International Journal of Rock Mechanics & Mining Sciences 78 (2015) 91–98

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International Journal ofRock Mechanics & Mining Sciences

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Three-zone characterisation of coupled strata and gas behaviour inmulti-seam mining

Qingdong Qu a, Jialin Xu b,c,n, Renlun Wud, Wei Qin b,c, Guozhong Hu b,c

a Coal Mining Research Program, CSIRO Energy, Pullenvale, Queensland 4069, Australiab State Key Laboratory of Coal Resource and Mine Safety, China University of Mining & Technology, Xuzhou, Jiangsu 221116, Chinac School of Mining Engineering, China University of Mining & Technology, Xuzhou, Jiangsu 221116, Chinad School of Resource and Safety Engineering, China University of Mining & Technology (Beijing), Beijing 100083, China

a r t i c l e i n f o

Article history:Received 10 November 2014Received in revised form14 April 2015Accepted 19 April 2015Available online 18 June 2015

Keywords:Co-extraction of coal and gasGas drainageGas migrationFractureKey stratumGreen mining

x.doi.org/10.1016/j.ijrmms.2015.04.01809/& 2015 Elsevier Ltd. All rights reserved.

esponding author at: State Key Laboratoryhina University of Mining & Technology, Xuzail address: cumtxjl@cumt.edu.cn (J. Xu).

a b s t r a c t

We propose here a three-zone conceptual model in overlying strata of a longwall panel that accounts forthe coupled behaviour of strata deformation and gas flow. The model comprises a fractured gas-interflowzone, a de-stressed gas-desorption zone, and a confined gas-adsorption zone. The fractured gas-interflowzone represents the area where mining-induced cross-strata fractures and bedding separations are welldeveloped with high permeability in both the vertical and horizontal directions. Coal seam gas can easilybe released from this lower zone to flow down into the mine workings. The de-stressed gas-desorptionzone, which lies above the fractured gas-interflow zone, is another significant gas-producing zone inwhich strata are highly de-stressed. However, mining-induced fractures in this zone are mainly createdin the form of bedding separations, which only increase horizontal permeability, and thus the gas cannoteasily flow vertically down to the mine workings. In the upper confined gas-adsorption zone, stratadepressurisation is limited; the major proportion of coal seam gas in this zone remains adsorbed andcannot be effectively captured. While both lower zones are the targets of gas drainage, the fractured gas-interflow zone is the main source of ventilation gas emission and the prime area of gas control. We havedeveloped an approach to determine the height of these three zones based on the hypothesis of keystratum in strata movement, and verified the approach using gas drainage experience at a Chinese coalmine. The applications of the three-zone concept in selecting appropriate gas drainage methods forvaried mining conditions, assessment of methane recovery efficiency, and gas drainage optimisation andmaximisation in a mining district of China are also discussed.

& 2015 Elsevier Ltd. All rights reserved.

1. Introduction

China is the world's largest coal producer, with an annualproduction of more than 3.5 billion tonnes in recent years. How-ever, a large proportion of China's coal mines produce massiveamounts of gas emissions, due to gassy conditions and multiplecoal seams within the mines. Because coal seams in China aregenerally of extremely low permeability and have low gas sa-turation [1], gas control and management are very difficult. Withdecades of gas control experience, China's coal mining industryhas found that the most effective solution is post gas drainage.Since coal seam methane is a clean energy source, co-extraction ofcoal and coal mine methane has been widely practiced in China inthe last decade [2–5]. This has improved mining safety,

of Coal Resource and Minehou, Jiangsu 221116, China.

productivity and gas utilisation. In 2012, about 10.3 billion cubicmetres of methane were extracted by Chinese coal mines, whichaccounted for about 81.7% of total coalbed methane production.However, gas drainage quality is often poor, with issues such aslow and unstable gas flow, low methane purity and low methanedrainage efficiency.

The co-extraction system integrates and harmonises coal pro-duction, gas drainage and gas utilisation. It is fundamentally de-signed to take advantage of the significant depressurisation ofsurrounding coal seams and increase of strata permeability causedby the action of mining, thereby enabling efficient gas drainagefrom low-permeability coal seams. Efficient implementation ofthis system relies on clear understanding of the coupled strata andgas behaviour in response to mining.

The overlying strata of a goaf are often categorised into variouszones with respect to characteristics of strata break and de-formation. Fig. 1 shows a conceptual model of strata zoning that iswidely used in China [6]. In the vertical direction, the overlayingstrata are divided into the caved zone (I), fractured water-inflow

Table 1Empirical expressions between the height of overlying fractured zone (Hd) and themining thickness (M) used in China [13].

Strength rank of overlying strata Height of the fractured zone (m)

Strong H 8.9dM

M100

1.2 2.0= ±∑

∑ +

Medium strong H 5.6dM

M100

1.6 3.6= ±∑

∑ +

Weak H 4.0dM

M100

3.1 5 . 0= ±∑

∑ +

Very weak H 3.0dM

M100

5 8 . 0= +∑

∑ +

Fig. 1. Conceptual model of mining-induced overburden deformation zones usedin China [6].

Q. Qu et al. / International Journal of Rock Mechanics & Mining Sciences 78 (2015) 91–9892

zone (II) and bending zone (III). In the direction of mining advance,the overlying strata are divided into the coal pillar supporting zone(A), separation zone (B) and re-compacted zone (C). Similar zoningconcepts can be found in many other countries [7–12]. Fig. 2shows another conceptual model, which divides the overburdeninto the caving zone (A), fracture zone (B), dilated zone (C) andconfined zone (D) [12]. The height of the overburden zones is oftenexpressed and estimated as a function of mining thickness[7,10,13]. Table 1 shows the empirical expressions of the height ofthe fractured water-inflow zone that are widely used in China [13].This simple method is commonly used in coal mines because itrequires no site measurements or computer simulations.

Coal seam gas flows in the goaf can be investigated by ap-proaches such as coupled modelling, computational fluid dy-namics simulations, site monitoring with a tube bundle system,and tests with tracer gases [11,14–18]. Although these approachescan provide detailed information about gas flow directions andvelocity, they cannot be easily adopted by coal mines. Instead, gasflows are often analysed by characterising mining-induced frac-tures and their distributions, since they differ in various zones interms of their density, opening direction and aperture. Fracturesthat are interconnected with mine workings, as described in [9],form channels for rapid flow of gas and water. On a plan section,the distribution of mining-induced fractures is described as an ‘O-shaped’ zone, which is likened to a ‘gas river’ [14]. To achieve acontinuous, high flow rate, gas drainage boreholes are thereforesuggested to be located in this zone. This kind of analysis is basedon assumptions that gas migration along mining-induced fracturesdominates the flow, and ignores gas migration through the in-situporous and fracture networks.

Post gas drainage is often designed and implemented followingthe three-zone concept of strata deformation (Fig. 1) and fracturedistribution characteristics. Since the fractured water-inflow zonerepresents the area where water can flow down into the workings,

Fig. 2. Overburden response to full extraction of mining [12].

it is also regarded as the zone of methane emission to the mineworkings and the prime target of gas drainage. However, from aresource development point of view, the concept of strata de-formation zones is incomplete. It does not fully provide guidancefor gas drainage optimisation and maximisation, because it doesnot directly indicate gas flow characteristics and gas drainability ofall zones. As a consequence, gas drainage is often targeted to thefractured water-inflow zone only, and coal seams beyond this zoneare neglected – disregarding whether or not they are drainable. Animproved approach to characterising both the strata and gas be-haviours would avoid these shortcomings and meet the goals ofthe co-extraction system.

2. A conceptual model of coupled strata and gas behaviours

Efficient co-extraction of coal and methane should effectivelycontrol gas to maximise coal production in a safety environment,and also maximise the capture of high-quality gas for further use.To achieve these goals, two aspects need to be essentially andclearly understood and characterised: (a) the extent to which coalseam gas can be effectively depressurised and released and (b) thedominative gas flow direction in the goaf and fractured strata.

The first aspect determines the coal seams from which largeamounts of gas can be desorbed and released. Gas desorption inresponse to mining is a consequence of gas depressurisation,which can be caused by either strata de-stressing or pore waterdraining. The highly de-stressed zone is often higher than thefractured water-inflow zone. In [19], the height with a verticalstress reduction level of more than 80% was about 160 m at alongwall panel: much higher than the estimated fractured water-interflow zone (60 m) according to empirical experience in themining area (i.e., 20 times the mining thickness). In unsaturatedcoal seam conditions such as those in China, a high degree of coalseam de-stressing is required to enable a high level of gasdesorption.

The second aspect reflects the extent to which the released gasfrom coal seams can flow down into the mine workings. This ex-tent is the major target of gas drainage for intercepting gas fromflowing into mine workings. It also reveals where methane con-centration can be diluted by ventilation air ingress. Although goafgas flow directions are also affected by many factors, such as re-servoir gas pressure, gas drainage and ventilation, permeabilitydominates. Reference [19] reveals the interactions betweenmining-induced stress, fracture and permeability changes. Theseshow that (a) the highly horizontal permeable zone is muchhigher than the highly vertical permeable zone, as shown in Fig. 3;and (b) the change of permeability relates directly to mining-in-duced stress changes and fractures.

Based on the above analysis, we propose a new conceptualmodel characterising both strata and gas behaviours, as shown inFig. 4. We call this model the ‘gas three-zone’ model to differ-entiate it from conventional models. The three zones characterised

Fig. 3. Mining-induced changes of (a) horizontal permeability and (b) vertical permeability about a longwall panel [19]. The legend is in orders of magnitude.

Q. Qu et al. / International Journal of Rock Mechanics & Mining Sciences 78 (2015) 91–98 93

in the overlying strata are the ‘fractured gas-interflow zone’ (I),‘de-stressed gas-desorption zone’ (II) and ‘confined gas-adsorptionzone’ (III). The characteristics of each zone are summarised below.

In the fractured gas-interflow zone, coal seams and rock strataare greatly de-stressed, and are rich in mining-induced fracturesthat are interconnected in both vertical and horizontal directions.Methane adsorbed in the coal seams within this zone can bedesorbed as a result of both de-stressing and water draining, andcan easily flow down into the mine workings through inter-connected fractures. The gas flow velocity within this zone ishigher than in other zones.

In the de-stressed gas-desorption zone, strata are significantlyde-stressed, and bed separations and rock dilation occur. Cross-strata fractures are rarely developed in this zone. Methane in coalseams within this zone can be desorbed to some degree as a resultof de-stressing, and gas mainly flows in the horizontal direction. Ifnot drained, the desorbed methane can be re-adsorbed when porepressure recovers due to goaf re-compaction.

In the confined gas-adsorption zone, strata undergo neither de-stressing nor fracturing. The methane in this zone maintains itsstatus of adsorption in the coal matrix.

The significance of the gas three-zone characterisation is that itclearly differentiates the fractured gas-interflow zone from the de-stressed gas-desorption zone. Both are methane-producing zones;however, the fractured gas-interflow zone represents where gascan flow vertically, and therefore is the major source of ventilationmethane emission. It is also a potential low-methane concentra-tion zone into which ventilation air can flush. In comparison, thede-stressed gas-desorption zone has different fracture and gasflow patterns. Methane produced from this zone does not flowdown easily into mine workings, and thus is not a source of ven-tilation methane emissions. For this reason, the fractured gas-

Coal seam

Coal seam

Coal seam

Mining seam

I: fractured gas-interflow zone; II: de-stressed gas-desorIII: confined gas-adsorption zone

Fig. 4. Three-zone characterisation for coupled strata

interflow zone is often neglected when designing gas drainage.Not all three gas zones may exist in a particular mining panel,

depending on mining and geological conditions such as miningdepth, thickness, panel dimension, stratigraphy, in-situ gas inplace, and in-situ permeability and porosity of coal seams. Inshallow mining, the confined gas-adsorption zone is not likely toexist, since the in-situ permeability and porosity in the coal seamsand surrounding strata are generally high due to natural cleats andfractures. Adsorbed gas in such conditions would be easily re-leased, even with a minor degree of strata de-stressing or hydro-static pressure release, and migrate through the in-situ inter-connections. In deep and thin mining, particularly with low per-meability and low saturation of gas adsorption, the de-stressedgas-desorption zone may not exist, because gas desorption re-quires a relatively high degree of depressurisation.

3. Estimation of the height of the three gas zones

To adopt the gas three-zone concept in gas drainage design, apractical and reliable method to discriminate the zones and esti-mate their extents is essential. Empirical methods, such as themethod shown in Table 1, are one option to estimate the fracturedgas-interflow zone, though they sometimes underestimate theextent. The hypothesis of key stratum or strata (KS) in overburdenstrata movement, developed for studying mining-induced stratamovement and fracture development, provides an alternative ap-proach to characterise strata de-stressing and fracturing patternswith detailed stratigraphy taken into consideration. Previous stu-dies [20,21] have revealed that gas emissions dynamically increasewith consequent breaks of KS, indicating the feasibility of usingthe KS hypothesis to characterise gas behaviours along with strata

No obvious mining-induced fracturesMethane not depressurised and desorbedMethane difficult to capture

Bedding separation and strata dilation occursMethane significantly depressurised and desorbedMethane can be captured within a certain timeMethane hardly migrates down into workingsHigh methane purity

Interconnected fractures in vertical and horizontalMethane largely depressurised and desorbedMethane can be effectively capturedMajor source of ventilation gas makeMethane purity may be low due to air ingress

ption zone;

deformation and gas flow in multi-seam mining.

Q. Qu et al. / International Journal of Rock Mechanics & Mining Sciences 78 (2015) 91–9894

fractures.

3.1. The key stratum (KS) hypothesis

Due to the variation of diagenetic time and mineral composi-tion, stratified rock masses exhibit highly anisotropic strength andvarious thicknesses. A key stratum is defined as a rock layer thatcan bear the weight of part or all of its overlying strata, and play acontrolling role in the deformation and break of these strata [21].The role of controlling means the strata deform simultaneouslyand in coordination with the KS. One or more KS may exist in theentire overlying strata of a mining panel, with the top stratumreferred to the primary key stratum (PKS), because it controls theentire strata from itself up to the ground surface. Overburdenstrata behave with a stepwise movement, with breaks of each KS.When the lowest KS breaks, its controlled strata break simulta-neously, resulting in bedding separation forming underneath thenext KS. This process repeats with each breach of the KS until thePKS. No bedding separations will be formed above the PKS, be-cause the whole overlying strata deform in coordination with it.Fig. 5 illustrates the rule of strata movement with three KS above amining panel.

KS can be identified by a computer programme named KSPB[22]. This enables practical application of the KS hypothesis inresearch and engineering fields. The main identifying criteria of aKS are its deforming and breaking characteristics, as describedabove, which are expressed as the following stiffness and strengthconditions [21]:

q q 1n n1 1 1( ) > ( ) ( )+

l l 2n1 1< ( )+

where qn 1( ) means the stress imposed on the bottom layer underan assumption that a number of n layers deform in coordination,i.e. the n 1+ layer is another KS; l is the periodic breaking span of arock layer caused by mining; and qn 1( ) is calculated based on thecomposite beam theory and is expressed as [21]

qE h h

E h 3n

in

i i

in

i i1

1 13

1

13

γ( ) =

∑

∑ ( )=

=

where Ei, hi and iγ are elastic modulus, thickness and specificweight of layer i, respectively.

Stratigraphic and geomechanical information at or near alongwall panel are required to use KSPB. The input parameters ofeach rock layer include lithology, thickness, density, elastic mod-ulus and compressive strength. The KSPB runs calculations andoutputs the locations of the identified KS, along with a column ofthe stratigraphic section.

3.2. Estimation of the height of fractured gas-interflow zone

Previous studies [23,24] on the effect of KS on fractured water-interflow zone revealed that if a KS was vertically fractured by

PKS

KS2

KS1

Fig. 5. Progress of strata movement under the effect of key strata (KS) showing (a

mining, then the overlying strata controlled by the KS will also bevertically fractured, and a KS located within �7–10 times themining thickness above the mining seam will be vertically frac-tured. An estimation method of fractured water-interflow zonewas developed based on this understanding [24]. These results arethen employed to estimate the height of the fractured gas-inter-flow zone. An estimation criterion is established and is that thezone extends up to the floor level of the first KS located higherthan 7–10 times the mining thickness above the mining seam. Thevalues of 7, 8, 9, and 10 times the mining thickness are speciallydefined in accordance with the overburden strength classificationof very weak, weak, medium strong and strong (as shown in Ta-ble 1), with mining thickness, panel width and mining methodtaken into consideration. The estimation procedure comprises thefollowing steps:

Step 1: Collect borehole logs and rock properties of overlyingrock units at or near the panel of interest and characterise theoverburden strength level.Step 2: Use KSPB to identify KS in the overlying strata on thebasis of borehole logs and rock properties.Step 3: Work out the height of each KS above the mining seamand the number of times of mining thickness.Step 4: Determine the height of the fractured gas-inflow zoneaccording to the estimation criterion; i.e., the fractured gas-interflow zone extends up to the floor level of the first KS lo-cated higher than 7–10 times the mining thickness above themining seam.

3.3. Estimation of the height of de-stressed gas-desorption zone

The de-stressed gas-desorption zone is the area where a largeportion of the overburden weight is removed. According to the KShypothesis, bedding separations and dilation occur in the strataunderneath the lowest KS that is not broken. In critical and su-percritical mining conditions, strata would break up to the top-most layer of the rock body of a panel, suggesting strata de-stressing would extend to the PKS. Thus, the height of the de-stressed gas-desorption zone in critical or supercritical longwallpanels is estimated up to the PKS. Its estimation procedure followssteps 1–3 as described in Section 3.2.

All coal seams located above the PKS in critical and super-critical mining conditions are within the confined gas-adsorptionzone. The height of these zones dynamically changes with miningadvance, and estimation using the above methods results in theirmaximum values.

4. Verification of the estimation methods

The estimation methods were verified with gas drainage ex-perience gained at a longwall panel in Yangquan Mining Group(YMG), China. The panel, K8206, was 1579 m long, 252 mwide and

PKS

KS2

KS1

PKS

KS2

KS1

) break of KS1, (b) break of KS2, and (c) break of primary key stratum (PKS).

Q. Qu et al. / International Journal of Rock Mechanics & Mining Sciences 78 (2015) 91–98 95

approximately 500 m deep. It excavated coal from Seam #15,which was 7.0 m thick on average and had a dipping angle of about5°. The gas content of this coal seam was about 7.13 m3/t, and gaspressure ranged from 0.2 to 0.3 MPa. The full thickness of the seamwas extracted using the top coal caving method. A total of 10 gas-bearing layers overlaid the mining seam (Fig. 6(a)), including fivecoal seams (#13, #12, #11, #8, and #3) and five limestone layers(K2, K3, K4, K6, and K7). No coal seams and gas-bearing rock unitsunderlaid the mining seam that contribute to longwall gas emis-sions. A gas drainage tunnel was developed along the roof of Seam#11 and connected to the longwall installation roadway throughan angled tunnel. The ventilation system was of the ‘U’ type, withan additional return airway developed along the roof of themining seam and offset by 30 m from the main ventilation return.

Four KS in overlying strata were identified using KSPB, basedon the stratigraphic column shown in Fig. 6(a). Their heights abovethe mining seam are 6.8 m (KS-1), 14.3 m (KS-2), 66.2 m (KS-3)and 145 m (PKS), and their relative heights to mining thickness are1.0, 2.1, 9.5 and 20.7 times, respectively. Given that sandstone,limestone and siltstone layers predominate the overlying strata,and that the longwall used the top coal caving method to extractthe full thickness of the 7.0-m-thick seam, a critical value of 10times of mining thickness is defined to identify which KS are lo-cated within the fractured gas-interflow zone. Based on thesedeterminations and the definitions and criteria given in Sections3.2 and 3.3, the fractured gas-interflow zone extends up to thefloor level of the PKS, which is 145 m above the mining seam, andno de-stressed gas-desorption zone exists. Overlying coal seams#12, #11, #8, and #3 are all within the fractured gas-interflowzone. Using the empirical estimation method shown in Table 1, thefractured gas-interflow zone is only about 58–76 m above themining seam, and Seam #3 is far beyond the fractured zone.

The methane-in-place of overlying coal seams and the lime-stone units within the fractured gas-interflow zone was assessed

Order Thickness(m) Depth(m) Lithology Position 50 340.34 340.34 Simplified load layer49 5.98 346.32 Siltstone48 4.39 350.71 Coarse Sandstone47 0.98 351.69 Siltstone46 3.33 355.02 Fine sandstone45 6.72 361.74 Siltstone44 4.00 365.74 Coarse Sandstone43 1.50 367.24 Siltstone42 6.79 374.03 Fine sandstone PKS41 1.00 375.03 Siltstone40 4.00 379.03 Medium sandstone39 1.45 380.48 Coal seam #338 1.70 382.18 Siltstone37 4.39 386.57 Mudstone36 4.80 391.37 Siltstone35 4.58 395.95 Siltstone34 5.14 401.09 Medium sandstone33 1.20 402.29 Siltstone32 4.60 406.89 Medium sandstone(K7)31 2.35 409.24 Siltstone30 2.67 411.91 Mudstone29 1.64 413.55 Siltstone28 6.80 420.35 Mudstone27 3.40 423.75 Mudstone26 1.50 425.25 Coal seam #825 2.03 427.28 Mudstone24 1.25 428.53 Mudstone23 2.50 431.03 Fine sandstone22 6.50 437.53 Coarse Sandstone21 0.60 438.13 Mudstone20 3.00 441.13 Medium sandstone(K6)19 11.74 452.87 Siltstone KS-318 2.44 455.31 Limestone(K4)17 0.30 455.61 Coal seam #1116 2.36 457.97 Medium sandstone15 5.34 463.31 Mudstone14 1.60 464.91 Coal seam #1213 1.20 466.11 Fine sandstone12 1.80 467.91 Limestone(K3)11 0.15 468.06 Coal seam #1310 8.30 476.36 Siltstone9 16.00 492.36 Coarse Sandstone8 0.30 492.66 Mudstone7 12.00 504.66 Coarse Sandstone KS-26 1.80 506.46 Mudstone5 5.74 512.2 Limestone KS-14 3.37 515.57 Mudstone3 2.31 517.88 Limestone(K2)2 1.15 519.03 Mudstone1 7.00 526.03 Coal seam #15

Fractured gas-interflow zone, 145 m

Confined gas-adsorption zone

Fig. 6. (a) Stratigraphic section of K8206 and the identified gas three zones, and

at about 350.5 m3/m2. Fig. 7 shows the methane flow rates in thegas drainage tunnel and the ventilation returns. The first 330 m ofretreat was not included in the verification for the vacuum pumpand the drainage pipeline did not work normally. A high drainageflow rate in the ‘Normal gas drainage period’ was maintained, andpeaked at 186 m3/min. According to this flow rate, gas capturedfrom the fractured gas-interflow zone was about 269.0 m3/m2 ofoverlying coal seams, or 76.8% of the estimated methane-in-place(350.5 m3/m2). This high extraction ratio indicates that the inter-connected fractures should have developed throughout the entirezone, suggesting the estimation method is effective and more re-liable than the empirical method.

Another phenomenon further verifying the effectiveness of thezones' height is methane emissions in the roadways of PanelK7209 in Seam #3, 138 m above Seam #15. Seam #3, which had agas content of 32.4 m3/t and a gas pressure of about 1.3 MPa, had ahigh risk of coal and gas outburst. Its permeability was extremelylow: about 0.00925�10�3 mD. Panel K7209 was perpendicular toPanel K8206, as shown in Fig. 7, and the overlapping area of K8206was not excavated. When the K8206 face advanced past K7209boundary by about 16.1 m, the methane concentration in K7209'sroadways increased immediately, and could not be effectively di-luted by ventilation. This resulted in the sealing of K7209 road-ways to draw gas, with a methane flow rate of up to 35 m3/min(Fig. 8). Later, when K7209 was in operation, the relative gasemission in the overlapping area was 8.99 m3/t, or 27.4% of the in-situ methane content of Seam #3. No outburst occurred whenmining this area and the daily coal production averaged 1393 t.Beyond this overlapping area, a number of small outbursts oc-curred, and the average daily coal production was more thanhalved to 563 t.

5. Applications of the gas three-zone in Yangquan Mining

Start-up

Auxiliary return

Main return

Gas drainage tunnel

50-6

0 m

(b) cross-section of the layout of the gas drainage and ventilation system.

Fig. 7. Drainage methane flow rate and methane emission rate in the ventilation returns at K8206. Panel K8206 extracted coal from Seam #15. Panel K7209 and K7207extracted coal from Seam #3, 138 m above Seam #15.

0

5

10

15

20

25

30

35

40

-60 -40 -20 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400

Flow

rate

, m.m

in

Dist past K7209 MG, m

Fig. 8. Methane emission in K7209 roadways when undermined by K8206.

Q. Qu et al. / International Journal of Rock Mechanics & Mining Sciences 78 (2015) 91–9896

Group

YMG is located in the Shanxi Province of China. It has gassy,multiple coal seams of low permeability. The major coal measurestrata in the mining area contain 16 coal seams, with a totalthickness of 13–15 m, a depth of 150–500 m, and a dip angle of 5–10°. All the coal seams are rich in gas, with gas content varyingbetween 7.13 and 32.41 m3/t. Coal seam gas pressure ranges from0.2 to 2.3 MPa, and some of the coal seams are prone to coal andgas outburst risks. Gas pre-drainage is not feasible, due to theextremely low permeability.

5.1. Selection of appropriate gas drainage methods

The two major gas drainage methods used in YMG are gasdrainage tunnels and cross-measure boreholes. The ‘UþL’ venti-lation system is generally adopted with cross-measure boreholes,and the ‘Uþ I’ system runs along with an upper gas drainagetunnel, as shown in Fig. 9. The ‘UþL’ and ‘Uþ I’ are named withrespect to the relative location of the auxiliary return to the mainreturn. Although gas drainage tunnels are more effective thancross-measure boreholes, particularly at the panels mining Seam#15, development of a tunnel is time consuming, costly and noteconomically feasible for all panels. Cross-measure boreholes arepreferred if their capacity meets the gas drainage requirements.

A key component of gas control in YMG is the selection be-tween these two gas drainage methods. The conventional selec-tion method, which is based on experience of gas drainage in oldmining leases, is ineffective in current longwall panels, whichdiffer significantly in terms of mining depth, panel width and gasreservoir parameters. Inappropriate selection can result in ex-cessive methane emissions to ventilation, extra drilling works, orlow-quality drainage gas for further use.

The appropriate selection of a gas drainage method relies uponan effective estimation of all gas emission sources and theiremission levels. This can be better assessed by the gas three-zoneconcept than previous gas drainage experience, and has been de-monstrated to be effective in several coal mines in YMG. Assess-ment using the gas three-zone concept allows discrimination ofthe height of each zone and estimation of methane emission ratefrom the fractured gas-interflow zone. If the estimated methaneemission rate is greater than 100 m3/min, a gas drainage tunnelmust be chosen; otherwise, cross-measure boreholes can be se-lected. The critical value of 100 m3/min was determined from gasdrainage performance and capacity of the two gas drainagemethods practiced at a large number of panels in YMG.

5.2. New factors to assess methane recovery efficiency

Gas drainage efficiency is generally used as a factor to assessgas drainage effectiveness in gas control. It is expressed as thepercentage of drainage methane volume of the total panel emis-sions (the sum of the drainage methane and ventilation methane).From the co-extraction point of view, this type of assessment doesnot reflect the degrees of gas release from coal seams, and is notconsidered suitable for the assessment of methane recovery ef-fectiveness. In addition, this assessment gives no information todetermine which coal seams need better gas drainage if overall gascontrol is not effective. We suggest the following new factorsbased on the gas three-zone concept:

rQ

GIP100%

4f

f

f= ×

( )

Intake Main return

Aux return

Gas drainage tunnel

Plan section

Vertical section

Longwall retreat direction

Gas

dra

inag

e tu

nnel

Aux

retu

rn

Mai

n re

turn

Intake

Intake Aux return Main return

Plan section

Vertical section

Longwall retreat direction

Main return

Ret

urn

Intake

Cross-measure boreholes

Cross-measure boreholes

Aux return

Fig. 9. Typical gas drainage and ventilation systems used in Yangquan Mining Group; (a) gas drainage tunnel plus ‘Uþ I’ ventilation and (b) cross-measure boreholes plus‘UþL’ ventilation.

Q. Qu et al. / International Journal of Rock Mechanics & Mining Sciences 78 (2015) 91–98 97

rQ

GIP100%

5d

d

d= ×

( )

rQ Q

GIP GIP100%

6

f d

f d=

++

×( )

where rf , Q f and GIP f are methane recovery ratio, drainage me-thane volume, and gas in place in fractured gas-interflow zone,respectively; rd, Q d and GIPd are methane recovery ratio, drainagemethane volume, and gas in place in de-stressed gas-desorptionzone, respectively; and r is the overall methane recovery ratio ofthe whole gas-producing zone.

These factors can also be used to assess methane reserves inabandoned goafs through identifying the height of the three gaszones and estimating the levels of gas release from coal seams indifferent zones. A project aimed to recover methane from aban-doned goafs in YMG has adopted these factors to estimate theremaining methane reserve in abandoned goafs [25].

5.3. Strategy of optimisation and maximisation of methane extrac-tion method

Gas drainage in coal mines is aimed more towards controllinggas emissions than recovering methane recovery for further use,and is therefore often targeted to fractured gas-interflow zonesonly. The de-stressed gas-desorption zone is also a methane-pro-ducing zone and rich in high-purity methane; therefore, gasdrainage from this zone would maximise and optimise gas drai-nage quantity and quality. Missing this opportunity would makemethane recovery from this zone difficult when it is re-pressurisedafter goaf consolidation.

According to gas flow characteristics in the de-stressed gas-desorption zone, drainage boreholes need to be intimately con-nected with coal seams in this zone to enable sufficient gas flow.At panels which have a de-stressed gas-desorption zone and use agas drainage tunnel, boreholes can be drilled from the tunnel asillustrated in Fig. 10. For such a system, gas drainage would extend

to coal seams in the de-stressed gas-desorption zone whether ornot additional pipelines and suction pressure are applied to theboreholes.

6. Conclusions

To provide a complete and simple understanding of the cou-pled behaviour of strata and gas in multi-seam mining conditions,we propose a three-zone conceptual model in overlying strata. Themodel comprises (i) a fractured gas-interflow zone, (ii) a de-stressed gas-desorption zone and (iii) a confined gas-adsorptionzone. They are characterised based on the characteristics ofmining-induced fractures, depressurisation, gas desorption andmigration. The lower two zones are the major gas-producing zonesduring mining, with gas from the lowest zone being the primesource of ventilation methane emission and the major target ofmethane emission control.

We have also established a method of discriminating the zonesand estimating their extent based on the KS hypothesis, and ver-ified that it is more reliable than the empirical method used inChina. Another advantage of our method is that it takes intoconsideration the detailed stratigraphy of a particular miningpanel.

Applications of the gas three-zone concept have been discussedbased on mining conditions in YMG. It improves the selection ofan appropriate gas drainage method, and has allowed the estab-lishment of new factors of methane recovery efficiency. We havealso suggested a strategy to maximise and optimise gas drainagequantity and quality for panels using gas drainage tunnels.

Note that only coal seams overlaying a mining panel are ad-dressed in this paper. However, a similar characterisationwould beexpected in coal seams underlying a mining panel.

#3

#5

#12

#8

K3 & #13

K2

LW retreat direction

Gas drainage tunnel

#15

K4 & #11

Consolidated area

Fractured gas-interflow zone

De-stressed gas- desorption

zone

Tunnel convergence

Fig. 10. Extension of gas drainage to the de-stressed gas-desorption zone with boreholes drilled from a gas drainage tunnel.

Q. Qu et al. / International Journal of Rock Mechanics & Mining Sciences 78 (2015) 91–9898

Acknowledgements

This paper is associated with the first author's PhD dissertationundertaken in China University of Mining and Technology and fi-nancially supported by the National Natural Science Foundation ofChina (Project no. 50834005) and the State Key Laboratory of CoalResource and Mine Safety (Project no. SKLCRSM08X05). The au-thors are grateful to Yangquan Mining Group for their support incollecting technical data. The first author would like to thank Prof.Minggao Qian from China University of Mining and Technology forhis co-supervision of the first author's PhD dissertation and Dr.Hua Guo from CSIRO for his support of writing this paper.

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