reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams

International Journal of Mining Science and Technology xxx (2017) xxx–xxx Contents lists available at ScienceDirect International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams Wang Liang a,b,c, Liu Shimin c, Cheng Yuanping b,d, Yin Guangzhi a, Zhang Dongming a, Guo Pinkun a,e,⇑ a State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China Key Laboratory of Gas and Fire Control for Coal Mine, School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China c Department of Energy and Mineral Engineering, G3 Center and Energy Institute, The Pennsylvania State University, University Park, PA 16802, USA d National Engineering Research Center for Coal and Gas Control, China University of Mining and Technology, Xuzhou 221116, China e Department of Civil Engineering, Logistical Engineering University, Chongqing 400041, China b a r t i c l e i n f o Article history: Received 12 December 2015 Received in revised form 12 March 2016 Accepted July 2016 Available online xxxx Keywords: Reservoir reconstruction Coalbed methane Multiple seam Surface well Gas drainage a b s t r a c t Multiple coal seams widely develop in the deep Chinese coal-bearing strata Ground in situ stress and coal seam gas pressure increase continuously with the increase of the mining depth, and coal and gas outburst disasters become increasingly severe When the coal is very deep, the gas content and pressure will elevate and thus coal seams tends to outburst-prone seams The safety and economics of exploited firstmined coal seams are tremendously restricted Meanwhile, the multiple seams occurrence conditions resulted in different methane pressure systems in the coal-bearing strata, which made the reservoir reconstruction of coal difficult Given the characteristics of low saturation, low permeability, strong anisotropy and soft coal of Chinese coal seams, a single hydraulic fracturing surface well for reservoir reconstruction to pre-drain the coalbed methane (CBM) of multiple seams concurrently under the different gas pressure systems has not yet gained any breakthroughs Based on analyses of the main features of deep CBM reservoirs in China, current gas control methods and the existing challenges in deep and multiple seams, we proposed a new technology for deep CBM reservoir reconstruction to realize simultaneous high-efficiency coal mining and gas extraction In particular, we determined the first-mined seam according to the principles of effectiveness and economics, and used hydraulic fracturing surface well to reconstruct the first-mined seam which enlarges the selection range of the first-mined seam During the process of mining first-mined seam, adjacent coal seams could be reconstructed under the mining effect which promoted high-efficiency pressure relief gas extraction by using spatial and comprehensive gas drainage methods (combination of underground and ground CBM extraction methods) A typical integrated reservoir reconstruction technology, ‘‘One well for triple use”, was detailed introduced and successfully applied in the Luling coal mine The application showed that the proposed technology could effectively promote coal mining safety and simultaneously high-efficiency gas extraction Ó 2017 Published by Elsevier B.V on behalf of China University of Mining & Technology This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Introduction Coal, as the primary energy source, accounts for 70% of the China’s total energy supply and more than 48% of the world’s coal consumption The annual coal production in China has increased significantly from 1299 million metric tons (Mt) in 2000 to 3870 Mt in 2014 and this makes China as the largest coal-producing country in the world It is envisioned that coal will continuously play a leading role in the Chinese energy mix in next a few decades With the rapid sustainable development of Chinese ⇑ Corresponding author at: State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China E-mail address: guopinkun@163.com (P Guo) economy and the continuous demand and dependence on coal production, the geology and technical conditions for coal exploitation are deteriorating [1] With excessive coal mining depth increase, both gas pressure and content in deep coal seams continue to increase along with much more complex coal geologic structures The gas related mine geo-hazards, particularly coal and gas outburst disasters, are becoming increasingly severe Currently, more than 1000 coal mines have been classified to be outburst risk mines, and annually there were more than 300 fatalities directly due to outbursts in China [2] Coal seam gas has become the key factor that constrains safe and efficient production of Chinese deep coal mines [3,4] As a by-product of coal production, coalbed methane (CBM) is not only a major hazard of coal mine production but also a clean http://dx.doi.org/10.1016/j.ijmst.2017.01.026 2095-2686/Ó 2017 Published by Elsevier B.V on behalf of China University of Mining & Technology This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Please cite this article in press as: Wang L et al Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.026 L Wang et al / International Journal of Mining Science and Technology xxx (2017) xxx–xxx and efficient energy resource [5], and an intense greenhouse gas with a Global Warming Potential (GWP) of 25, i.e., 25 times the environmental impact of carbon dioxide [6] The energy released in the combustion of m3/t methane is 35.9 MJ, equivalent to the combustion of 1.2 kg of standard coal [3,5] As the geological resources of CBM at above 2000 m depth in China amount to 36.81 trillion m3, which ranks 3rd in the world, therefore, measures to control coal mine methane (CMM) in China bear the multiple purposes of promoting mining safety, recovering methane resources and abating greenhouse gas emissions [3,6] China’s known coal reserves amount to 5.57 trillion metric tons, 63% of which is occurred at a depth of 800–2000 m, with coal reserves of 3.25 trillion metric tons In addition, 70% of Chinese coal reserves are multiple seams [7], as summarized in Table Coal is a self-sourced reservoir rock, which is always referred as a CBM reservoir in areas for oil-gas exploitation [8,9] And the multiple-seam coal regions are usually rich in CBM resources in China and present a good development prospect for CBM production [10] In order to guarantee the safe and efficient exploitation of coal resources, CBM extraction should be initially conducted before the actual mining to reduce the overall gas content and to mitigate the gas-related mining hazards [11,12] However, the Chinese deep coal reservoirs share the common characteristics with low permeability, low saturation, under pressure and strong anisotropy [13,14] The pre-mining degasification and gas drainage in the deep coal seams is technically challenging, therefore, CBM reservoir stimulation emphasizing on the permeability enhancement is required for the mine degasification and for the methane energy recovery as well [15,16] Based on the unique features of deep coal seams, the current coal gas extraction technologies were summarized and reviewed in the article and the existing challenges for each technology were also analyzed In order to effectively drain the gas from multiple coal seams, a new CBM reservoir stimulation to enhance the permeability was proposed by stress-relief and/or unloading fracturing through adjacent seam mining This new technology has been implemented in a deep gas mine and it was proven to be very effective to ensure gas hazard mitigation with additional benefit of gas energy recovery A case study was finally provided and outcomes were summarized and analyzed for Luling coal mine in China Main features of deep CBM reservoirs in China Most Chinese coal originated during the Carboniferous-Permian [6,17] Thereafter, the coal underwent a number of strong tectonic movements that destroyed the original fracture/cleat networks in the coal seams As a result, the coal became structurally complicated, high degree of metamorphism, mechanically soft, and very low gas deliverability due to low reservoir permeability Table summarizes the virgin coal seam reservoir properties for the Table Coal seam groups occurrence of partial mines in China [6] Coalfield Coal seam conditions Huainan Huaibei Shenyang Yangquan Pingdingshan Yaojie Hedong Xishan Tiefa Laochang Seam Seam Seam Seam Seam Seam Seam Seam Seam Seam C14, C13, B11b, B10, B8, B7a, B6, B5, B4, A3 and A1 4, 5, 6, 7, 8, 9, 10 7, 11, 12, 13 2, 3, 6, 8, 9, 12, 15 14, 15, 16, 17 1, 2, 2, 3, 4, 02, 03, 2, 4, 6, 7, 8, 12, 13, 14, 15, 16, 17 C2, C3, C4, C7 + 8, C8, C9, C13, C16, C19 Table CBM reservoir property comparison for Chinese, US and Australian coal seams Coalfield Huainan Huaibei Tianfu Yangquan Zhenzhou Shenyang Yaojie Jincheng USA (San Juan) Australia (Bowen) Virgin coal reservoir properties Permeability (mD) Gas pressure (MPa) Gas content (m3/ t) 0.00028 0.00121 0.00106 0.00037 0.00118 0.00035 0.00244 1.55 10–100 1–10 6.5 5.1 13.6 2.3 2.6 8.3 7.3 1.25 – – 10–40 10–35 10–40 10–40 8–20 10–35 10–30 5–50 9–19 8–17 Note: The CBM parameters in the table were measured or calculated in the current mining condition Chinese, US and Australian CBM reservoirs The permeability of Chinese coal is usually on the magnitude of 10À4–10À3 mD except for Jincheng coal field As comparison, permeability of San Juan coal in US is four orders of magnitude higher than most Chinese coals and Bowen basin coal in Australia is three order of magnitude higher than most Chines coals This explained why the San Juan and Bowen coals are very successful in CBM production and commercialization Although CBM extraction has a long history in China, it is still not yet up to the expected commercialization level Therefore, tax incentives is being distributed in major CBM states in China Currently, coal mine gas pre-drainage techniques face many challenges for large scale commercial production and thus the advanced reservoir stimulation technologies are required to increase the gas drainage efficiency [18,19] The China national coal mining depth is going deeper and deeper as the average annual rate of 10–50 m [20] The mining depth of many coal mines in Mid-East of China has reached 800–1200 m below surface (e.g 800 m in Huaibei, 850 m in Huainan, and 1000 m in Xuzhou) At these mining depths, the coal seam overburden stress ranges from 22 to 33 MPa and the coal seam gas pressure and gas content can exceed MPa and 20 m3/t respectively Most of these coal seams have the virgin permeability at order of 0.001 mD or even lower, which makes CBM extraction inefficient and even not possible without secondary reservoir stimulation Surface borehole gas drainage is widely used as a conventional CBM extraction technology in today’s world [19,21–23] However, low virgin coal permeability limits the efficiency of the primary reservoir depletion at which the borehole drainage area/volume is very constrained Meanwhile, deep Chinese coal exhibits a strong anisotropy of permeability which will further decrease the depletion efficiency for any given well patterns Additionally, the Chinese main geological CBM accumulations are generally located in the Carboniferous-Permian coal-bearing regions where the coal ranks are known to be partially high due to the metamorphic effect CBM resources in high rank coals account for >27.6% of the total resources, which owns the features of low permeability and difficulty in desorption, limiting the application of conventional CBM extraction technology [6] In contrast, US high gas bearing coal seams are mainly distributed in the Cretaceous laver with thick coal seams, and rich-gas coalfields are often formed with high to low volatile bituminous coals associated with high reservoir pressure and high permeability, making larger production from a single surface well [24] In Chinese coal mines, the coal-bearing strata develop into multiple and relatively stable low-permeability compartments, which serve as seals to prevent the vertical reservoir fluid exchange Coal Please cite this article in press as: Wang L et al Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.026 L Wang et al / International Journal of Mining Science and Technology xxx (2017) xxx–xxx seam groups are usually in multi-independent fluid pressure systems, making grouping CBM reservoir accumulation [10,25] High-efficiency and safe mining in firstmined seam Current CBM extraction methods and their challenges in deep and multiple seams Reduce gas emissions in firstmine seam There are two types of energy in tight and gassy coal seams, namely solid coal and gaseous methane If only the methane gas is exploited, the low permeability would restrict rate and efficiency of methane extraction If only the coal resource is exploited, the gas-associated mine hazards could lead to gas explosions or outbursts The permeability enhancement is one of the key challenges for CBM extraction in deep coal seam with low permeability [15,16] A coal reservoir stimulation technique should be chosen based on the in situ coal geological conditions For multiple coal seam conditions, a unique permeability enhancement technique, mining induced ground stress relief, was successfully proposed and implemented in the deep coal mines In multiple coal seams, a coal seam with absence or relatively low outburst risks should be selected as a first-mined coal seam After mining this seam, the ground stress in the adjacent seams (above and/or below) could be relieved, thus, coal permeability increases, facilitating good conditions for high-efficiency CBM extraction The mining induced permeability enhancement technique was conceptually illustrated in Fig Effective extraction of gas in adjacent gassy seam(s) could significantly decrease the gas content and pressure in the seam, thus realizing the objective of extraction of both coal and methane in a safe environment [6,11,18,26,27] The coalmethane co-exploitation technology for multiple gassy coal seams is shown in Fig 2, which was considered as an effectively underground reservoir reconstruction method This innovative technology was termed as protective seam mining In recent years, the protective seam mining in multiple seams is accepted widely as a key technology in the coal mining industry [2,11,18,26] With progressive increase of overall mining depth, each deep coal seams are commonly outburst prone [20,28,29], which results in that the first-mined seam (protective seam) is hard to choose and the outburst elimination of the whole coal region becomes difficult as well On this occasion, we could only choose some thin seams with high ash content or even non-coal soft rock as the first mined seams In order to gain an effective mining induced permeability enhancement, the extraction height of the first-mined seam needs to reach a specific value On the contrary, the higher the mining height of the first mined seam, the more gangue will be produced from working face which is not cost effective for the whole mining system Therefore, a key considera- Deformation and fractures in roof and floor of first-mined Bending zone Extreme distant pressure relief seam Distant pressure relief seam Fractured zone Caving zone Fractured zone First-mined seam Lower pressure relief seam Deformation zone Fig The strata movement and fractures development after mining the firstmined seam Desorption-diffusion of gas flow High-efficiency and safe coal exploitation Gas drainage in the adjacent seam High-efficiency and safe mining adjacent seams Eliminate outburst risk in adjacent seams Decrease gas content in adjacent seams High-efficiency and safe gas exploitation Fig A coal-methane co-exploitation model for multiple gassy coal seams tion for the reservoir reconstruction technologies application in coal seam gas recovery is the selection of the protective seam According to Chinese criterions on the gas control requirement, the related CBM parameters of the first-mined seam with outburst risk must be decreased to critical values, that is, gas pressure 0.74 MPa or gas content m3/t [20,30] The underground gas pre-drainage method is mostly adopted by using penetration borehole or bedding borehole for the first-mined seam, which requires large engineering quantities and long drilling and drainage duration [1,26] Hydraulic fracturing technology was initially applied in petroleum/natural gas industry in order to increase productivity of surface wells [31,32] Then it was widely used in CBM extraction in many countries, such as the U.S., Australia and China This has been proved to be very effective for the deep and gassy coal seams The fracking stimulation has become the most important technique for reservoir reconstruction in deep outburst seams, especially in Chinese CBM reservoir with the characteristics of low saturation, low permeability, strong anisotropy and soft-toplastic coals Meanwhile, the multiple seam occurrence conditions resulted in different methane pressure systems in the coal-bearing strata make the reservoir reconstruction difficult The technical difficulties involved in coal fracking include: difficulties in concentrated reconstruction using single surface well, difficulties in drainage and pressure lowering, layout with small spacing, low production from single well and high cost Therefore, further studies are essential for deep CBM reservoirs in terms of reservoir reconstruction and stimulation Key technologies for reservoir reconstruction in deep and multiple seams We proposed a new approach for CBM reservoir reconstruction In particular, we determined the first-mined coal seams based on the extraction effectiveness and economics The coal seam with relatively low outburst risk, good pressure-relief effect and economical costs for mining was selected as the first mined coal seam The fractured surface well was used to reconstruct the first-mined seam This process can enhance the permeability of first-mined seam, thereby being beneficial to the extraction of CBM in firstmined seam and the elimination of its outburst risk Along with the mining of the first-mined seam, the adjacent coal seams can be reconstructed due to the stress-relief induced by mining The permeability of adjacent seam would increase significantly which will increase the efficiency of methane extraction The surface wells (or the underground penetration boreholes) were then used to extract the pressure relief methane of adjacent coal seams, and the outburst risk of the adjacent seams could be eliminated as well The technical layout for the proposed technique is illustrated in Fig Please cite this article in press as: Wang L et al Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.026 L Wang et al / International Journal of Mining Science and Technology xxx (2017) xxx–xxx Joint with underground gas drainage method Pressure relief gas drainage Surface wells Reservoir reconstruction for the adjacent seams Deep coal reservoir reconstruction Unloading damage by mining effect Selection of reconstruction target (first-mined seam) Ensure mining safety of firstmined seam Integrated plannings of coal mining and gas control Enlarge the selection range of first-mined seam Mining the firstmined seam (protective seam) Fig CBM reservoir reconstruction technical layout The reservoir reconstruction technology for deep and multiple seams mainly includes three aspects: (1) Selection of the first mined coal seam: Generally, the seam which has the lowest gas content and little or no outburst risk is the ideal seam when the mining depth is shallow As the mining depth increases, all coal seams become outburst-prone and there is no suitable seam to be mined first with gas drainage In the new reservoir reconstruction system, we could only choose the first-mined seam based on principles of both effectiveness and economy The effectiveness of pressure relief could be calculated according to the occurrence of the coal strata, mining parameters [33], and stress and deformation fields of adjacent seams could be obtained by numerical simulation The economy of mining need to consider the thickness of coal seam, coal quality, extraction engineering quantities, coal pressure relief quantities, and so on [34] If the first-mined seam has coal and gas outburst risks, localized gas control methods should be used to eliminate this risk prior to mining According to the integrated planning of coal seam occurrence, excavation system, gas control and drainage engineering, a CBM reservoir with good occurrence and economical mining conditions, even existing outburst risk, should be selected as the first-mined seam (2) Reservoir reconstruction technology for the first-mined seam: Surface well using hydraulic fracturing technology was used to stimulate the first-mined seam initially (Fig 4) The drilling locations of the surface well were determined by the layout of the working face, which often located at the middle of the panel, about 50 m away from air return roadway The well patterns were arranged as square or rectangle with borehole spacing less than 300 m After the hydraulic fracturing with fracturing fluids, the sand proppant was injected for propping fractures For improving the carrying capacity of fracturing fluid, CO2 and N2 could be used as accompanying injection materials [35] Then permeability of the first-mined seam could be enhanced resulting in high gas extraction efficiency Then the outburst risk of the first-mined seam could be eliminated which could ensure the mining safety and high-efficiency of the mining system The fractured well could be used for several years, which broadens the selection range of the first-mined seam (3) Reservoir reconstruction technology for the adjacent seams: In the process of exploiting the first-mined seam, the upper and lower adjacent seams experience the sequential phenomena of stress concentration, unloading damage, and stress recovery [16,36], and the adjacent seams could be reconstructed by mining induced stress relief Exploitation Fig A sketch of hydraulic fracturing surface well for the first-mined seam reconstruction of the first-mined seam will increase the gas deliverability in the adjacent coal seams by the permeability enhancement [36–38] The integrated gas drainage technology by combining both underground and surface drainage methods in the pressure relief area can effectively realize high-efficiency gas extraction from the adjacent protected seams and eliminate their outburst risk, thereby promoting the safe and efficient exploitation of adjacent seams [39] (Fig 5) The pre-drilled surface boreholes with hydraulic stimulation can be triple-purpose wells under certain geological condition (lower protective seam mining), which was a typical integrated reservoir reconstruction technology, namely, pre-drainage of gas before mining, gas extraction of working and adjacent seams during mining, and post goaf gas drainage This concept is illustrated in Fig showing the application in the Luling coal mine, and then we discuss this further in detail in the next section This technology not only considered the commercial exploitation and utilization of CBM, but also solved the gas control problems during mining in deep multiple seams Fig Spatial and comprehensive gas drainage methods for adjacent seams Please cite this article in press as: Wang L et al Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.026 L Wang et al / International Journal of Mining Science and Technology xxx (2017) xxx–xxx First-mined seam Fig ‘One well for triple use’ in the Luling coal mine Thickness (m) 1000 km Luling coal mine Coal Columnar seam 0.29 − 2.22 1.19 0.14 − 16.62 9.56 0.29 − 9.82 3.65 2.89 K2 − 4.30 1.92 10 Lithology Mudstone Packsand Medium sandstone Mudstone Packsand Mudstone Coal seam Mudstone Packsand Mudstone Coal seam Mudstone Coal seam Mudstone Packsand Key bed Medium sandstone Sandstone Siltstone Mudstone Siltstone Mudstone Medium sandstone Huaibei coalfiled Hefei Packsand Mudstone Coal seam Mudstone 200 km Anhui Fig The location and coal strata histogram of the Luling coal mine Table Key reservoir characteristics in the Luling coal mine Parameters Coal seam thickness (m) Hardness coefficient (f) Permeability coefficient (m2/(MPa2Ád)) Maximum gas pressure (MPa) Limiting adsorption amount (Langmuir volume) (m3/t) Gas content (m3/t) Maximum value of the initial speed of gas diffusion (DP, mmHg) Largest outburst Nos and coal seams No 10 coal seam Value Characteristics Value Characteristics 9.56 m on average (No 8), 15.3 m in composited seams (Nos and 9) 0.11–0.46, 0.26 on average 0.0277 (or 0.0007 mD) 5.7 (at 800 m depth, predicted) Extremely thick 1.92 m on average Medium-thick seam Extremely soft Low permeability High outburst risk 0.8–1.13, 0.9 on average Hard Relative low outburst risk 37.6 2.8 (at 800 m depth, predicted) 15.69 25 30 9.1 8.9 Coal 10,500 t, gas 1.23 Mm3 Note: (a) The initial speed of gas diffusion (DP) representing the gas emission capacity of the coal was measured by a device (WT-I, Fushun Coal Science Research Institute, Liaoning, China) with particle sizes ranged from 0.2 to 0.25 mm according to the AQ 1080-2009 test method The hardness coefficient (f) representing the ability to resist damage of the coal was measured using the drop weight method with particle sizes ranged from to mm according to the MT 49-87 test method Gas pressure and gas content were tested in the field according to the AQ 1066-2008 and AQ/T 1047-2007 test methods respectively (b) Gas pressure, gas content and the initial speed of gas diffusion (DP) are the key factors dictating the outburst risk, which critical values are 0.74 MPa, m3/t and 10 mmHg, respectively Please cite this article in press as: Wang L et al Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.026 L Wang et al / International Journal of Mining Science and Technology xxx (2017) xxx–xxx Flac3D was used to simulate stress and deformation distribution and evolution of adjacent seams The length, width and height of the model are 500 m, 500 m and 300 m; while the mining scopes are from 160–340 m on the trend and 180–320 m on the strike, and the mining height is 1.92 m Based on the numerical simulation results, the stress reduced from 17.7 MPa to 3.34 MPa in the No seam in the pressure relief area (Fig 8a) The maximum expansive deformation ratio was 0.321% (Fig 8b), which exceeded the Chinese standard (0.3%), and the coal seam permeability increased from its original mean amount of 0.0007–4.33 mD, which showed an obvious pressure relief effect (Fig 8c) It is proved that the mining of the long-distance lower protective seam A case study in the Luling coal mine The Luling coal mine is located in the Sunan mining area in Anhui Province The coal mine runs 8.2 km along the strike and 3.6 km along the trend There are three primary mineable coal seams from top to bottom: Nos 8, and 10 coal seams The average thicknesses of the three seams are 9.56 m, 3.65 m and 1.92 m respectively The spacing between the No and No coal seams ranges from to 5.3 m, with an average of 3.2 m The two seams have similar properties The spacing between the No and No 10 coal seams ranges from 60 to 110 m, with an average of 80 m The lithologic components in the roof and floor of coal seams are mainly mudstone, sandstone and packsand The location and coal strata histogram are shown in Fig The primary mineable coal seams (Nos 8, 9, 10) of the Luling coal mine are all outburst-prone coal seams 26 coal and gas outburst accidents have occurred in coal seam Nos and An extremely large coal and gas outburst accident occurred on April 7th, 2002 which threw out 10,500 t of coal-rock masses and 1.23 Mm3 of gas The spraying holes phenomenon usually appears during the process of drilling, and the mean spraying coal amount is 15.0 t As shown in Table 3, the middle group coal seams (Nos and 9) are typical tectonic coal with characteristics of extremely soft and low permeability It is also characterized by high gas pressure, high gas content and rapid diffusion rate While the outburst risk of the No 10 coal seam is relatively little and with a good occurrence, which is chosen as the first-mined seam (protective seam) of Nos and coal seams Gas concentration (%) 100 80 60 40 40 20 20 100 200 200 Y (m 300 400 ) 500 (a) m) X( 400 50 100 150 200 250 Time (day) Fig 10 Gas output of mining pressure relief area well in the Luling coal mine Expansive deformation ratio Z (%) 0.4 0.3 0.2 0.1 -0.1 500 Y SZZ (Pa) -3.0E+6 -4.0E+6 -5.0E+6 -6.0E+6 -7.0E+6 -8.0E+6 -9.0E+6 -1.0E+7 -1.1E+7 -1.2E+7 -1.3E+7 -1.4E+7 -1.5E+7 -1.6E+7 -1.7E+7 -1.8E+7 -1.9E+7 -2.0E+7 -2.1E+7 80 60 Z X 100 Gas concentration Amount of gas drainage Amount of gas drainage (m3/min) 400 300 200 Y (m ) 100 300 (b) 100 200 400 500 m) X( Coal seam permeability Z (mD) 500 400 300 Y (m ) 400 500 300 200 100 (c) 100 200 ) X (m Fig Changes of stress and seam permeability after mining No 10 seam after completing mining the No 10 coal seam (about 1.9 m) ((a) The stress distribution cloud; (b) expansion deformation ratio; (c) coal seam permeability) LG-3 790 LG-2 30 m 700 840 LG-5 LG-4 790 740 200 m LG-6 (a) Locations of surface wells LG-7 Volume of gas drainage (m3) 4000 880 3500 3000 2500 2000 1500 1000 500 50 100 150 200 250 300 Time (day) (b) Gas output of LG-6 350 400 Fig Locations and gas output (LG-6) of hydraulic fracturing surface wells in the Luling coal mine Please cite this article in press as: Wang L et al Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.026 L Wang et al / International Journal of Mining Science and Technology xxx (2017) xxx–xxx Table Prediction on gas drainage effect of surface well Coal thickness (8 + + 10) (m) Coal density (m3/t) Well space (m) Original average gas content (m3/t) Original gas content (m3/t) Duration of drainage (year) Residual gas content (m3/t) Drainage rate (%) Note 14.4 1.4 25 2.7 4.3 14.4 1.4 12 6.5 14.4 1.4 15 8.1 23.8 13.4 8.6 37.9 21.3 13.7 57.3 32.2 20.6 71.4 40.2 25.2 Gas output 1500 m3/d 1.4 11.34 20.20 31.50 11.34 20.20 31.50 11.34 20.20 31.50 11.34 20.20‘ 31.50 14.4 150 200 250 150 200 250 150 200 250 150 200 250 25 25 25 (No 10 coal seam) could effectively affect the middle group coal seam (Nos and 9) However, when the mining deepened to the third level (below 900 m), the No 10 coal seam turns to be outburst seam It is necessary to eliminate this risk before continuing mining, which requires the arranging of a set of hydraulic fracturing surface wells for the deep coal seam of the Luling coal mine Surface wells for gas pre-drainage should be constructed in the No 10 coal seam more than 10 years in advance Although the gas output cannot reach a commercial scale, the daily production could still reach 1000 m3/ d as predicted Thus, we can weaken or eliminate the No 10 coal seam’s outburst risk obviously, and further gas control measures will be much easier In addition, we can use the surface wells as mining pressure relief area wells (with little technical transformation) to drain the pressure relief gas from Nos and coal seams When mining the No coal seam, the wells could be used for third time as goaf wells to drain post goaf gas Thus, following the procedure of ‘one well for triple use’, the gas pressure and gas content can be minimized, and high-efficiency mining can be realized Because of the long period of ‘one well for triple use’, the three different well patterns are practiced separately In 2007, the Luling coal mine applied the hydraulic fracturing surface wells for gas pre-drainage Six surface wells with spacing of 200–300 m were constructed with a layout designed in a rectangular well pattern (as shown in Fig 9a) The fracturing radius was 120–130 m on average From April 2008 to June 2011, fracturing gas drainage amount has reached 1.726 Mm3 in total with the maximum gas output reaching 6706 m3/d and the daily output kept stable at approximately 4700 m3, among which the LG-6 well kept at 1500 m3/d during the stable period (as shown in Fig 9b) The mining pressure relief area well was practiced in the II 1048 working face with depth 587 m, and gas drainage maintained for 10 months and amounted to a total 2.484 Mm3 of gas The maximum daily output was 46,656 m3, and the monthly output was 100,000–450,000 m3, as shown in Fig 10 The drainage radius was between 200 m and 300 m Two goaf wells were drilled at face II 825-1, and the termination depths of the holes were 483 m and 475 m separately The gas output of a single well was approximately 100,000 m3 The maximum daily output was as much as 2938 m3, and the average daily output was 1120 m3 The gas drainage lasted three months, and the drainage radius was between 100 m and 150 m Besides, when the hydraulic fracturing surface wells were used for the whole deep coal seams, the drainage durations and effects could be different Drainage effect predictions of surface wells are shown in Table We set the daily output amount to be 1500 m3/d of fracturing drainage before mining in deep coal seams The prediction conditions have three different situations, which represent three different well spacing: 150 m, 200 m, and 250 m The extraction durations are 5a, 8a, 12a, 15a The original average gas content of the coal seam is 25 m3/t We found that drainage rate with the 150 m well pattern distance could be 70% after 15a of gas extraction, and the gas content would drop to m3/t or less Unless the technology makes significant development, the possibility of eliminating the outburst risk of Nos and coal seams in short time is very little, when we only use the hydraulic fracturing surface wells on these coal seams Combining with the protective seam mining (No 10 coal seam), the pressure relief effect is more significant which could provide good condition for gas drainage Thus, the outburst risk of the middle group coal seams could be eventually eliminated Conclusions (1) In China, multiple seam existence accounts for 70% in coal measure strata CBM reservoir presents the characteristics of low saturation, low permeability, strong anisotropy and soft coal Coal seam groups are usually in multiindependent fluid pressure systems, making grouping CBM reservoir accumulation (2) When the coal is very deep, the gas content and pressure will elevate and thus coal seams tend to outburst-prone seams The safety and economics of exploited first-mined coal seams are tremendously restricted Meanwhile, the multiple seam occurrence conditions resulted in different methane pressure systems in the coal-bearing strata, which made the reservoir reconstruction of coal difficult (3) We proposed a new integrated technology for deep CBM reservoir reconstruction, namely, hydraulic fracturing surface well was used to reconstruct the first-mined seam which enlarges the selection range of the first-mined seam During the process of mining first-mined seam, adjacent coal seams could be reconstructed under the mining effect which promoted high-efficiency pressure relief gas extraction by using spatial and comprehensive gas drainage methods (4) A typical integrated reservoir reconstruction technology, ‘‘one well for triple use”, was introduced and successfully applied in the Luling coal mine The application showed that the proposed technology could effectively promote coal mining safety and simultaneously high-efficiency gas extraction Acknowledgments This research was supported by the National Key Research and Development Program of China (No 2016YFC0801406), the National Natural Science Foundation of China (No 51674252), the Visitor Foundation of State Key Laboratory of Coal Mine Disas- Please cite this article in press as: Wang L et al Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams 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Definition, theory, methods, and applications of the safe and efficient simultaneous extraction of coal and gas Int J Coal Sci Technol 2015;2(1):52–65 Please cite this article in press as: Wang L et al Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.026 ... surface wells in the Luling coal mine Please cite this article in press as: Wang L et al Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams Int J Min Sci Technol... vertical reservoir fluid exchange Coal Please cite this article in press as: Wang L et al Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams Int J Min Sci... seam mining), which was a typical integrated reservoir reconstruction technology, namely, pre-drainage of gas before mining, gas extraction of working and adjacent seams during mining, and post