistivity method system. 2.2. Detection principles Equivalent to conventional DC methods, the multielectrode resistivity method is a geophysical prospecting method that is based on the difference in electrical conductivity between an underground object and surrounding rock (Wilkinson et al2005, Loke et al2013). When a DC current is applied to the electrodes, the electric-field distribution is observed on the ground by the corresponding instrument and the geological problem is solved by studying the distribution of the artificial electric field (AEF). An analytic method is usually used to solve the electric field distribution under simple geological conditions, namely, solving the Laplace equation according to the given boundary conditions (van Schoor 2005, Chambers et al2007): ∇2U=0, 1 where U is the potential. This equation summarizes the basic experimental law and reflects the inherent law of the stable current field. When the multielectrode resistivity method is used, the resistivity is obtained by supplying power to the AB poles and by measuring the potential difference ∇V between the MN poles (figure 2). In practical work, the apparent resistivity at one point is obtained according to equation (2), ρS=K∇VI, 2 where ρs is the apparent resistivity of the media in the detection area; I is the supply current; ∇V is the potential difference; and K is the coefficient of equipment, which depends on the arrangement of A, B, M and N, according to this relationship (Liu et al2009, Giovanni et al2014, Karaoulis et al2014): K=2π1AM-1AN-1BM+1BN, 3 where AM, AN, BM and BN are the distances between the four electrodes. Figure 2. View largeDownload slide Schematic diagram of any quadrupole device. A, B are power supply electrodes, andM, N are measuring electrodes. Figure 2. View largeDownload slide Schematic diagram of any quadrupole device. A, B are power supply electrodes, andM, N are measuring electrodes. The resistivity method of an AEF is one of the most important electrical detection methods. Its basic principle of operation is as follows: the DC power-supplying electrodes A and B are buried in a detection area to supply power, and an AEF is built in the detection area. Different distribution states of the AEF appear because of the different occurrences of media (coal, soil, water, etc) in the detection area. If the medium is homogeneous, the AEF will show a uniform hemispherical shape, but because of the tectonic influence in the detection area, high- and low-resistivity ores of different conductivity will appear. A ‘rejection’ phenomenon occurs when the AEF passes through a high-resistivity orebody, an ‘attraction’ phenomenon occurs through a low-resistivity orebody, and the electric field turns into a uniform distribution from the hemisphere. The inhomogeneous distribution of electric field in the detection area is reflected by the apparent-resistivity nephogram that is achieved from inversion, and thus, geological bodies of different conductivities within the detection area may be determined by analysing the inhomogeneous distribution of the electric field. 2.3. Brief introduction to the instrument The experimental instrument is a distributed-network parallel electrical apparatus made by Huizhou Geological Safety Research Institute Co., Ltd, and consists mainly of the host, main wire, collection base stations and plug wire and electrodes, as shown in figure 3. Sixteen acquisition boards were set up in the host, and the instrument can be divided into a distributed and a centralized system. The distributed system needs to be connected with a base station that has 16 channels, and the number of base stations that is used depends on the number of channels that is required according to the task. The host can be connected with the system in two ways. One method is by connecting the communication plugs of the host and the base station using the 485 communication lines (14 cores), and the other is by connecting the thread of the main cable directly to the communication plug of the host (14 cores). The host and the base station are equipped with an ‘ABN’ air jack, and the ‘ABN’ line at infinity can be connected to the host or base station. The centralized system requires only 16 monitoring cables or tracking plugs instead of base stations. Figure 3. View largeDownload slide Components of multielectrode resistivity method system. Figure 3. View largeDownload slide Components of multielectrode resistivity method system. This instrument includes the measuring method of traditional electrical equipment, and has the ability to conduct parallel, massive, and high-efficiency data receiving and collection, which can improve the field-detection efficiency significantly. The Wenner array is usually applied in the field and in the laboratory; and its working principle is shown in figure 4. Electrodes are spaced a certain distance within the detection area to enable the AEF to cover the region to be detected. The electrodes are connected with a multicore-cable switch and the automatic conversion of power-supply electrode, electrode distance and electrode device form are achieved by applying the host-control switch. The receiving electrode stores data to the host computer, and the apparent-resistivity nephogram is obtained after specific application software is used to analyse the data. Figure 4. View largeDownload slide Detection principle of winner method. Figure 4. View largeDownload slide Detection principle of winner method. 3. Field tests 3.1. Overview of test sites The 11703 working face was selected as the test region where hydraulic flushing was conducted, and testing electrodes were installed in the roof of the 11703 transportation bedding-plane tunnel. Layouts of the 11703 working face and hydraulic-flushing drilling holes are shown in figures 5 and 6, respectively. Figure 5. View largeDownload slide Layout drawing of 11703# working face. Figure 5. View largeDownload slide Layout drawing of 11703# working face. Figure 6. View largeDownload slide Layout drawing of hydraulic flushing drilling in 11703# haulage gate. Figure 6. View largeDownload slide Layout drawing of hydraulic flushing drilling in 11703# haulage gate. 3.1.1. Coal-seam occurrence of 11703 working face Coal 19# next to the 11703 transportation bedding-plane tunnel is stable, and the average coal thickness is approximately 0.7 m, the coal-seam dip angle is approximately 6°, and the direct roof is B3 limestone. Coal 17# is located in the middle of the Longtan Formation, 22 m from B2 limestone and 0.5 m from the B3 argillaceous limestone above, of which the thickness is 0.65–8.99 m, and averages 4.16 m. Most of the coal-seam thickness exceeds 3.5 m, and it is slightly thinner in the east, to the north and near the 13 line. The direct roof of coal 17# is mostly mudstone and silty mudstone, partly mud limestone (B3) and the floor is generally mudstone or silty mudstone. 3.1.2. Gas occurrence in coal seam Laboratory data analysis of samples revealed that coal 17# lies in the methane zone. Its initial gas pressure was 0.91–1.15 MPa and averaged 1.03 MPa. The initial gas content was 14.60–16.11 m3 t-1 and averaged 15.37 m3 t-1. The initial gas flow from a one-hundred meters borehole was 0.429–0.733 m3 min-1, the gas-flow attenuation coefficient was 0.1433–0.7108 d-1, the permeability coefficient was 0.016429–0.021527 m2 MPa-2 d-1, the initial rate of methane emission ΔP was 18–28, the consistent coefficient was 0.16–0.30 and the porosity was 11.19%–11.72%. 3.2. Test scheme 3.2.1. Arrangement of drilling field The vertical distance between the 11703 haulage gate and the 11703 transportation bedding-plane tunnel was 10 m, the horizontal distance was 26 m and the straight-line distance was approximately 28 m. Before hydraulic flushing, drilling fields were constructed in the 11703 transportation bedding-plane tunnel, at a spacing of 35 m, and the first drill field was 10 m away from the head of the 21701 haulage gate. 3.2.2. Testing steps Thirty-two holes were drilled on both sides of the drilling field midline, with an aperture of not less than 10 mm and a depth of 200 mm. The distance between the boreholes was 2 m, and the aperture was directed to the coal and rock inside the tunnel. Clay soil mixed with water was used to seal holes, and electrodes were inserted into clay soil to link and couple electrodes with rock. The main wire and the base stations were arranged, and the interfaces on the main wire were connected with electrodes. The main wire was connected with the host and the communication interface couplings were checked. The end of the measuring line was pulled to approximately 450 m away as the infinity. The communication statuses of all the instruments were checked, then a test was conducted to collect the data before and after hydraulic flushing. After data collection, dedicated software was used for data processing and an apparent-resistivity nephogram was obtained by Surfer to analyse the apparent-resistivity variation of coal and rock in the hydraulic-flushing area. 4. Results and analysis 4.1. Temporal evolution of apparent resistivity To learn about the apparent resistivity in the test area before and after hydraulic flushing in detail and to acquire more data for analysis, a multiperiod data-collection method was used. The apparent resistivity was monitored before and after drilling holes to determine the effect of drilling on the resistivity of coal and rock. One hour before flushing, the background apparent resistivity was collected. From the beginning to the end of flushing (which lasted two hours), the data were collected once every 20 min; within the eight-hour flushing period, data were collected every hour and data were collected every eight hours during an 8–48 h period. After the data acquisition, the device was connected to the host computer for data processing and the apparent-resistivity nephograms were prepared using Surfer, as shown in figure 7. Figure 7. View largeDownload slide Nephograms of pre- and post-fracturing apparent resistivity values. (a)–(e) respectively are before drilling, 1 h pre-fracturing, during hydraulic flushing, 24 h post-fracturing, and 48 h post-fracturing nephograms. Figure 7. View largeDownload slide Nephograms of pre- and post-fracturing apparent resistivity values. (a)–(e) respectively are before drilling, 1 h pre-fracturing, during hydraulic flushing, 24 h post-fracturing, and 48 h post-fracturing nephograms. Figure 7(a) shows the apparent-resistivity nephogram before drilling. The test area shows a high resistivity state and the apparent resistivity ranges from 114 to 152 Ω m. Because drilling has not been carried out, in general, the area is in a primitive state of stress, the value of resistivity depends mainly on the coal and rock’s characteristics, so the overall apparent resistivity is higher. In the area below 18 m, the nephogram shows a trapezoidal layer distribution, and the apparent resistivity decreases gradually from the bottom to the top. The lower part of the nephogram is at the roof of 11703 transportation bedding-plane tunnel where three zones (falling, fractured and bending zones) form during roadway tunnelling. The base of the nephogram is located in the falling zone where rock collapses heavily, numerous large fractures exist and the stress is small, so the apparent resistivity in this zone is the highest. The apparent resistivity decreases with height because the fractured and bending zones appear successively and there are fewer fractures and a smaller stress. However, the coal conductivity is stronger than that of the surrounding rock, so the apparent resistivity in the coal seam is almost the smallest in the test area and obvious layers exist as represented by the green area in the nephogram (a). The overlying strata above the coal seam are not disturbed by tunnelling and exist in the state of original stress, and the apparent resistivity is distributed steadily and uniformly. Small local areas of high and low resistivity exist, probably because the rock contains a small amount of water and other components. Figure 7(b) shows the apparent-resistivity nephogram after drilling, which is compared with the nephogram before drilling and as the background before hydraulic flushing. The two nephograms before and after drilling holes appear mostly the same, but some differences exist between them. The apparent resistivity of the area that is marked by an ellipse in (b) is approximately 142 Ω m, and higher than that in (a), of approximately 126 Ω m. The reason for this phenomenon is that drilling holes caused coal and rock fracture in this area, and many pores and fissures appeared, which increased the resistivity in the area sharply. The apparent-resistivity distribution during hydraulic flushing is shown in figure 7(c). The apparent resistivity in the flushing region is reduced sharply, from approximately 140 Ω m before flushing to approximately 90–100 Ω m during hydraulic flushing, as shown in the circled region, which shows the expected area of effect. Thirty-five flushing holes were arranged as an ellipsoid, and only coals could be flushed out during hydraulic flushing because of the hard rock. The average thickness of 17# coal seam was approximately 4.16 m, and was much smaller than the effect length along the coal seam, so the expected area of effect is an ellipsoid in space. Consequently, a two-dimensional oval profile forms in the nephogram. Besides the coal in the expected area of effect, the rock around the coal is also affected by hydraulic flushing and the apparent resistivity decreased significantly. This occurs because, under the action of hydraulic flushing, coal and rock in the area fracture, many cracks appear and water channels form, so the coal and rock could fill with water, and reduce the apparent resistivity significantly. Figure 7(d) shows that after 24 h, the ruptured coal and rock became a high-resistivity region, which indicates that coal and rock in this area fractured and that water flowed out through the cracks and drilling holes in coal and rock under the effect of gravity. After a long drainage and evaporation, very little water existed in the ruptured coal and rock, so the apparent resistivity increases and a high-resistivity region forms. A comparison of figures 7(d) and (e) shows no obvious difference between the nephogram after 24 h and that after 48 h. This result indicates that the effect of hydraulic flushing on the resistivity of coal and rock changes little and becomes stable 24 h after flushing. 4.2. Spatial evolution of apparent resistivity To research the changing regularity of the apparent resistivity at different locations with time, eight representative points in the coal and rock resistivity profile were chosen. Their coordinates and distribution are shown in table 1 and figure 8, respectively. The apparent resistivity–time curves at different points are drawn in figure 9 according to the acquired data. Table 1. Coordinates of points in the detection area. Axial direction O A B C D E F G X/m 59 60 45 57 71 60 80 60 Y/m 20 26 20 13 20 34 20 6 Axial direction O A B C D E F G X/m 59 60 45 57 71 60 80 60 Y/m 20 26 20 13 20 34 20 6 View Large Table 1. Coordinates of points in the detection area. Axial direction O A B C D E F G X/m 59 60 45 57 71 60 80 60 Y/m 20 26 20 13 20 34 20 6 Axial direction O A B C D E F G X/m 59 60 45 57 71 60 80 60 Y/m 20 26 20 13 20 34 20 6 View Large Figure 8. View largeDownload slide Locations of monitoring points. Figure 8. View largeDownload slide Locations of monitoring points. Figure 9. View largeDownload slide Curves of apparent resistivity in various points with time. I, II are the stages before and during hydraulic flushing, respectively; III and IV are both the stage after hydraulic flushing. Figure 9. View largeDownload slide Curves of apparent resistivity in various points with time. I, II are the stages before and during hydraulic flushing, respectively; III and IV are both the stage after hydraulic flushing. As shown in figure 9, although differences exist between the changes in apparent resistivity with time at different points, the overall trend of the curves is almost the same. The variation trend can be divided into four stages. (I) In the preparation stage before hydraulic flushing, the apparent resistivity is mostly constant. (II) In the sharply decreasing stage, hydraulic flushing starts, coal and rock in the area fracture, many cracks appear and water channels form, high-pressure water flows into the fractures and coal and rock are filled with water during flushing. The coal and rock conductivity increases rapidly, so the apparent resistivity drops to a minimum and is maintained at this value until the end of hydraulic flushing. (III) In the rapid rising stage, because of the end of hydraulic flushing, no high-pressure water exists. A large amount of water in the coal and rock mass will flow out along the fissures and holes, so the apparent resistivity increases rapidly in a short time. Because of the flow resistance, this stage lasts longer at approximately 6 h as shown in figure 9. (IV) In the stable stage, most water in coal rock has drained out and after a few hours of evaporation the apparent resistivity is no longer affected by moisture, achieves a maximum and maintains this level. The apparent-resistivity variations with time at points O, A, C and D agree with the four stages above, whereas those at B, E, F and G are significantly different. Despite the stage, the apparent resistivities of points E, F and G exhibit no obvious change and fluctuate around certain values. According to figure 9, points E, F and G exist far from the area that is expected to be affected by hydraulic flushing, and the apparent resistivities at these points have not changed. This consistency shows that the coal and rock have not ruptured and no water flowed into the area, which confirms that the three points exist outside the influence of flushing. The apparent-resistivity variation of point B agrees only with the rules in stages I and II, but it increases slowly after hydraulic flushing and is maintained at a small value. Figure 8(e) shows a small blue zone of low resistivity around point D, and that when flushing was finished, a small hole filled with residual water. The original apparent resistivities at points A, C, E and G are higher than those at O, B, D and F, mainly because the coal conductivity is stronger than that of rock. In stage II, the apparent resistivity at all points decreases to a minimum, and ρminO < ρminB < ρminC < ρminA < ρminD < ρminF < ρminE <ρminG, whereas ρmaxO > ρmaxC > ρmaxA > ρmaxG >ρmaxE > ρmaxD > ρmaxF > ρmaxB in stage IV. Therefore, point O occurs at the centre of the expected area of effect, so the flushing effect and fracturing caused by high-pressure water are heaviest at this point. Thus, the apparent resistivity is lowest in stage II and highest in stage IV. Point B occurs at the edge of the expected area of effect; the flushing effect is weaker than at point O but it is stronger than that at other points, so its apparent resistivity also decreases significantly in stage II, and remains at a very low level long after hydraulic flushing because of the water. Points A and C exist in rocks above and under the coal seam, respectively, and the rock strength is stronger than that of coal, so less fracturing occurs and less water flows into the rocks. Thus, the apparent-resistivity decrease in stage II and increase in stage IV are small. Point D exists on the right edge of the expected area of effect. As shown in figures 8 and 9, the effect of hydraulic flushing here is weaker than in the centre. It can be concluded that the actual area of effect is smaller than expected. 4.3. Verification by conventional methods According to article 52 of the ‘Provisions on prevention of coal and gas outburst’, methods that use the residual gas content, residual gas pressure or other indices as verified experimentally can be used to evaluate the effect of eliminating coal and gas outbursts. To verify the feasibility of the multielectrode resistivity method in the evaluation of hydraulic flushing in a coal seam, two indices, the residual gas content and the residual gas pressure, were measured in the testing area. Gas in this area was prepumped after hydraulic flushing, and after three months of gas drainage, coal samples at points B, O, D and F were obtained by drilling test holes and the residual gas content and residual gas pressure were determined, with results as shown in table 2. Table 2. Evaluation results of residual gas content and gas pressure. Test site Residual gas content/m3 t-1 Residual gas pressure/MPa Evaluation result B 6.48 0.54 Below the critical value in ‘Basic index of coal mine gas drainage and exploitation’ and ‘Provisions on prevention of coal and gas outburst’ O 6.15 0.50 D 6.72 0.58 F 14.82 1.02 Test site Residual gas content/m3 t-1 Residual gas pressure/MPa Evaluation result B 6.48 0.54 Below the critical value in ‘Basic index of coal mine gas drainage and exploitation’ and ‘Provisions on prevention of coal and gas outburst’ O 6.15 0.50 D 6.72 0.58 F 14.82 1.02 View Large Table 2. Evaluation results of residual gas content and gas pressure. Test site Residual gas content/m3 t-1 Residual gas pressure/MPa Evaluation result B 6.48 0.54 Below the critical value in ‘Basic index of coal mine gas drainage and exploitation’ and ‘Provisions on prevention of coal and gas outburst’ O 6.15 0.50 D 6.72 0.58 F 14.82 1.02 Test site Residual gas content/m3 t-1 Residual gas pressure/MPa Evaluation result B 6.48 0.54 Below the critical value in ‘Basic index of coal mine gas drainage and exploitation’ and ‘Provisions on prevention of coal and gas outburst’ O 6.15 0.50 D 6.72 0.58 F 14.82 1.02 View Large The original gas pressure of 17# coal seam was 0.91–1.15 MPa, and averaged 1.03 MPa. The original gas content was 14.60–16.11 m3 t-1, and averaged 15.37 m3 t-1. After hydraulic flushing and three months of gas predrainage, the residual gas content at B, O and D decreased to 6.48 m3 t-1, 6.15 m3 t-1 and 6.72 m3 t-1, respectively, which were all below the limit value (8 m3 t-1) in the ‘Basic index of coal mine gas drainage and exploitation’. The residual gas pressure dropped to 0.54, 0.50 and 0.58 MPa, which was also below the critical value (0.74 MPa) in the ‘Provisions on prevention of coal and gas outburst’. The residual gas content and gas pressure at point F were 14.82 m3 t-1 and 1.02 MPa, respectively, which exceeded the limit value and exhibited almost no change with respect to the initial value. It can be concluded that hydraulic flushing improved the efficiency of coal-seam gas predrainage extensively, especially in the expected affected area, which agrees with the results of the multielectrode resistivity method, and verifies the rationality and feasibility of the multielectrode resistivity method in the evaluation of coal-seam hydraulic-flushing effects. 5. Conclusion We applied for the first time the multielectrode resistivity method to evaluate coal-seam hydraulic flushing in Nuodong Coal Mine, Guizhou, China. Based on a field test, data analysis and a comparison of methods, the following conclusions were drawn: The apparent resistivity of coal and rock is influenced by water and its degree of fragmentation. With high-pressure water flushing, more fractured coal and rock contains more water, and the apparent resistivity decreases more. These decreases in apparent resistivity during hydraulic flushing correlate with higher apparent resistivities after stabilization. During hydraulic flushing, the apparent resistivity trend can be divided into four stages: (I) the preparation stage, (II) the sharp decreasing stage, (III) the rapid increasing stage and (IV) the stable stage. During stage I, the apparent resistivity remains at the initial value. During stage II, coal and rock in the area starts to fracture, many cracks appear, and water channels form, and so the apparent resistivity decreases rapidly to a minimum and remains level until the end of hydraulic flushing. During stage III, the apparent resistivity increases with water flowing out from the cracks and drilling holes. During stage IV, the apparent resistivity reaches a maximum. After three months of gas predrainage, the residual gas content and residual gas pressure in the testing area were measured. The residual gas content and residual gas pressure at points B, O and D in the high-resistivity zone decreased extensively to below the limit value in the ‘Basic index of coal mine gas drainage and exploitation’ and in the ‘Provisions on prevention of coal and gas outburst’, whereas almost no change resulted at point F. These results verify the rationality and feasibility of the multielectrode resistivity method in the evaluation of coal-seam hydraulic-flushing effects. This paper reports on the first attempt at applying the multielectrode resistivity method to evaluate coal-seam hydraulic-flushing effects. 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TI - Spatio-temporal evolution of apparent resistivity during coal-seam hydraulic flushing
JF - Journal of Geophysics and Engineering
DO - 10.1088/1742-2140/aaa228
DA - 2018-06-01
UR - https://www.deepdyve.com/lp/oxford-university-press/spatio-temporal-evolution-of-apparent-resistivity-during-coal-seam-va6LEMxDjW
SP - 707
VL - 15
IS - 3
DP - DeepDyve
ER -