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Improvement of intensive in-seam gas drainage technology at Kirova mine in Kuznetsk coal basin.

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The Kirova mine with its 3 Mt production in 2019 is one of the coal mining leaders in Russia. The available mining equipment has the potential to significantly increase the output, however, gas is a limiting factor on the way to this. The customary approaches to coal seam degassing have already been petered out. The miners and mine science are facing a challenge to validate and test an alternative technology to ensure effective in-seam gas drainage prior to vigorous mining. This article gives an account of the improvement track record of the in-seam gas drainage technology used to pre-treat coal seam for intensive and safe extraction. This technology suggests, at the first stage, hydraulic slotting of the target coal seam through wells drilled from the surface (SHS), then hydraulic fracturing of the coal seam through the boreholes drilled from underground development headings, followed by methane extraction from the created high-permeability coal-gas reservoir through standard in-seam gas dranage underground wells. Presented in this paper are the results of field testing of improved SHS technique. Findings are presented on the effective parameters of the hydraulic fracturing technology. Methodological recommendations are offered for selecting viable in-seam gas drainage technology.
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1
Improvement of intensive in-seam gas drainage technology at Kirova mine in
Kuznetsk coal basin.
Sergey Slastunov1, Konstantin Kolikov1, Andrian Batugin1*, Anatoly Sadov2,
Adam Khautiev2
1 National University of Science and Technology «MISiS», Moscow 119049,
Leninsky Prospect, 4. Russia
2 JSC SUEK-Kuzbass, Leninsk-Kuznetsky 652507, Vasilyeva str.1.Russia
*corresponding author, +79037571047; E-mal: as-bat@mail.ru
Abstract The Kirova mine with its 3 Mt production in 2019 is one of the coal
mining leaders in Russia. The available mining equipment has the potential to
significantly increase the output, however, gas is a limiting factor on the way to
this. The customary approaches to coal seam degassing have already been petered
out. The miners and mine science are facing a challenge to validate and test an
alternative technology to ensure effective in-seam gas drainage prior to vigorous
mining. This article gives an account of the improvement track record of the in-
seam gas drainage technology used to pre-treat coal seam for intensive and safe
extraction. This technology suggests, at the first stage, hydraulic slotting of the
target coal seam through wells drilled from the surface (SHS), then hydraulic
fracturing of the coal seam through the boreholes drilled from underground
development headings, followed by methane extraction from the created high-
permeability coal-gas reservoir through standard in-seam gas dranage underground
wells. Presented in this paper are the results of field testing of improved SHS
technique. Findings are presented on the effective parameters of the hydraulic
fracturing technology. Methodological recommendations are offered for selecting
viable in-seam gas drainage technology.
2
Key words intensive coal mining, integrated in-seam gas drainage, hydraulic slotting through
surface wells, hydraulic fracturing from underground boreholes, in-situ tests, improvement of
technology.
Introduction. SUEK-Kuzbass JSC is one of the leaders in the coal industry in
Russia. In 2018, the company set a world record for monthly production from one
longwall face - 1.7 million tons. The Kirova mine with its 3 Mt production in 2019
is one of the key assets of the company. It is currently a single LW mine operating
in 2.2 m thick Boldyrevsky Seam. The gas content in this seam averages 16 m3/t.
The company management aim at further increase of production while maintaining
the conditions for safe mining. The key limiting factor for further production
intensification is gas factor, which sets the task of ensuring effective and early in-
seam gas drainage.
Dozens of gas drainage methods and technologies are known
internationally (Gas 2013; Hu at al. 2018; Karacan 2011; Kurlenya at al. 2015;
Slastunov at al. 2015; Slastunov at al. 2016; Szott at al. 2018; Wu 2020; Yan
2015; Zhang at al. 2017; Zhao at al. 2014). The most effectively used methods of
gas drainage are those where methane is drained from the stress-relieved coal
seam. The effectiveness of these methods is over 80%, which from a cause-and-
effect view is due to extraction of liberated gas. Whatever issues arising thereat
do not represent conceptual scientific problems and their solution depends on
competent engineering support.
The situation is different with coal seams still exposed to strata pressure,
where gas drainage efficiency is significantly lower. Worldwide, a common
approach to achieve high production from a face while assuring gas related safety
of the mining operations is to apply the in-seam gas drainage technology operation
schedules and parameters thereof are defined with due account for the properties
and behaviour of the target coal seam, time allowance for its drainage and the size
of "gas barrier". Yet at the depth of 400 meters and below, the effectiveness of the
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in-seam gas drainage technology at SUEK-Kuzbass JSC mines is only 10÷15%.
Therefore, gas factor is the obvious bottleneck in the face output and impedes more
intensive mining in high-gas coal seams, hence, the challange is to develop new
effective coalbed gas drainage technologies.
In recent years experimental and implementation work at Kirova mine to
substantiate effective technological gas drainage systems have been and are being
carried out, at the initiative of the authors and with support of the company's
management.
In this paper, we consider the experience of developing new for Russia
integrated schemes for the stepwise degassing of development areas using the
example of Kirova mine.
2. Methodology
2.1 Improving the in-seam gas drainage technology prior to intense
underground mining.
Layout of mine workings in Boldyrevsky Seam is provided in Figure 1. The
extraction blocks 2458, 2459, 2460, and 2462 were mined out in recent years;
extraction blocks 2463 and 2464 have been in operation in 20192020. In blocks
2458, 2459, 2460 and 2462, standard technique, i.e. in-seam gas drainage from
development headings, was applied as a baseline scheme since it has been well
proven in the mines of Russia. The parameters of this standard technology are set
out in the current guideline document Gas Drainage Instructions (2013).
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Fig. 1 Mine workings layout in Boldyrevsky Seam
In the block 2458 with intense coal production this baseline gas drainage
scheme was supplemented by seam hydraulic fracturing technology. In this
production block hydraulic fracturing was performed from 12 underground wells
(UgHF technology). The effectiveness of this auxiliary technological scheme was
evaluated during coal-face operations. Following factors were taken into account:
gas emission rate, resultant quantity of methane captured from the in-seam gas
drainage wells, reduced amount of gas in production workings, reduced longwall
downtime due to gas factor, and increased output from the production face.
An important aspect of the work carried out on this technology was the
search for effective parameters of UgHF technology, primarily the effective length
of UgHF wells. So, the plan of exploratory experimental studies on in-seam gas
drainage in the extraction block 2460, Boldyrevsky Seam, includes the hydraulic
fracturing operations with effective lengths of hydraulic fracturing wells varying at
5, 35, 70 and 110 meters (Table 1).
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Table 1 Technical specifications for extraction block 2460 in Boldyrevsky
seam.
Parameter
Unit
Hydrofracture
location -
vent. raise 2460
1.
Effective length of hydrofracture
boreholes
m
110 (for bh.9-13)
70 (for bh.14-18)
35 (for bh.19-23)
5 (for bh.24-28)
2.
Drill bit dia
mm
93-132
3.
Casing pipe dia
mm
70
4.
Pump pressure at seam hydrofracture
MPa
12÷30
5.
Fluid pumping rate
l/s
до 10
6.
Seam hydro-conditioning radius
m
25
7.
Quantity of fluid for hydrofracture
m3
5…50
8.
Min time of seam treatment
considering varied pumping rate
min
80
As shown in Table 1, four zones were laid out in the extraction block 2460 where
in-seam gas drainage was done by underground hydraulic fracturing through wells
with various effective lengths (i.e. lengths of the open/uncased part of the
borehole). In the first group of wells No. 60/9-60/13 (zone 1 in Fig. 2), the
effective length of the hydraulic fracturing wells was 110 m; in the second group
of wells No. 60/14-60/18 (zone 2 in Fig. 2) this parameter was 70 m, in the third
group of wells No. 60/19-60/23 (zone 3 in Fig. 2) it was 35 meters, and in the
fourth group of wells No. 60/24-60/28 (zone 4 in Fig. 2) it was 5 meters
respectively.
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Fig. 2 Location of UgHF wells of various lenght at extraction block 2460
The length of the uncased part increases only when hydraulic fracturing
(hydraulic slotting/hydrojacking) mode is achieved, i.e. when pressure is stabilized
at a certain level while working fluid is being injected into the seam.
The design amount of working fluid for injection is 50m3. The injection
volume allowable, for various technological reasons (mainly liquid breakout into
the mine working, water drip from under the roof bolts), can vary between 5 and
50 m3.
Concurrently, the technology of auto-pneumatic impact on a coal seam was
initially tested in extraction block 2455 (Slastunov and Yutyaev 2017). During
auto-pneumatic impact (API) on a coal seam, the sliding valve in gas drainage well
was shut off from time to time. When the valve is shut, the pressure in the well
rises to the value which hypothetically levels off with that in the seam, hence
increases the coal seam permeability in the near-wellbore zone.
3. Results
3.1 Results of combining in-seam gas drainage baseline scheme with
seam hydrofracture technology
The very first results of underground hydrofracture technique application in
intensive coal extraction block 2458 demonstrated 3 to 4-fold long-term (7-8
months prior to face breakthrough) increase in methane yield attributed to the in-
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seam gas drainage holes drilled in the fracture target areas. The average value of
the relative gas content at the LW face was reduced by 30%, process shutdowns
due to “gas barrier” decreased by 42% and coal production was increased by 21%
on average. Subsequently, the technology of underground hydraulic fracturing was
successfully used in extraction blocks 2459, 2460 and 2462.
3.2 The results of the underground hydrofracture parameter
optimization
One of the key parameters of the underground hydrofracture technique is the
effective length of UgHF hole. The time related changes in the amount of methane
removed from wells in 2460 ventilation raise with different effective lengths is
shown in Figure 3.
Fig. 3 Time related changing flow rate of methane from various effective
length boreholes observed in ventilation raise 2460
The methane yield analysis shows that the integrated in-seam gas drainage
process is most effective when the length of the wells is 70 meters (red curve in
Figure 3).
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At the present stage of research, our recommendation is the range of
effective UgHF well lengths between 35 and 70 meters.
3.3 Results of combining in-seam gas drainage baseline scheme with the
technology of pneumatic impact on a coal seam
Technological try out of seam pneumatic impact in block 2455 provided
following data. In eight trial gas drainage wells, after the valve was shut off, the
pressure rose to 20÷25 atmospheres, thereafter the methane flow rate increased
from 3÷8 l/min to 22÷27 l/min, which significantly improved the effectiveness of
in-seam gas drainage.
The pneumatic impact technology probed is recommended as a method to
stimulate gas release from slow functioning typical underground wells of in-seam
gas drainage.
3.4 Recommended Practice for selecting the most viable method of in-
seam gas drainage for high producion mining panels
Based on the results obtained, recommendations were drafted on the
selection of in-seam gas drainage methods for high producion mining panels,
including two phases.
At the first stage of drafting recommendations, the required performance of
in-seam gas drainage is determined where the predicted gas yield, the time
available for in-seam gas drainage and the scheduled face output are taken into
account (Fig. 4). At the second stage (Table 2), the specific integrated in-seam gas
drainage technology is selected (eg: the conditions of Kirova mine with its
extraction blocks 2458, 2459, 2460 and 2462).
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Fig. 4 Methodological approach to defining the required effectiveness of in-seam
gas drainage (Phase 1)
Table 2 Recommended Practice for selecting the most viable method of in-
seam gas drainage for high producion mining panels (Phase 2).
Factors to be taken into account
and technological operations
In-situ conditions
Drilling-in method
Boreholes drilled from
development headings
Baseline scheme
In-seam gas drainage from
underground workings
Support technology
Hydraulic fracturing of coal seam
(UgHF)
Recommended in-seam gas
drainage enhancement technique
Auto-pneumatic impact (API)
4. Discussion and further development of the integrated in-seam gas
drainage technology
10
4.1 Justification of experimental work on testing new integrated
technology for gas drainage conditioning of the coal seam
The results obtained on enhancing the effectiveness of in-seam gas drainage
in hydraulically fractured zones in longwalls 2455, 2458, 2459, 2460 and 2462
served as the basis for launching a larger-scale program of gas drainage trial
operations in extraction block 2463 in 2019. It is here that it became possible to
implement the already proven technology of early in-seam gas drainage through
wells drilled from the surface (SHS) in combination with hydrofracture of the
Boldyrevsky coal seam (UgHF).
The purpose of such integration is to achieve even higher performance of gas
drainage operations hence ensure intense recovery of coal seams. The main idea of
this experimental technology is to improve the reservoir properties (porosity and
permeability) of a coal seam by creating a network of technogenic fractures by
injecting water under high pressure.
It is assumed that by way of hydraulic slotting from surface wells (SHS), in-
seam fractures shall open up and integrate into a common hydraulic system, which
will result in many-fold increase of the coal seam permeability in the well coverage
radius. Associated with this would be replacement of part of methane in the coal
sorption volume by working fluid, forcing methane off and blocking it outside the
mining range of influence.
In this regard, following tasks were set during the experimental work:
- ensure radius of SHS well influence of 120-150 m and more;
- use physical properties of the coal seam (swelling, shrinkage, dissolution,
creeping, plasticity) to achieve high conductivity of the fissures expanded by
hydrofracture;
- use shrinkage, associated with stress relief, to achieve enhanced
permeability, as well as substitution of part of methane in the coal sorption volume
for subsequent profound gas drainage from the coal-bearing stratum.
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Many-fold increase of permeability and gas release potential of coal seams,
achieved by their hydraulic slotting is due to their changed stress-strain condition
and physical properties caused by injection of working fluid in regimes exceeding
natural intake capacity of the seams. Such injection results in widening of natural
fissures and/or development of artificial fissures, all of them making up an
integrated hydraulic system oriented towards the well, which becomes a canal for
subsequent extraction of liquid and gas onto the surface.
Early gas drainage through surface wells is normally effective when methane
extraction period is more than 3 years (Interim 1991; Nozhkin 1979). It is possible
to reduce this time to less than 3 years, draining hydraulic slotting zone by
discharge of water into the mine workings.
The coal seam gas drainage work procedure whereby hydraulic slotting is
performed using surface boreholes most commonly includes three main phases: I -
hydrodynamic impact; II - well completion (removal of working fluid from the
seam using pumps or in-seam wells, gas extraction); III - gas extraction from the
worked out space after wells have been negotiated by the longwall operations.
Surface wells are located at a distance of at least 300m from the existing
workings in the direction of the main system of natural fissures in coal seams and
not more than radius of faults influence with amplitudes exceeding the thickness of
the target seam.
At the stage of experimental search at Kirova mine, process water shall be
used as working fluid.
To make a hydraulic slotting, a well is drilled to a target depth of 600-620 m,
working (ultimate) core bit diameter being 159 mm; annular space shall be
overlapped and throughly grouted to the entire depth including overlying worked
out formations. Wells are drilled from the surface, 0.5 m well depth span below the
top of the seam is cased and coal seam undergoes hydraulic slotting.
4.2 Experimental site
The first experimental area within the extraction block 2463 at Kirova mine
covers six seam hydraulic slotting boreholes (SHS) №№ 1-6 drilled from the
12
surface. Part of the program is to test a simplified SHS procedure whereby the
target coal seam is not screened by a perforated pipe. After the well has been
cased, the entire complex of equipment is moved down the hole and seam
hydraulic slotting is performed by high pressure pumps with a total flow rate of up
to 100 l/s.
Pressure and flow rate are constantly monitored by gauges capable to record
and visualize data from a safe distance. The volume of water required for injection
into one well is at least 500 to 1000 m3.
After hydraulic slotting is completed, the wells are left inactive for up to 8-
12 months or until the approach of mining operations. At the experimental phase, it
is assumed that withdrawal of working fluid will be in the form of SHS well
spouting with associated gas removal, however, at a later phase, water shall be
discharged into underground working.
As soon as longwall operations approach SHS well influence zone, working
fluid shall be discharged into ventilation raise 2463. For this purpose, a cluster of 3
to 5 directional wells shall be drilled towards SHS well zone with water inflow
control therein. If needed, the number of wells can be increased.
At the present phase of work (extraction block 2463), the plan is to apply the
integrated gas drainage option based on SHS wells, whereby water shall be
discharged into mine workings, with subsequent effective functioning of standard
underground in-seam wells in the impact zones. The time lag between drilling
advance in-seam gas drainage wells in SHS zone and those in the control area shall
not exceed 2 months. The completion of both groups of wells shall be at
approximately the same time, with a time lapse of no more than 2-3 weeks.
In the future, we intend to explore if it is possible to alter the grid of both
SHS wells and underground gas drainage boreholes, and if it is viable to reduce the
time needed for in-seam gas drainage in SHS zones.
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4.3 The selection of feasible in-seam gas drainage technology for
extraction block 2463
Selection of viable in-seam gas drainage technology for individual
geological and mining conditions is one of the most challenging issues. The key
point here is the magnitude of the “gas barrier”, hence, adequate in-seam gas
drainage effectiveness shall be defined.
The nomogram on the required methane extraction as a result of in-seam gas
drainage in the Boldyrevsky coal seam is shown in Figure 5 and allows to make an
adequate decision about the in-seam gas drainage option depending on the required
output of the production face.
Fig. 5 Methane extraction required to enhance production output in
dependence on baseline gas content of the coal seam 1) 18 m3/t 2 ) 16m3/t 3 ) 14
m3/t 4) 12 m3/t
With actual gas content at 16 m3/t in the extraction block 24-63 of the
Boldyrevsky seam and estimated production target of 14,000.00 tpd, the required
volume of methane removal in the process of in-seam gas drainage is 5 m3/t. Based
on the available data on the actual efficiency of in-seam gas drainage, the updated
technology for early degassing of coal seams through surface wells (SHS) was
14
advised; the proven efficiency thereof is 0.3 ÷ 0.5 and it can provide methane
extraction at the level of 5 m3/t. The technology of in-seam gas drainage from
underground development headings in SHF zones with an extended grid of in-seam
wells was recommended as a support measure. The parameters of in-seam gas
drainage should also take into account the availability and location of tectonically
stressed and tectonically relieved zones that could be identified by geodynamic
zoning (Batugina and Petukhov 1990; Yutyaev 2019). The latter aspect is the
subject of additional research.
Conclusions
1. Analysis made for a number of production blocks of the Kirova mine on
restrictions imposed on face output by gas factor shows that in-seam gas drainage
with rated capacity of up to 0.3 (30%) is required for majority of production faces.
Under favorable conditions this can be achieved by means of advance gas drainage
operations carried out from development headings (in particular, the technological
version of integrated degassing using underground hydraulic fracturing, shown in
Table 2). This was confirmed by successful practices in the extraction blocks 2458,
2459 and 2460 at Kirova mine. (Slastunov at al. 2018; Slastunov at al. 2019).
2. Mining is currently carried out at extraction block 2462, where in-seam
gas drainage facilities include UgHF and advance in-seam gas drainage wells. Gas
is drained from extraction block 2463, where the in-seam gas drainage complex
also includes hydraulic slotting through surface wells. Studies on the relative and
absolute gas content in production faces, downtime of mining equipment due to
gas factor and mining intensity will make it possible to provide adequate
assessment of how viable the tested in-seam gas drainage technologies are and
estimate their feasibility and scope of their application.
In conclusion, it should be noted that Kirova mine with their large-scale gas
drainage project is the first ever mining company to pioneer such practice in Russia
15
and this practice is definitely of great interest for specialists in the field of assuring
methane safety for high performing underground coal mines.
The authors acknowledge the invaluable contribution to the present work
from SUEK-Kuzbass JSC Director General Dr. Yutyayev E.P., we also
appreciate the help of Gas Drainage and Utilazation Department team who
collaborated in this work at Kirova mine, the Company headed by Dr. E. P.
Yutyaev.
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In coal seam group mining, there is a growing concern on protective seam mining to solve the problems of coal and gas outburst. For upper protective seam mining, the effect of pressure relief and permeability increase in lower protected layers is determined by the distance between upper protective seams and lower protected seams (seam spacing). The level of pressure relief and permeability increase in lower protected seams is stronger at smaller distances. However, more relief gas flows to the working faces of protective seams at shorter distances, making gas control of the working faces more difficult. In this study, taking working face 62114 of Xinzhuangzi Mine, Huainan as the engineering background, the gas emission laws of the working faces of the upper protective seams in close range are analyzed, and gas control measures are proposed. Furthermore, the gas extraction results of the close range mining face on the upper protective seam of Huainan Xinzhuangzi Mine are analyzed. The results show that ① with the implementation of the protective seam mining, in the mining influence area, the underlying rockstrata can be divided into floor heave fracture zone and floor heave deformation zone, and gas from the lower protected seams in the floor heave fracture zone tends to migrate to the protective seam goaf; ② Y-ventilation can be used to solve gas accumulation and gas overflow in the upper corner at the mining face of close range upper protective seams; ③ in principle, the penetrating borehole spacing of lower protected seams should be less than or equal to the seam spacing in order to effectively prevent flows of releasing gas of protected seams into the working face of protective seams. © Indian Pulp and Paper Technical Association. All rights reserved.
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