modelling ground subsidence at an underground coal ... · underground coal gasification (ucg) 1)...

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)


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.


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


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


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


properties

2D slice images (3×3 mm)

Coarse Sandstone

Fine Sandstone


properties

Micro pore structure analysis

Coarse Sandstone


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


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


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


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


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



Sandstone_thermalShale_nonthermal



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)


A B

Shale_nonthermal



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)


20% reduction 82mm


spacing, Point B)


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

)


Sub. at 0m -orginal property

Sub. at 100m- orignal property

Sub. at 0m-10% property reduction

Sub.at 100m-10% property reduction


Sub. at 100m -20% property reduction

A B

Shale_nonthermal



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

)








Subsidence at the central point (5m

spacing)


20% reduction 108mm


spacing)


20% reduction 21mm

A B

Shale_nonthermal



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

)








Subsidence at the central point (5m

spacing) Point A


20% reduction 98mm


spacing, Point B)


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)







Subsidence at the central point

(5m spacing)(Point A)


20% reduction 158mm

Subsidence at 100m away

(5m spacing) (Point B)


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


300mm

Displacement at the top of

model (B)

50mm

A

B



Sandstone_thermal

Shale_nonthermal

Z=94

X=200Y=60


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


00-043-0685 (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


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)



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

modelling ground subsidence at an underground coal ... · underground coal gasification (ucg) 1)...

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