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

Reservoir reconstruction technologies for coalbed methane recovery indeep and multiple seams Wang Lianga,b,c, Liu Shiminc, Cheng Yuanpingb,d, Yin Guangzhia, Zhang Dongminga, Guo Pinkuna,

Reservoir reconstruction technologies for coalbed methane recovery in

deep and multiple seams

Wang Lianga,b,c, Liu Shiminc, Cheng Yuanpingb,d, Yin Guangzhia, Zhang Dongminga, Guo Pinkuna,e,⇑ a

State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China

b

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, G 3

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

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 5 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 out-burst 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 first-mined 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 ani-sotropy and soft coal of Chinese coal seams, a single hydraulic fracturing surface well for reservoir recon-struction 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 accord-ing to the principles of effectiveness and economics, and used hydraulic fracturaccord-ing surface well to recon-struct 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 inte-grated reservoir reconstruction technology, ‘‘One well for triple use”, was detailed introduced and suc-cessfully 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/)

1 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

economy and the continuous demand and dependence on coal pro-duction, 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 out-burst 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/ ).

⇑ 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).

Contents lists available atScienceDirect

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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 1 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,

mea-sures to control coal mine methane (CMM) in China bear the

mul-tiple 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 1

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

produc-tion[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

perme-ability, 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

reser-voir 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

per-meability was proposed by stress-relief and/or unloading

fractur-ing through adjacent seam minfractur-ing 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

out-comes were summarized and analyzed for Luling coal mine in

China

2 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

compli-cated, high degree of metamorphism, mechanically soft, and very

low gas deliverability due to low reservoir permeability.Table 2

summarizes the virgin coal seam reservoir properties for the

Chinese, US and Australian CBM reservoirs The permeability of Chinese coal is usually on the magnitude of 10 4–10 3mD 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 com-mercialization 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 dee-per 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 over-burden stress ranges from 22 to 33 MPa and the coal seam gas pressure and gas content can exceed 6 MPa and 20 m3/t respec-tively 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 deple-tion efficiency for any given well patterns Addideple-tionally, the Chi-nese 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 conven-tional CBM extraction technology[6] In contrast, US high gas bear-ing 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 mul-tiple and relatively stable low-permeability compartments, which serve as seals to prevent the vertical reservoir fluid exchange Coal

Table 1

Coal seam groups occurrence of partial mines in China [6]

Coalfield Coal seam conditions

Huainan Seam C14, C13, B11b, B10, B8, B7a, B6, B5, B4, A3 and A1

Huaibei Seam 4, 5, 6, 7, 8, 9, 10

Shenyang Seam 7, 11, 12, 13

Yangquan Seam 2, 3, 6, 8, 9, 12, 15

Pingdingshan Seam 14, 15, 16, 17

Yaojie Seam 1, 2, 3

Hedong Seam 2, 3, 4, 5

Xishan Seam 02, 03, 2, 4, 6, 7, 8, 9

Tiefa Seam 12, 13, 14, 15, 16, 17

Laochang Seam C2, C3, C4, C7 + 8, C8, C9, C13, C16, C19

Table 2 CBM reservoir property comparison for Chinese, US and Australian coal seams Coalfield Virgin coal reservoir properties

Permeability (mD)

Gas pressure (MPa)

Gas content (m 3

/ t)

Australia (Bowen)

Note: The CBM parameters in the table were measured or calculated in the current mining condition.

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seam groups are usually in multi-independent fluid pressure

sys-tems, making grouping CBM reservoir accumulation[10,25]

3 Current CBM extraction methods and their challenges in deep

and multiple seams

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

out-bursts 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 inFig 1 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

coal-methane co-exploitation technology for multiple gassy coal seams

is shown inFig 2, which was considered as an effectively

under-ground reservoir reconstruction method This innovative

technol-ogy 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

min-ing induced permeability enhancement, the extraction height of

the first-mined seam needs to reach a specific value On the

con-trary, 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-tion for the reservoir reconstrucconsidera-tion technologies applicaconsidera-tion 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 8 m3/t [20,30] The underground gas pre-drainage method is mostly adopted by using penetration bore-hole or bedding borebore-hole for the first-mined seam, which requires large engineering quantities and long drilling and drainage dura-tion[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 tech-nique for reservoir reconstruction in deep outburst seams, espe-cially in Chinese CBM reservoir with the characteristics of low saturation, low permeability, strong anisotropy and soft-to-plastic 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 dif-ficulties involved in coal fracking include: difdif-ficulties in concen-trated 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 stud-ies are essential for deep CBM reservoirs in terms of reservoir reconstruction and stimulation

4 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 eco-nomical 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 first-mined 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 3

Extreme distant pressure relief seam

Distant pressure relief seam

Lower pressure relief seam

Bending zone

Fractured zone

Caving zone

Fractured zone

Deformation zone

First-mined seam

Fig 1 The strata movement and fractures development after mining the

first-mined seam.

High-efficiency and safe mining in first-mined seam

High-efficiency and safe mining adjacent seams

High-efficiency and safe coal exploitation

Reduce gas emissions in first-mine seam

Gas drainage in the adjacent seam

Eliminate outburst risk in adjacent seams

Decrease gas content in adjacent seams Desorption-diffusion

of gas flow

Deformation and fractures in roof and floor of first-mined

High-efficiency and safe gas exploitation

Fig 2 A coal-methane co-exploitation model for multiple gassy coal seams.

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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

effec-tiveness 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

min-ing need to consider the thickness of coal seam, coal quality,

extraction engineering quantities, coal pressure relief

quan-tities, 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

inte-grated 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

deter-mined 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

rect-angle with borehole spacing less than 300 m After the

hydraulic fracturing with fracturing fluids, the sand

prop-pant was injected for propping fractures For improving the

carrying capacity of fracturing fluid, CO2 and N2could be

used as accompanying injection materials[35] Then

perme-ability 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

phe-nomena of stress concentration, unloading damage, and

stress recovery [16,36], and the adjacent seams could be

reconstructed by mining induced stress relief Exploitation

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 combin-ing both underground and surface drainage methods in the pressure relief area can effectively realize high-efficiency gas extraction from the adjacent protected seams and elim-inate 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 dur-ing mindur-ing, and post goaf gas drainage This concept is illustrated

inFig 6showing 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

Integrated plannings of coal mining and gas control Selection of

reconstruction target (first-mined seam) Surface wells

Mining the first-mined seam (protective seam)

Reservoir

reconstruction for

the adjacent seams

Deep coal reservoir reconstruction

Unloading damage by mining effect Pressure relief gas drainage

underground gas drainage

selection range of first-mined

Ensure mining safety of first- mined seam

Fig 3 CBM reservoir reconstruction technical layout.

Fig 4 A sketch of hydraulic fracturing surface well for the first-mined seam reconstruction.

Fig 5 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

First-mined seam

Fig 6 ‘One well for triple use’ in the Luling coal mine.

200 km

Hefei

Anhui

1000 km

Luling coal mine

Huaibei coalfiled

Thickness (m) Coal Columnar Lithology

7

8

9

K2

10

Medium sandstone

2.89

Mudstone

Medium sandstone

Mudstone

Medium sandstone Mudstone

Mudstone Mudstone

Mudstone

Mudstone Mudstone Mudstone Mudstone

Mudstone Packsand

Packsand

Packsand

Packsand

Packsand Coal seam

Coal seam Coal seam

Coal seam

Key bed Sandstone Siltstone Siltstone

0.29 2.22 1.19

−

0.14 16.62 9.56

− 0.29 9.82 3.65 −

0 4.30 1.92

−

Fig 7 The location and coal strata histogram of the Luling coal mine.

Table 3

Key reservoir characteristics in the Luling coal mine.

Coal seam thickness (m) 9.56 m on average (No 8), 15.3 m in

composited seams (Nos 8 and 9)

Extremely thick 1.92 m on average Medium-thick seam Hardness coefficient (f) 0.11–0.46, 0.26 on average Extremely soft 0.8–1.13, 0.9 on average Hard

Permeability coefficient (m 2 /(MPa 2
d)) 0.0277 (or 0.0007 mD) Low permeability

Maximum gas pressure (MPa) 5.7 (at 800 m depth, predicted) High outburst risk 2.8 (at 800 m depth,

predicted)

Relative low outburst risk Limiting adsorption amount (Langmuir

volume) (m 3

/t)

Gas content (m 3

Maximum value of the initial speed of gas

diffusion ( DP, mmHg)

Largest outburst Coal 10,500 t, gas 1.23 Mm 3

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 1 to 3 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, 8 m 3 /t and 10 mmHg, respectively.

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5 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, 9 and 10 coal seams The

aver-age thicknesses of the three seams are 9.56 m, 3.65 m and 1.92 m

respectively The spacing between the No 8 and No 9 coal seams

ranges from 0 to 5.3 m, with an average of 3.2 m The two seams

have similar properties The spacing between the No 9 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 inFig 7

The primary mineable coal seams (Nos 8, 9, 10) of the Luling

coal mine are all outburst-prone coal seams 26 coal and gas

out-burst accidents have occurred in coal seam Nos 8 and 9 An

extre-mely large coal and gas outburst accident occurred on April 7th,

2002 which threw out 10,500 t of coal-rock masses and

1.23 Mm3of gas The spraying holes phenomenon usually appears

during the process of drilling, and the mean spraying coal amount

is 15.0 t As shown inTable 3, the middle group coal seams (Nos 8

and 9) are typical tectonic coal with characteristics of extremely

soft and low permeability It is also characterized by high gas

pres-sure, 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 8 and 9 coal seams

Flac3Dwas 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 simula-tion results, the stress reduced from 17.7 MPa to 3.34 MPa in the

No 8 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

Z

-3.0E+6 -5.0E+6 -7.0E+6 -9.0E+6 -1.1E+7 -1.3E+7 -1.5E+7 -1.7E+7 -1.9E+7 -2.1E+7 SZZ (Pa)

400 200 0

400 500 300 200 100 0

X (m)

0

300 200 100

500

100 200 300 400 500

X (m)

Y (m)

X (m)

Y (m)

0

400 300 200 100

500

100 200 300 400 500

0.4 0.3 0.2 0.1 0 -0.1

4 3 2 1

Expansive deformation ratio

Coal seam permeability

(a)

(b)

(c) Fig 8 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

880

840

790 740

LG-4

LG-7 LG-2 790

700

300 m

200 m

LG-6

LG-5

500 1000 1500 2000 2500 3000 3500 4000

0 50 100 150 200 250 300 350 400

Time (day)

3 )

(a) Locations of surface wells (b) Gas output of LG-6

Fig 9 Locations and gas output (LG-6) of hydraulic fracturing surface wells in the Luling coal mine.

Time (day)

20 40 60 80 100

20 40 60 80 100

0

3 /min)

Gas concentration Amount of gas drainage

Fig 10 Gas output of mining pressure relief area well in the Luling coal mine.

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(No 10 coal seam) could effectively affect the middle group coal

seam (Nos 8 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

nec-essary 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

transforma-tion) to drain the pressure relief gas from Nos 8 and 9 coal seams

When mining the No 8 coal seam, the wells could be used for third

time as goaf wells to drain post goaf gas Thus, following the

pro-cedure 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 inFig 9a) The fracturing radius was 120–130 m on

average From April 2008 to June 2011, fracturing gas drainage

amount has reached 1.726 Mm3in 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 inFig 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 Mm3of gas The

maxi-mum 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

out-put 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

drai-nage 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 inTable 4 We set the daily output amount to be 1500 m3/d

of fracturing drainage before mining in deep coal seams The

pre-diction conditions have three different situations, which represent

three different well spacing: 150 m, 200 m, and 250 m The

extrac-tion duraextrac-tions 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 extrac-tion, and the gas content would drop to 8 m3/t or less Unless the technology makes significant development, the possibility of elim-inating the outburst risk of Nos 8 and 9 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

6 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 multi-independent 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 sur-face 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-Table 4

Prediction on gas drainage effect of surface well.

Coal thickness (8

+ 9 + 10) (m)

Coal density (m 3 /t)

Well space (m)

Original average gas content (m 3

/t)

Original gas content (m 3

/t) Duration of drainage (year)

Residual gas content (m 3

/t) Drainage rate (%)

Note

1500 m 3 /d

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

ter Dynamics and Control (Chongqing University) (No.

2011DA105287-FW201405), the Qing Lan Project, the Sponsorship

of Jiangsu Overseas Research & Training Program for University

Prominent Young & Middle-Aged Teachers and Presidents, the

Pro-ject Funded by the Priority Academic Program Development of

Jiangsu Higher Education Institutions, and the Fundamental

Research Funds for the Central Universities of China (No

106112015CDJXY240001)

References

integrated coal production and gas extraction J China Coal Soc 2009;34

engineering applications on coal mine gas control Xuzhou: China University of

emissions and responding strategies in China Int J Greenhouse Gas Control

a coal-methane co-exploitation model: analysis and projections Resourc

coalbed methane reservoirs in the Lu’an mining area (China) based on a fluid

controlling factors of a vertical superimposed coalbed methane system in the

of long distance and pressure relief rock mass upon exploited coal seam J

diffusion model used for the fast desorption process of methane in coal Chaos:

approach to mining-enhanced permeability for simultaneous exploitation of

characteristic of mining-enhanced permeability in deeper gassy coal seam J

and utilization practices with benefits to mining safety and to greenhouse gas

of coal and pressure relief gas in thick and high-gas seams J China Univ Min

state-level demonstration project of coalbed methane development: an overview of such high-tech and commercial project in the southern Qinshui Basin Nat Gas

of deep coal-seam gas pressure and its application in coal mines Saf Sci

recovery via numerical simulations and data envelopment analysis Int J Min

upper Silesia Basin coal seams’ drainage ahead of mining Int J Min Sci Technol

simultaneous exploitation of coal and gas in China J China Coal Soc 2014;39

the excavation of coal and gas outburst-prone coal seams in deep shafts Int J

accurate and rapid measurement of underground coal seam gas content J Nat

sandstone on coalbed methane production J Nat Gas Sci Eng 2015;22

and full cost: the case of the Luling coal mine, Huaibei coalfield, China J Nat

channel and gas flow properties of overlying stratum in high intensity mining.

process in mining-induced crack network Int J Min Sci Technol 2012;22

methods, and applications of the safe and efficient simultaneous extraction of

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