modelling ground subsidence at an underground coal ... · underground coal gasification (ucg) 1)...
TRANSCRIPT
Modelling ground subsidence at an
underground coal gasification site
University of the Witwatersrand, Johannesburg
South Africa
Thushan Chandrasiri Ekneligoda
• offers a significant potential contribution to the future energy demand.
• is an in situ process. Therefore, it leaves a less carbon foot print in the
environment.
• provides a method to bring energy from thin coal layers in the
subsurface(difficult extract using traditional method).
• leaves only the cavity in the subsurface(traditional mining leaves waste on
the ground.
• offers a cavity that can be used to store CO2(CCS).
Underground coal gasification (UCG)
The process involves two wells, one serving as the injection well and
the other as the production well.
(https://en.wikipedia.org/wiki/Underground_coal_gasification.2017)
Underground coal gasification (UCG)
Four different phases in UCG
1 Drilling the injection and production wells from the surface to the coal
seam.
2 Establishment of a highly permeable path between the two wells is
ensured.
Methods such as hydraulic fracturing and explosives are
used for the purpose.
3 The injection of air and/or oxygen through the injection well is made to start
the ignition of coal.
4 Finally (Fourth phase), the extraction of produced syngas by the
production well is carried out.
Underground coal gasification (UCG)
1) The variation of the geo-mechanical properties due to high temperature
Properties such as uni-axial compressive strength, Young’s modulus, Cohesion
and friction angle may vary due to the high temperature expected at the gasification
area.
This presentation covers four different approaches to capture the variation of the
properties ;
(a) Multi stage tri-axial test,
(b) CT* analysis together with image analysis
(c) XRD and Thermo gravimetric analysis
(c) Uni-axial compressive strength test at elevated temperature
2) Ground subsidence and the cavity development.
It is important to predict the development of the cavity during the UCG process as
well as surface induced subsidence after the UCG process
Coupled Numerical approach(Mechanical-Thermal) was used in this study
CT* -Micro computed tomography (CT)
Some of the concerns of Underground coal
gasification (UCG)
The variation of the geo mechanical
properties
• 16 intact core samples were obtained from the Core No. 3a collected from the
underground trial site at Wieczorek trial site in Poland
• The original core contained mainly fine sandstone and coarse sandstone
• Intact core samples have been cored with 37mm diameter and 74mm height
• Samples have been grouped and pretreated with 20oC, 400oC, 800oC, 1000oC
• Multi-stage triaxial test were performed on the pretreated samples with confining
stress of 0, 6, 9, 12 MPa
(a) Multi-stage Triaxial Test
Stress-strain curves obtained from multistage
triaxial tests
Coarse sandstone
Finesandstone
(b) Micro computed tomography (CT) analysis
• Micro computed tomography is X-ray imaging in
3D(by the same method used in hospital CT
scans, but with higher resolutions with smaller
samples)
• Two samples with 10 mm diameter and
approximately 20 mm height were prepared for
the CT scanning
• CT scan was performed on the prepared rock
samples after being subjected to heat treatment
of 20, 400, 800, and 1000°C
The variation of the geo mechanical
properties
Micro CT analysis (Cont…)
• Scan was conducted in the centre of
the sample with a dimension of 5
mm diameter and 5 mm height
• 1,024 images with a pixel size of 5
μm were recorded
• 2D raw images were processed with
Avizo 9.0, an advanced 3D
visualization and analysis software
application
Nottingham University Xradia micro CT system
The variation of the geo mechanical
properties
Steps of acquisition of 3D rock pore structure
a) The filtered 2D slices were stacked into 3D images of the block sample
b) Through binarisation, the greyscale images were transferred into binary
images with only interior pores in blue and exterior materials in black
c) The 3D pore structure of the sample was separated out by volume rendering
from the binarized images
d) In order to obtain statistics on the pore space data, pores were separated
and represented with different colours
The variation of the geo mechanical
properties
2D slice images (3×3 mm)
Coarse Sandstone
Fine Sandstone
The variation of the geo mechanical
properties
Micro pore structure analysis
Coarse Sandstone
The variation of the geo mechanical
properties
XRD analysis
The mineral composition of both coarse and fine sandstone mainly
consists of Quartz, Kaolinite, Orthoclase and Illite at room
temperature
3, 20
00-046-1045 (*) - Quartz, syn - SiO2 - Y: 40.23 % - d x by: 1. - WL: 1.5406 - Hexagonal - a 4.91344 - b 4.91344 - c 5.40524 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P3221 (154) - 3 - 113.010 - I/Ic PDF 3.4 - F30=539(
00-043-0685 (I) - Illite-2M2 - KAl2(Si3Al)O10(OH)2 - Y: 0.47 % - d x by: 1. - WL: 1.5406 - Monoclinic - a 9.01700 - b 5.21000 - c 20.43700 - alpha 90.000 - beta 100.400 - gamma 90.000 - Base-centered - C2/c (15) - 4 - 944.328 - I/Ic PD
00-031-0966 (*) - Orthoclase - KAlSi3O8 - Y: 1.18 % - d x by: 1. - WL: 1.5406 - Monoclinic - a 8.55600 - b 12.98000 - c 7.20500 - alpha 90.000 - beta 116.010 - gamma 90.000 - Base-centered - C2/m (12) - 4 - 719.122 - F30= 55(0.0148,
01-080-0886 (C) - Kaolinite - Al2(Si2O5)(OH)4 - Y: 3.54 % - d x by: 1. - WL: 1.5406 - Triclinic - a 5.15770 - b 8.94170 - c 7.39670 - alpha 91.672 - beta 104.860 - gamma 89.898 - Primitive - P1 (1) - 329.571 - I/Ic PDF 1.1 - F30=1000(0.0
Operations: Import
3, 20 - File: 3, 20.raw - Type: 2Th/Th locked - Start: 10.000 ° - End: 60.000 ° - Step: 0.020 ° - Step time: 2. s - Temp.: 25 °C (Room) - Time Started: 10 s - 2-Theta: 10.000 ° - Theta: 5.000 ° - Chi: 0.00 ° - Phi: 0.00 ° - X: 0.0 mm - Y: 0.0 m
Inte
nsity
(cps
)
0
100
200
300
400
500
2 Theta (º)
10 20 30 40 50 60
0
500
250
Kaolinite
Orthoclase
Quartz
Illite
20°C
10 20 30 40 50 60
2θ (°)
Identification of Mineral composition
(C ) High Temperature UCS test
• Accepts testing samples with diameter up to 50 mm, length up to 100 mm
• The temperature controller provides programmable temperature control
• Maximum operating temperature: 1000 °C
The variation of the geo mechanical
properties
The variation of average uni-axial compressive strength
against temperature
0
10
20
30
40
50
60
70
80
0 200 400 600 800 1000
Un
i axi
al C
om
pre
ssiv
e S
tre
ngt
h (
MP
a)
Temperature (oC)
Approach 1
Approach 2
Approach 1- Using the new apparatus
Approach 2- Traditional method
The variation of the geo mechanical
properties
Determination of the cavity using a Conductive-Mechanical
coupled model
The factors that govern the extent
of fracturing and the likelihood of
subsidence include:
• Thickness of the coal seam
extracted
• Width of the coal seam
extracted
• Depth and strength of the
overlying geology.
Typical profile where subsidence affects the surface
Ground Subsidence induced by
UCG(2nd part of the study
Conductive-mechanical coupled analysis
• The temperature doesn't spread beyond 6 m from the coal layer due to the low
thermal conductivity (ITASCA software, FLAC 3D analysis)
• Coupled analysis was carried out only to the layers shown below(Ekneligoda et
al., 2016)
• Temperature dependent material properties were set for coupled analysis
27.5
6
12
32
6
761.5
11
5
x=100y=60
z=9
1
Dimension in m
Shale non-thermalCoal non-thermalSandstone non-thermalShale thermalCoal thermalSandstone thermal
27.5
6
12
32
6
761.5
11
5
x=100y=60
z=9
1
Dimension in m
Shale non-thermalCoal non-thermalSandstone non-thermalShale thermalCoal thermalSandstone thermal
395m below the ground surface
Determination of the cavity using a
Conductive-Mechanical coupled model
1. The elements in the coal seam start burning when the temperature rises to the
ignition point of coal (which is assumed to be 200oC in this study).
2. During the ignition period, the elements in the coal seam emit energy according
the calorific value of the coal. (2000 MJ/m3, Q = 337C + 1442(H - O/8) + 93S,)
3. The energy emission is represented by using a decay function as the energy
emission due the coal burning reduces gradually with time.
4. The movement of the coal burning head is set at 2m/day.
5. The elements that are burnt are ignored from the calculation after 1 hour.
6. Temperature dependent material properties are considered.
Fish(Computer language used in FLAC 3D) functions were developed to incorporate
all the above mentioned features.
(Six Special features of the model)
The cavity development in the horizontal
plane (plan view)
Cavity
developmentDuration Cavity
1 day
Maximum dimension 3 m x 1.5 m x 1.5 m
5 day
Maximum dimension 7 m x 2.5 m x 2.5 m
10 day
Maximum dimensions 12 m x 3.5 m x 4 m
15 day
Maximum dimensions 17 m x 4.5 m x 5 m
Maximum dimension
3m x 1.5m(Y) x 1.5m
X (Burning head) Y Z
Maximum dimension
7m x 2.5m x 2.5m
X Y Z
Maximum dimension
12m x 3.5m x 4m
X Y Z
Maximum dimension
17m x 4.5m x 5m
X Y Z
1 day
5 day
10 day
15 day
Modelling of the long term effect of the
UCG process.
• Burning distance does not spread more than 10m after the 15 days of burning
(From the previous study)
• Maximum possible dimension of the cavity is 30(X) x 12(Y) x 6(Z) m
• Instantaneous removal of the burnt zone (This represents the worst case
scenario as the coal burning process is a gradual process).
Displacement at the roof (A)
90mm
Displacement at the top of model (B)
23mm
Shale_nonthermalCoal_nonthermal
Sandstone_nonthermalShale_thermalCoal_thermal
Sandstone_thermal
Shale_nonthermal
Z=94
X=200Y=60
(A)
(B)27.5
6
12
32
6
761.5
11
5
x=100y=60
z=9
1
Dimension in m
Shale non-thermalCoal non-thermalSandstone non-thermalShale thermalCoal thermalSandstone thermal
Parallel burning
• Importance of parallel burning
• Increase the production
• One of the concerns of parallel burning
• Ground subsidence
• Selection of minimum distance between two burning panels is important to
control the ground subsidence
Geometry of the total coal removal
• Dimension of the excavated region 12(X) x 30(Y) x 6(Z) m
Arrangement of five burning
panels
Arrangement of seven
burning panels
395 m
XY
Z
Shale_nonthermal
Shale_nonthermalCoal_nonthermal
Sandstone_nonthermalShale_thermalCoal_thermal
Sandstone_thermalShale_nonthermal
Shale_nonthermalCoal_nonthermal
Sandstone_nonthermalShale_thermalCoal_thermal
Sandstone_thermal
Variation of ground subsidence at 395m level for
5 burning panels
5 10 15 200
10
20
30
40
50
60
70
Su
bsid
en
ce
me
asu
red
at 3
95
m
be
low
th
e s
urf
ace
(mm
)
Minimum distance between two burning panels(m)
Subsidence vertically
above the gasification point
Subsidence 100m from
gasification point
Subsidence at the central
point (5m spacing, Point A)
Original property 71mm
Subsidence at 100m away(5m
spacing, Point B)
Original property 8mm
A B
Shale_nonthermal
Shale_nonthermalCoal_nonthermal
Sandstone_nonthermalShale_thermalCoal_thermal
Sandstone_thermal
dGasification cavity Measuring point
Property variation due to high temperature and
subsidence• Property at the neighbour zones can reduce due to high
temperature at the gasification reactor
• All the mechanical properties were reduced up to 20% in steps of
10%
• Ground subsidence was monitored similar to
the previous caseTemperature
affected area
Variation of ground subsidence at 395m level for
5 burning panels
Subsidence at the central point
(5m spacing, Point A)
Original property 71mm
20% reduction 82mm
Subsidence at 100m away(5m
spacing, Point B)
Original property 8mm
20% reduction 8mm
5 10 15 200
20
40
60
80
Sub
sid
en
ce
me
asu
red
at 3
95
m b
elo
w
the
su
rfa
ce
(mm
)
Minimum distance between two burning panels(m)
Sub. at 0m -orginal property
Sub. at 100m- orignal property
Sub. at 0m-10% property reduction
Sub.at 100m-10% property reduction
Sub. at 0m-20% property reduction
Sub. at 100m -20% property reduction
A B
Shale_nonthermal
Shale_nonthermalCoal_nonthermal
Sandstone_nonthermalShale_thermalCoal_thermal
Sandstone_thermal
Variation of ground subsidence at 395m level for 7
burning panels
5 10 15 200
20
40
60
80
100
Su
bsid
en
ce
me
asu
red
at 3
95
m b
elo
w
the
su
rfa
ce
(mm
)
Minimum distance between two burning panels(m)
Sub. at 0m -orginal property
Sub. at 100m- orignal property
Sub. at 0m-10% property reduction
Sub.at 100m-10% property reduction
Sub. at 0m-20% property reduction
Sub. at 100m -20% property reduction
Subsidence at the central point (5m
spacing)
Original property 88mm
20% reduction 108mm
Subsidence at 100m away(5m
spacing)
Original property 16mm
20% reduction 21mm
A B
Shale_nonthermal
Shale_nonthermalCoal_nonthermal
Sandstone_nonthermalShale_thermalCoal_thermal
Sandstone_thermal
Fractured roof
• Fractures in the roof can produce a weak layer.
• Exact arrangement of fractures can be difficult to determine and it is
unknown.
• Different arrangement of fractures can be modelled with
UDEC(Universal Discrete Element Code).
• This approach is two dimensional
• Three different fracture orientations were considered
Fracture properties(Typical properties)
• Strength properties of joints/fractures
▪ Cohesion 5MPa
▪ Friction angle 15o
• Fracture spacing 0.5 m
• Fractures are modelled using Coulomb Slip model
• Elastic properties of joints/fractures
▪ Kn 10 GPa/m
▪ Ks 5 GPa/m
Three different arrangements of fractures
in the roof were considered
• Horizontal Fractures(dip 0o)
• Inclined fractures (dip 60o)
• Randomly oriented fractures
Arrangement of horizontal Fractures
(three panels burning)
Fracture spacing 0.5m
Dip angle of the fractures 0o
Property variation due to high
temperature and subsidence
• Properties can reduce due to high temperature at the
neighbouring area
• The mechanical properties of the sandstone roof layer
were reduced up to 20% in steps of 10%
• Ground subsidence was monitored similar to
the previous cases
Fractured roof (three panels burning)
10 15 200
20
40
60
80
100
Sub
sid
en
ce
me
asu
red
at 3
95
m b
elo
w
the
su
rfa
ce
(mm
)
Minimum distance between two burning panels(m)
Sub. at 0m -orginal property
Sub. at 100m- orignal property
Sub. at 0m-10% property reduction
Sub.at 100m-10% property reduction
Sub. at 0m-20% property reduction
Sub. at 100m -20% property reduction
Subsidence at the central point (5m
spacing) Point A
Original property 96mm
20% reduction 98mm
Subsidence at 100m away(5m
spacing, Point B)
Original property 5mm
20% reduction 8mm
A B
Fractured roof (Five panels)
10 15 200
20
40
60
80
100
120
140
160
Sub
sid
en
ce
me
asu
red
at 3
95
m b
elo
w
the
su
rfa
ce
(mm
)
Mininum distance between two burning panels(m)
Sub. at 0m -orginal property
Sub. at 100m- orignal property
Sub. at 0m-10% property reduction
Sub.at 100m-10% property reduction
Sub. at 0m-20% property reduction
Sub. at 100m -20% property reduction
Subsidence at the central point
(5m spacing)(Point A)
Original property 148mm
20% reduction 158mm
Subsidence at 100m away
(5m spacing) (Point B)
Original property 19mm
20% reduction 20mm
A B
Inclined fractures(dip 60o, spacing 0.5m)
Two different panel burning were
considered
– Single panel
– 3 Panel with 20m spacing
Single panel burning(displacement)
discontinuum Vs continuum
Displacement at the roof (A)
300mm
Displacement at the top of
model (B)
50mm
A
B
Shale_nonthermalCoal_nonthermal
Sandstone_nonthermalShale_thermalCoal_thermal
Sandstone_thermal
Shale_nonthermal
Z=94
X=200Y=60
Displacement at the roof (A)
90mm
Displacement at the top of
model (B)
23mm
B
3 Panel with 20m spacing (Displacement)
Displacement at the roof (Point A)
420mm
Displacement at the surface(Point B)
100mm A
B
Generation of randomly oriented fractures
Four variables are important to derive the
fractures
– Fracture density
– Fracture length(Minimum and Maximum)
– Fracture orientations
– Fisher coefficients
Parameter Joint Set 1 Joint Set 2 Joint Set 3 Joint Set 4
Joint Mean Angle (o) 8 32 69 78
Standard Deviation in Angle (o) 20 20 20 20
Joint Minimum Length (m) 20 20 20 20
Joint Maximum Length (m) 48 48 48 48
Length Distribution Exponent (D) 2.2 2.2 2.2 2.2
Joint Density (m-2) 2.6 2.6 2.6 2.6
Li = Lmin-D - Ri(Lmin
-D- Lmax-D) -1/D
Where
Ri Random number
Lmin Minimum Length
Lmax Maximum Length
D Length distribution exponent
Conclusions
• Maximum Strength increases up to 400oC and then decreases upon
further increasing the temperature (Approach 2).
• Young modulus decreases by increasing the temperature from 400oC
to 800oC in both fine sandstone and coarse sandstone.
• Micro pore structure and the variation of porosity can be detected by
CT analysis
• Continuous reduction of uni axial compressive strength was observed
of the samples that were tested at elevated temperatures(1).
Conclusions
• Gradual reduction of Young’s modulus were observed with the increment of
temperature
• Maximum stress increased up to 800oC and then decreases upon further
increasing the temperature in fine sandstones with different confining stresses.
• XRD results revealed the chemical changes and recrystallization taking place
during the heating process.
• TGA confirmed the recrystallization process at different temperatures
• Analysis in micro pore structure can be used to explain the change in strength
after 800oC
Conclusions – continuum model
• We have numerically predicted the shape of the cavity
• The maximum dimension of the cavity is 12 x 7 x 4m after 10 days
• Parallel burning was modelled and the subsidence was monitored at the top of the
model (395m below the surface).
5 Panels 7 Panels
At the centre point(mm) 71 88
At 100m away(mm) 8 15
• The Variation of the properties was considered
• Three different fracture orientations in the roof were incorporated
• Horizontal fractures
• Slant fractures
• Randomly oriented fractures
Conclusions
5 Panels 7 Panels
Property reduction 0% 20% % 0% 20% %
At the centre
point(mm)
71 82 15% 88 108 23%
At 100m away(mm) 8 8 0 8 8 0
• Comparison of continuum and discontinnum modelling
5 Panels
(Continuum)
5 Panels
(Discontinuum-
Horizontal fractures)
Property reduction 0% 20% % 0% 20% %
At the centre
point(mm)
71 82 15% 148 158 6.8%
At 100m away(mm) 8 8 0 19 20 0
Thank you very much
TGA analysis
25 - 400 °C: Release of physical absorbed water in pores and on the surface
occurs.
400 - 800 °C: Dehydration of kaolinite and formation of metakaolinite takes
place
> 800 °C: Recrystallization to form Mullite takes place
Coarse sandstone Fine sandstone
Dehydration of kaolinite begins at temperatures between 500°C to 600°C.
The loss of lattice water breaks up the regular crystal structure of Kaolinite
and produces a dehydrated phase with an amorphous structure, known as
Metakaolinite.
Al2O3·2SiO2·2H2O (Kaolinite) → Al2O3·2SiO2 (Metakaolinite) + 2H2O↑
Amorphous structures can not be detected in XRD analysis
0
500
250
Orthoclase
Quartz
Illite
800°C3, 800
00-046-1045 (*) - Quartz, syn - SiO2 - Y: 40.23 % - d x by: 1. - WL: 1.5406 - Hexagonal - a 4.91344 - b 4.91344 - c 5.40524 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P3221 (154) - 3 - 113.010 - I/Ic PDF 3.4 - F30=539(
00-043-0685 (I) - Illite-2M2 - KAl2(Si3Al)O10(OH)2 - Y: 0.47 % - d x by: 1. - WL: 1.5406 - Monoclinic - a 9.01700 - b 5.21000 - c 20.43700 - alpha 90.000 - beta 100.400 - gamma 90.000 - Base-centered - C2/c (15) - 4 - 944.328 - I/Ic PD
00-031-0966 (*) - Orthoclase - KAlSi3O8 - Y: 1.18 % - d x by: 1. - WL: 1.5406 - Monoclinic - a 8.55600 - b 12.98000 - c 7.20500 - alpha 90.000 - beta 116.010 - gamma 90.000 - Base-centered - C2/m (12) - 4 - 719.122 - F30= 55(0.0148,
Operations: Import
3, 800 - File: 3, 800.raw - Type: 2Th/Th locked - Start: 10.000 ° - End: 60.000 ° - Step: 0.020 ° - Step time: 2. s - Temp.: 25 °C (Room) - Time Started: 10 s - 2-Theta: 10.000 ° - Theta: 5.000 ° - Chi: 0.00 ° - Phi: 0.00 ° - X: 0.0 mm - Y: 0.0
Inte
nsity
(cps
)
0
100
200
300
400
500
2 Theta (º)
10 20 30 40 50 60
XRD analysis
10 20 30 40 50 60
2θ (°)
XRD analysis3, 1000
00-015-0776 (I) - Mullite, syn - Al6Si2O13 - Y: 0.32 % - d x by: 1. - WL: 1.5406 - Orthorhombic - a 7.54560 - b 7.68980 - c 2.88420 - alpha 90.000 - beta 90.000 - gamma 90.000 - Primitive - Pbam (55) - 167.353 - F30= 60(0.0135,37)
00-046-1045 (*) - Quartz, syn - SiO2 - Y: 63.70 % - d x by: 1. - WL: 1.5406 - Hexagonal - a 4.91344 - b 4.91344 - c 5.40524 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P3221 (154) - 3 - 113.010 - I/Ic PDF 3.4 - F30=539(
00-031-0966 (*) - Orthoclase - KAlSi3O8 - Y: 1.18 % - d x by: 1. - WL: 1.5406 - Monoclinic - a 8.55600 - b 12.98000 - c 7.20500 - alpha 90.000 - beta 116.010 - gamma 90.000 - Base-centered - C2/m (12) - 4 - 719.122 - F30= 55(0.0148,
Operations: Import
3, 1000 - File: 3, 1000.raw - Type: 2Th/Th locked - Start: 10.000 ° - End: 60.000 ° - Step: 0.020 ° - Step time: 2. s - Temp.: 25 °C (Room) - Time Started: 10 s - 2-Theta: 10.000 ° - Theta: 5.000 ° - Chi: 0.00 ° - Phi: 0.00 ° - X: 0.0 mm - Y: 0.
Inte
nsity
(cps
)
0
100
200
300
400
500
2 Theta (º)
10 20 30 40 50 60
0
500
250
Orthoclase
Quartz
Mullite
10 20 30 40 50 60
2θ (°)
The metakaolinite transforms to a spinel structure and amorphous silica at a
temperature around 800°C – 900°C.
Al2O3·2SiO2 (Metakaolinite) → SiAl2O4 (spinel) + SiO2 (amorphous)
Upon further heating up to 1000 °C, recrystallization to form Mullite takes
place
SiAl2O4 (spinel) + SiO2 (amorphous) → 1/3 (3Al2O3·2SiO2) (Mullite) + 4/3SiO2 (amorphous)
1000°C