Annular overlying zone for optimal gas extraction in multi-seam Longwall mining

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1 NOTICE: this is the author s version of a work that was accepted for publication in International Journal of Rock Mechanics and Mining Sciences. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in International Journal of Rock Mechanics and Mining Sciences, 54, DOI: Annular overlying zone for optimal gas extraction in multi-seam Longwall mining Hua Guo a Liang Yuan b Baotang Shen a Qingdong Qu a Junhua Xue b a. The Commonwealth Scientific and Industrial Research Organisation (CSIRO), PO Box 883, Kenmore, Queensland 469 Australia b. National Engineering Research Centre for Coal Mine Gas Control, Huainan, Anhui, 2321 China *corresponding author: Tel: ; Fax: address: Hua.guo@csiro.au Abstract This paper presents key results from a recent comprehensive study that integrates the behaviour of strata movement, fluid flow and gas migration in a deep underground coal mine in Anhui, China. The study includes field monitoring of mining induced overburden displacement, stress and pore pressure changes at the longwall panel 1115 (1) of the Guqiao Mine. In addition, coupled 3D modelling of strata and fluid behaviours using COSFLOW software, and gas flow simulations at the longwall panel with CFD software are carried out. This research has resulted in many new insights into the complex dynamic interaction between mining induced stress changes, fracturing, and gas flow patterns. Based on the findings from the field monitoring and numerical modelling, a new concept, namely the Annular Overlying Zone (AOZ), is developed to identify the region suitable for optimal methane extraction. A practical method that helps define the geometry and boundary of this zone is proposed. This study provides a new methodology and a set of engineering principles for the design of optimal co-extraction of coal 1

2 and methane. Keywords: Deep mining; Multiple coal seams; Co-extraction of coal and methane; Annular Overlying Zone 1. Introduction Co-extraction of coal and methane is the future trend of efficient coal mining. It effectively combines the two previously separate operations of extracting coal and methane gas. The coal mining operation will enhance desorption and migration of methane in surrounding coal seams, as is necessary for gas extraction. The roadway system developed for mining transport and ventilation provides access and space for the gas extraction operation. Gas extraction will reduce the methane concentration in the vicinity of underground workings and the methane content of adjacent coal seams; both of which can be of significant benefit in preventing gas explosions and outbursts, promoting a safer and more productive coal mining operation. Past mining experiences have demonstrated that co-extraction of coal and gas is an effective method for mining coal seams of low permeability. This method has been fast developing in China, Australia and many other countries with a coal industry [1~9]. However, as a complex theoretical and engineering system, this method is not yet sufficiently validated by actual field measurement data, in particular on the dynamic evolution and interaction between the mininginduced stress field, the fractured zone and the gas migration zone. To address the issues above, Huainan Coal Mine Group (HCMG) and The Commonwealth Scientific and Industrial Research Organisation (CSIRO) established a research programme. This programme took Panel 1115(1) of Guqiao Mine as the experimental site and carried out a systematic study on the fundamental theory of deep coal mine co-extraction by means of field measurements, numerical modelling and theoretical development. This paper describes the key achievements from that study. 2. Mining condition of Panel 1115(1) Guqiao Mine is located in the central to western part of the Huainan Pan-Xie Coal Field, 2 km west of Fengtai City of Anhui Province, China. It was designed to have an annual production of 1 Mt. The key geological features at Guqiao Mine include thick alluvium layers, deep coal seams, high methane content in minable seams and high geothermal temperature. The coal-bearing geological section has a total thickness of 734m, and it contains 33 coal 2

3 seams, 9 of which can be mined with a total thickness of 24.11m. The key five minable seams include Seams 13-1, 11-2, 8, 6-1, and 1. Currently the mine is extracting Seams 11-2 and 13-1, both classified as low permeable seams with a methane content of 1.96~13.2m 3 /t and 2.7~12.9m 3 /t, respectively. The working seam at Panel 1115(1) is Seam It has an average thickness of 3.m, an average dip angle of 5 ranging from 3~8. Overburden depth ranges between 64~76m with a 4~45m thick alluvium section below the surface. The panel length is 26m and panel width is 22m. It was mined by the retreat longwall mining method with full seam extraction. The longwall face was ventilated using the Y type of ventilation design with maingate (belt road) and inby part of the tailgate (material transport road) being the intake roadways and the outby part of the tailgate being the return roadway. The tailgate behind the longwall face was kept open by constructing a concrete wall to replace the mined-out roadway wall, named as Goafside Retained Gateroad. The coal seam within a distance of 2m from either side of the Panel had not been mined before this panel. The mining seam is overlain by Seam 13-1 (thickness = 3.5m) at a distance of 75m above and Seam 17-2 (thickness = 1.4m) at a distance of 179m above. The plan view of this panel and its geological settings are shown in Fig.1. Tailgate Goafside retained gateroad Concrete wall 22m LW face Goaf Start-up Maingate Inflow (a) Panel layout and air flow directions Outflow Layers Lithology (b) Geological setting Thickness /m Floor depth /m Alluvium Rock Height above LW/m Coal Rock Coal Rock Coal Fig.1. Plane view of Panel 1115(1) and its geological setting. 3. Field monitoring of strata movement, stress change and fluid pressure at Panel 1115(1) 3.1. Monitoring design 3

4 For the purpose of an in-depth investigation of stress field, fracturing zone, gas migration zone and their dynamic interaction, an integrated real-time monitoring system was designed, see Fig.2, based on the specific geological conditions at Huainan. The monitoring programme focused on the region at a retreat distance of 13m from the longwall start-up. The monitoring programme included: (1) Monitoring stress change and displacement of roadway roof: 1 monitoring stations were set up in a 45m section of the tailgate, located at a retreat distance of 11 ~ 155m from longwall start-up. At each station, one multi-point roof extensometer was installed which has 5 anchors located at a depth of 8m 6m 4m 2m and 1m into the roof. At every second monitoring station, three uniaxial stressmeters were installed in vertical and inclined upward boreholes that measure the stress change in the roof of the roadway and sidewalls. (2) Monitoring overburden strata movement: Two multi-point surface extensometers were installed in the panel at a distance of 9m and 3m respectively from the tailgate. Twenty anchors in each extensometer borehole were installed over the key range of overburden strata to measure their displacement. (3) Monitoring fluid pressure change in overburden strata: Seven piezometers were installed in two deep boreholes drilled at a distance of 5m outside the longwall panel to avoid damage from mining-induced caving. The piezometers were located in aquifers in the rock and alluvium strata between seam floor and ground surface, and were grouted in the boreholes using special grout designed to isolate any hydraulic linkage between them. Data logger BH#4( 1 piezometer) 5m BH#3( 7 piezometers) 1m Panel 1115(1) 3m 9m BH#2 ( 2 anchors) BH#1 ( 2 anchors) retreat direction 45m 13m Extensometer boreholes Piezometer boreholes Monitoring stations for both stress and displacement of roadway roof Monitoring stations for only displacment of roadway roof Fig.2. Integrated real-time monitoring plan at Panel 1115(1). All monitoring systems included automatic data acquisition and recording with a logging interval of 1 hour. 4

5 3.2. Key monitoring results Stress change in roadway roof Fig.3 shows typical monitoring results of the change of inclined stress in the abutment roof and horizontal stress in roadway roof in Panel 1115(1). The abutment stress zone can extend to 3m ahead of the longwall face. At 28m ahead of the face, the stress change reached its peak. The inclined stress increased initially and then decreased, whereas the horizontal stress in the roadway direction kept decreasing. 2 6º LW 7~15m Test location and direction Stress change/mpa Distance ahead LW face/m -.5 (a) Inclined stress change in the abutment roof Distance ahead LW face/m m LW Test location and direction Stress change/mpa (b) Horizontal stress change in roadway roof Fig.3. Measured stress change in roadway roof Relative displacements in roadway roof Fig.4 shows the results from roadway roof displacement monitoring. Within the measurement depth of 8m in the roof, bed separations occurred at a distance of 25m behind the longwall face. At a distance of 17m~2m, roof displacement showed another obvious increase before stabilising. It appears that the strata movement occurred within a zone of - 2m behind the longwall face. 5

6 8m 6m 4m 2m Sandstone Claystone 1m Claystone Gateroad Displacement/mm m 1m 8m 6m 4m -2 Distance behind LW face/m Fig.4. Measured relative displacement in roadway roof Movement of overburden strata The two surface deep-hole extensometers had both been affected by borehole instability. As a result, only limited data were obtained for the depth range of 473m~595m (i.e. 152~274m above the mining seam), see Fig.5. The limited data indicate that: (1) the trend of strata movement in the panel centre is generally consistent with that in the panel side; (2) strata within a vertical distance of 152m~274m above the mining seam have a similar displacement trend; (3) Most of the strata displacement occurred within 17m behind the longwall face; (4) There was a 65mm relative displacement between the overburden rock strata and ground surface. Dispacement in BH #1/mm BH #1 Roof 22m BH #1 Roof 152m BH #2 Roof 192m BH#2 Roof 192m BH #2 Roof 222m BH #2 Roof 274m BH #1 Roof 274m Displacement in BH #2/mm Distance behind LW face/m Fig.5. Measured overburden displacements Pore pressure change in surrounding strata The measured pore pressure change in the overburden strata is shown in Fig.6. The key conclusions from the measurement results are: (1) The strata with a significant pore pressure drop can extend up to 145m above the mining seam, implying that the zone of rock fracturing has extended to this height. 6

7 (2) Strata at different heights showed a different trend in pore pressure change, indicating the effect of mining-induced stress changes and rock fracturing. It is postulated that the roof strata within 7~43m have been fractured subvertically and that the fractures are linked hydraulically in different strata. This zone is also called the inter-strata fracturing zone. Within the height range of 43~145m is the bed separation zone where fractures developed mainly horizontally. At a height of m, rock mass deformed continuously without significant fracturing. This zone is also called the deformation zone. Future up in the alluvium layers, the key aquifers were not affected by mining. (3) Pore pressure started to increase at a distance of 3m ahead of the longwall face, indicating that mining induced stress can extend to 3m ahead. Pore pressures dropped rapidly between 1m ahead of the longwall face and 17m behind the longwall face, implying that mining-induced fractures are developed within this region. Further behind the longwall face the pore pressure was either stabilised or started to recover, suggesting that the mining induced fractures are being compacted in this zone. 2 1 Measurement BH Surface Pressure/mH Roof 466m Distance behind LW face/m Piezometer Roof 466m Alluvium Pressure/mH m Roof 237m 555m Roof 237m Claystone Sandstone Clay&Sand Coal Distance behind LW face/m Roof 145m Sand set Pressure/mH m Roof 18m Roof 7m -2 Roof 15m 735m Roof 43m -3 Roof 88m Roof 145m -4 Distance behind LW face/m Roof 88m Roof 43m Roof 15m Roof 7m Floor 18m Clay set Sandstone Coal 13-1 Clay set Sand set Coal 11-2 Fig.6. Measured pore pressure change and its relationship with rock fracturing characteristics. 7

8 4. Modelling the effect of mining on surrounding rock mass and gas migration In order to extrapolate the localised monitoring results to the entire Panel 1115(1), a 3D numerical study was carried out. The numerical investigation employed a software called COSFLOW, developed by CSIRO, NEDO and JCOAL of Japan, and the commercial software CFD. The numerical study was aimed to investigate the zones of mining-induced stress change, rock fracturing and gas migration. 4.1 COSFLOW model COSFLOW is a three dimensional finite element code which couples the simulation of the rock mechanical response with two phase flow and can be run in parallel on many computers. It employs the Cosserat Theory and has a special advantage in efficiently modelling stratified rock mass [1]. The finite element mesh was based on a 3D geological model of Guqiao Mine which was constructed using the data from 136 boreholes with help of geological software such as Minescape and Gocad. The rock mechanical properties were based on the results of laboratory tests on specimens collected one piezometer borehole. The final COSFLOW model is shown in Fig.7. It simulates a region of size m(length width height), and the mesh has elements. Fig.7. COSFLOW model and mesh. 8

9 4.2 Dynamic development of mining-induced stress. Fig.8 shows the modelled vertical stress distribution in the seam roof of Panel 1115(1) along the longwall face and along the panel length. (1) Along the panel length direction, the vertical stress shows a 4-stage process marked by increase decrease recovery - stabilisation (see Fig.8(a)). The abutment stress can extend up to 3m ahead of the face. The stresses reduction zone is -8m behind the face, followed by stress recovery zone until 15~2m behind the face. Farther than 2m is the stress stable zone. (2) Along the face direction, the abutment stress zone can also extend up to 3m outside the panel. The vertical stress in the central part of the panel recovers gradually with the mining retreat. However, a de-stressed zone remains at either side of the panel even after the completion of the mining (see Fig.8(b)). (3) There exists an annular shaped zone where rock mass is fully de-stressed (see Fig.8(c)), and its width reduces with increasing height into the roof (see Fig.8(d)) Stress/M Pa Roof 7m Roof 3m Roof 88m Roof 226m Ahead LW face/m -5 Behind LW face/m (a) Stress variation along the panel length direction 3 25 Stress/MPa m behind LW face 5m behind LW face 1m behind LW face 2m behind LW face 4m behind LW face Lateral location of LW/m (b) Stress variation along the face direction 9

10 (c) Stress distribution in a plane section 3m above face (d) Stress distribution in a cross-panel-length section Fig.8. Modelled vertical stress distribution in overburden strata. During the process of methane absorption-desorption in coal seams, methane at its maximum absorption status can be desorbed if the stress in coal seam is reduced below a critical value. σ z r = 1 σ z (1) where σ is the post-mining vertical stress, σ is the in situ vertical stress. z z Based on the COSFLOW modelling results, a conceptual model is constructed to demonstrate the stress reduction factor in the overburden strata of Panel 1115(1), see Fig.9. r =.2 r =.2 r =.5 r =.5 r =.8 r =.8 A-A A B-B r =.8 Startup B r =.2 LW face B A Fig.9. Conceptual model of stress reduction factor in overburden strata. 1

11 4.3 Permeability change of surrounding rock mass Whether effective gas extraction can be achieved will not only depend upon the degree of methane desorption but also the permeability of the coal seam and surrounding rock mass. Fig.1 shows the modelled horizontal and vertical permeability distribution in the overburden strata of Panel 1115(1). The horizontal permeability is predicted to increase significantly in the de-stressed zone. A horse-saddle shaped horizontal permeability distribution is predicted across the panel width, where the permeability in strata above the panel central region are predicted to increase by about 6 orders of magnitude, then decrease by an order of magnitude and finally stabilise as the face advances. Near the sides of the panel, however, the permeability is again predicted to increase by about 6 orders of magnitude and be relatively stable at that level (see Fig 1(a)). In a plan view, an annular region with high permeability can be seen (see Fig 1(b)). Fig 1(c) and Fig 1(d) indicate how the horizontal and vertical permeability changes 6m behind the face Orders changed m behind LW face 5m behind LW face 1m behind LW face 2m behind LW face 4m behind LW face Lateral location of LW/m (a) Horizontal permeability variation across the panel width (b) Horizontal permeability distribution in a plane section 8m above face (c) Horizontal permeability distribution in a cross-panel section 6m behind face (d) Vertical permeability distribution in a cross-panel section 6m behind face Fig.1. Modelled permeability distribution in overburden strata. 11

12 The change of permeability relates directly to mining-induced stress and fracturing. COSFLOW modelling results indicate that mining-induced fractures open and close following the stress decrease and recovery, causing permeability increase and decrease in the same manner. Two key aspects of the permeability in overburden strata are its magnitude and direction, both playing an important role in gas extraction. There exist zones with high inter-strata permeability and zones with high horizontal permeability. In the inter-strata permeable zones, subvertical and horizontal fractures are both well developed, and hence both the horizontal and vertical permeability are high, encouraging gas migration in both horizontal and vertical directions. In the horizontal permeable zone, bed separation fractures are dominant and the horizontal permeability is much higher than the vertical permeability. Gas will mainly flow horizontally in these zones. Based on the discussion above and the relationship between permeability and de-stressing and fracturing, a conceptual model is suggested to describe the characteristics of permeability distribution in the overburden strata of Panel 1115(1), see Fig.11. Horizontal permeable zone Cross-strata permeable zone A-A A B-B Start -up Fracture compacted zone B Horizontal permeable zone LW face B A Horizontal permeability Vertical permeability Fig.11. Conceptual model of permeability distribution in overburden fractured zone. 4.4 CFD modelling of gas migration Mining induced gas migration is a combined consequence of methane desorption, ventilation and gas drainage. To understand the process of mining-induced gas migration, a 3D numerical study was carried out using Computational Fluid Dynamics (CFD) technique. The study was built on the monitoring results and the COSFLOW results, and it was focused on the dynamic process of gas migration under various drainage conditions. 12

13 4.4.1 CFD model The CFD model includes Panel 1115(1) only, see Fig.12. The side boundaries have an inclination angle of 8, based on the permeability distribution from the COSFLOW model. 1m Coal 13-1 Goafside retained roadway Coal 11-2 Fig.12. CFD model and mesh. Tailgate LW face Maingate The input parameters including methane source, fluid parameters and ventilation parameters are based on the actual methane emission measurement and operational experience at Huainan Coal Field [11]. The modelled total methane release is 25 m 3 /min,of which 25% is from the longwall face and coal production, 5% from the remaining coal in the goaf, and 7% from the adjacent Seam The modelled total ventilation airflow is 272m 3 /min, of which 22m 3 /min is from the maingate and 52m 3 /min from the tailgate. The modelled permeability distribution is from COSFLOW results and CSIRO s past modelling experience [12]. The input parameters were further calibrated from gas monitoring measurements using bundle tubing and tracer element tests at Panel 1115(1) to increase the reliability of the modelling results Gas concentration The predicted methane concentration in Panel 1115(1) increases with the height, the distance behind the face and the lateral distance from the panel sides (i.e. roadways). Low methane concentration (<3%) exists in the vicinity of the longwall face and the roadways. The modelled methane distribution for Y type of ventilation is shown in Fig.13. Tailgate Tailgate outby Maingate (a) Plan view 13

14 25m Tailgate (b) Cross section Fig.13. Modelled methane concentration Gas migration under drainage Fig.14 shows the modeled migration pattern when gas is drained from a vertical borehole in the goaf. The key findings are: (1) gas flows mainly along the edge of the goaf zone, forming an annular flow channel; (2) gas released from adjacent coal seam flows horizontally at first to the edge of the goaf zone and then migrates through the annular flow channels. Tailgate outby Drainage borehole Start -up Flow rate m/s (a) Horizontal plane 5m above the goaf Coal 13-1 Coal 11-2 (b) Cross-panel width section 2m behind face Fig.14. Modelled gas migration pattern under drainage condition. 5. Annular Overlying Zone (AOZ) for optimal gas extraction in multi-seam mining. 5.1 Method to define the optimal zone for gas extraction 14

15 Optimal gas extraction relies on two key factors: constantly high gas flow and high methane concentration. To achieve such a goal, the drainage holes should be located in the zone with high methane desorption, high permeability and high methane concentration. Based on the understanding of overburden de-stressing, permeability distribution, and gas migration process under drainage, it is suggested that the following methods be used to define the optimal gas extraction zone: (1) High methane desorption: Determine the stress reduction for significant methane desorption in the coal seam under specific geological conditions; then use the conceptual destressing model to estimate the zone of high methane desorption. (2) High permeability: Define the fractured zone which is starting to consolidate, and then use the permeability distribution model to estimate the zones with high inter-strata permeability and/or horizontal permeability. The optimal zone is often confined by the top boundary of the horizontal permeable zone. (3) High methane concentration: Define the minimum required methane concentration in the drainage stream, and then based on the gas migration pattern, determine the zone for optimal gas extraction. The above steps are considered to be a practical method for determining the optimal gas extraction zone for multi-seam mining. 5.2 Determination of optimal gas extraction zone in overburden strata of Panel 1115(1) Estimate of critical stress reduction factors Seam 13-1 which overlays the mining seam at Panel 1115(1) has a methane content of 5.7m 3 /t and a gas pressure of 1.4~5.9MPa. The isothermal absorption curve shows that methane desorption occurs only when the pore gas pressure is reduced to below.7mpa. Hence, the required stress reduction in this seam is high for gas extraction. Based on the past experience at Huainan [11] and COSFLOW modelling results, it is estimated that the critical stress reduction factor for Seam 13-1 is.8, which implies that the in situ vertical stress at the seam level needs to be reduced by 8% for efficient gas extraction. Mining-induced fractures at a distance of more than 17m behind the longwall face in Panel 1115(1) are mostly closed due to consolidation. The stress reduction factor at this location is about.2. This value can be considered as the threshold value for consolidation Estimate of the upper boundary of the optimal gas extraction zone 15

16 The field monitoring measurements showed that the fractured zone can extend to a height of 145m from the mining roof. The COSFLOW modelling results indicate that the zone with a stress reduction factor of.8 is about 139~16m above the mining seam. The zone with high horizontal permeability is about 15m. From both the field measurements and modelling results, it is estimated that the upper boundary of the optimal gas extraction zone is 145m above Seam Estimate of the lower boundary of the optimal gas extraction zone CFD modelling results suggest that the zone with low methane concentration (<3%) under drainage can extend 3m above the mining seam. The COSFLOW model indicates that the zone with high inter-strata permeability can extend up to 54m. It is estimated that the lower boundary of the optimal gas extraction zone is 3m above the mining seam Estimate of side boundaries of the optimal gas extraction zone Using the critical stress reduction factors of.8 for drainage and.2 for consolidation, and based on COSFLOW modelling results and the conceptual model of overburden de-stressing, it is estimated that the angle of de-stressed zone is 75 in the mining direction and 78 along the face direction. The angle of consolidation zone is 79 in the mining direction and 91 along the face direction. The optimal gas extraction zone has a length of 17m along the panel length and 75m along the panel width at the goaf level. 5.3 Annular Overlying Zone (AOZ) for optimal gas extraction in multi-seam mining. Based on the discussion in Section 4.2, a zone for optimal gas extraction in multi-seam mining can be defined as shown in Fig.15. This zone has an annular shape and is confined in height and width and bounded by sides with certain inclination angle. This zone is named Annular Overlying Zone (AOZ). 16

17 17-2 Start-up Ⅱ Ⅲ Rear section Central section Ⅱ 145m 3m 78 o 23m Ⅰ-Ⅰ 78 o Ⅰ Tailgate Front section Ⅲ LW face Ⅰ Maingate 78 o 91 o 75m Ⅱ-Ⅱ 17m Front section Ⅲ-Ⅲ o 78 o m 75 o 79 o o 78 o 75m Rear section Fig.15 Annular overlying zone for optimal gas extraction at Panel 1115(1). The Annular Overlying Zone (AOZ) has the following characteristics. (1) Along the panel length, it can be divided into three sections: front, central and rear sections. The front section, adjacent to the longwall face, is the key region where major methane desorption occurs. The central section is linked with the front and rear section from its sides which are the major flow channel for large scale gas migration. The rear section is in the vicinity of longwall installation road, and is smaller in size than the front section and similar to the central section in shape. (2) In the cross section along the panel length, the AOZ is confined by inclined side boundaries. The internal and external side boundaries have different inclination angles. (3) In the cross section along panel width, the AOZ is located at a certain height from the goaf, and is confined by inclined side boundaries. The front section is larger than the rear section. The concept of the AOZ provides a systematic and visualised definition of the optimal gas extraction zone in the overburden strata. It can be applied to Panel 1115(1) of Guqiao and other mines with similar condition to guide the co-extraction operations Parameters of AOZ for optimal gas extraction in multi-seam mining The parameters of the AOZ should be determined by means of field monitoring, laboratory tests, numerical modelling and the specific geological and mining conditions. 17

18 a. The height of lower boundary: It can be determined from the methane concentration from the drainage operations. It is often located at the mid-height of the zone of high inter-strata permeability. b. The height of upper boundary: It can be determined from the height of de-stressed zone and that of high horizontal permeability. c. Inclination angle of external side boundaries: They can be estimated from the shape of destressed zone. d. Length and width of AOZ and internal side boundaries. They can be estimated from the shape of consolidation zone. 6. Conclusions In this study we have taken Panel 1115(1) of Quqiao Mine as the experimental site and conducted a systematic investigation by means of field monitoring, numerical modelling and theoretical analysis. The key achievements from this study are listed below. (1) For the first time in a Chinese mine, we have obtained the systematic data of mininginduced stress change, overburden strata movement, pore pressure change and gas migration. They provide a base for developing the theoretical system of the co-extraction of coal and gas in Seam 11-2 at Quqiao Mine. The key findings include: 1) The influence zone of abutment stress can extend 3m from the panel; strata movement and mining-induced fracturing occur within 17m behind the longwall face, and farther away from 17m rock fractures are likely to be compacted. 2) The height of the fractured zone can extend up to 145m above the seam roof. Pore pressure reduces significantly within this zone. (2) We have systematically investigated the mining-induced stresses, rock fracturing, gas migration and the dynamic interaction between them. The key findings include: 1) the processes of overburden de-stressing is better understood, and a conceptual model is developed; 2) the zone of high inter-strata permeability and the zone of high horizontal permeability are defined; 3) There exists an annular gas flow channel. High concentration methane is mainly distributed in the middle and upper part of this channel. (3) We have determined the characteristics of overburden de-stressing, permeability distribution and the pattern of gas migration, which are the key factors controlling gas extraction. Based on the detailed study of the zone for optimal gas extraction in the overburden strata of Panel 1115(1), we have developed the Annular Overlaying Zone concept and suggested methods to determine its geometry and location. 18

19 The Annular Overlaying Zone for multi-seam mining was developed based on the specific mining condition of Seam 11-2 at Guqiao Mine. We are currently conducting similar studies for different mining conditions in both Huainan and Australia, aiming to develop a more universal theory and design guidelines for co-extraction of coal and gas. References [1] Guo, H., Ishihara, N., Fujioka, M., et al. Integrated Simulation of Deep Coal Seam Mining Optimisation of Mining and Gas Management. In: Australia Japan Technology Exchange Workshop in Coal Mining 21, Hunter Valley, 21. [2] Guo, H., Balusu, R., Adhikary, D.P. Coal mine gas drainage and recent developments in Australia. In: China International Conference on Coal Mine Gas Control and Utilisation, Huainan, 28. [3] Guo, H., Mallett, C., Xue, S., et al. Predevelopment Studies for Mine Methane Management and Utilisation. Exploration and Mining Report 699C, Brisbane, Australia, 2. [4] Romeo M. Flores. Coalbed methane: From hazard to resource. International Journal of Coal Geology 1998; 35:3~26. [5] Carol J.Bibler, James S. Marshall, Raymond C.Pilcher. Status of worldwide coal mine methane emissions and use. International Journal of Coal Geology 1998;35: [6] QIAN Ming-gao, XU Jia-lin, MIAO Xie-xing. Green techniques in coal mining. Journal of China University of Mining and Technology 23; 32: [7] QIAN Minggao, XU Jialin. Study on the O shaped circle distribution characteristics of mining induced fractures in the overlaying strata. Journal of China Coal Society 1998; 23: [8] YUAN Liang. Theory and practice of integrated pillarless coal production and methane extraction in multiseams of low permeability. Engineering Scineces 29; 11:72-8. [9] YUAN Liang. The technique of coal mining and gas extraction by roadway retaining and borehole drilling. Journal of China Coal Society 28;33: [1] Guo, H., Adhikary, D.P., Craig, M.S. Simulation of mine water inflow and gas emission during longwall mining. Rock Mechanics and Rock Engineering 29; 42: [11] YUAN Liang. Theory and technology of gas drainage and capture in soft multiple coal seams of low permeability. China Coal Industry Publishing House, Beijing, 24. [12] Ren, T. X., Balusu, R. CFD modelling of goaf gas migration for control of spontaneous combustion in longwalls. Journal of the Australasian Institute of Mining and Metallurgy 25; 6:

20 Figure Captions Fig.2. Plane view of Panel 1115(1) and its geological setting Fig.2. Integrated real-time monitoring plan at Panel 1115(1) Fig.3. Measured stress change in roadway roof Fig.4. Measured relative displacement in roadway roof Fig.5. Measured overburden displacements Fig.6. Measured pore pressure change and its relationship with rock fracturing characteristics Fig.7. COSFLOW model and mesh Fig.8. Modelled vertical stress variation in overburden strata Fig.9. Conceptual model of stress reduction factor in overburden strata Fig.1. Modelled permeability distribution in overburden strata Fig.11. Conceptual model of permeability distribution in overburden fractured zone Fig.12. CFD model and mesh Fig.13. Modelled methane concentration Fig.14. Modelled gas migration pattern under drainage condition Fig.15 Annular overlying zone for optimal gas extraction at Panel 1115(1) 2