Modelling ground subsidence at an underground coal gasification site

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1 Modelling ground subsidence at an underground coal gasification site Thushan Chandrasiri Ekneligoda University of the Witwatersrand, Johannesburg South Africa

2 Underground coal gasification (UCG) 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 CO 2 (CCS).

3 Underground coal gasification (UCG) The process involves two wells, one serving as the injection well and the other as the production well. (

4 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.

5 Some of the concerns of 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)

6 The variation of the geo mechanical properties (a) Multi-stage Triaxial Test 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 20 o C, 400 o C, 800 o C, 1000 o C Multi-stage triaxial test were performed on the pretreated samples with confining stress of 0, 6, 9, 12 MPa

7 Stress-strain curves obtained from multistage triaxial tests Coarse sandstone Fine sandstone

The variation of the geo mechanical properties (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

8 The variation of the geo mechanical properties (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

9 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

10 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

11 The variation of the geo mechanical properties 2D slice images (3 3 mm) Coarse Sandstone Fine Sandstone

12 The variation of the geo mechanical properties Micro pore structure analysis Coarse Sandstone

13 XRD analysis Identification of Mineral composition C 3, 20 Kaolinite Orthoclase Quartz Illite 300 Intensity (cps) , 20 - File: 3, 20.raw - Type: 2Th/Th locked - Start: End: Step: Step time: 2. s - Temp.: 25 C (Room) - Time Started: 10 s - 2-Theta: Theta: Chi: Phi: X: 0.0 mm - Y: 0.0 m Operations: Import 2θ ( ) 2 Theta (º) (C) - Kaolinite - Al2(Si2O5)(OH)4 - Y: 3.54 % - d x by: 1. - WL: Triclinic - a b c alpha beta gamma Primitive - P1 (1) I/Ic PDF F30=1000( (*) - Orthoclase - KAlSi3O8 - Y: 1.18 % - d x by: 1. - WL: Monoclinic - a b c alpha beta gamma Base-centered - C2/m (12) F30= 55(0.0148, (I) - Illite-2M2 - KAl2(Si3Al)O10(OH)2 - Y: 0.47 % - d x by: 1. - WL: Monoclinic - a b c alpha beta gamma Base-centered - C2/c (15) I/Ic PD (*) - Quartz, syn - SiO2 - Y: % - d x by: 1. - WL: Hexagonal - a b c alpha beta gamma Primitive - P3221 (154) I/Ic PDF F30=539( The mineral composition of both coarse and fine sandstone mainly consists of Quartz, Kaolinite, Orthoclase and Illite at room temperature

14 The variation of the geo mechanical properties (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

15 Uni axial Compressive Strength (MPa) The variation of the geo mechanical properties The variation of average uni-axial compressive strength against temperature Approach 1- Using the new apparatus Approach 2- Traditional method Approach 1 Approach Temperature ( o C)

$Ground Subsidence induced by UCG(2 nd part of the study Determination of the cavity using a Conductive-Mechanical coupled model The factors that govern the extent of fracturing and the$

16 Ground Subsidence induced by UCG(2 nd part of the study 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

17 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) 395m below the ground surface y=60 x=100 Dimension in m z=91 Shale non-thermal Coal non-thermal Sandstone non-thermal Shale thermal Coal thermal Sandstone thermal Temperature dependent material properties were set for coupled analysis 100 z=91 Shale non-thermal Coal non-thermal Sandstone non-thermal Shale thermal Coal thermal Sandstone thermal

18 Determination of the cavity using a Conductive-Mechanical coupled model (Six Special features of the 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 200 o C 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/m 3, Q = 337C (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.

19 The cavity development in the horizontal plane (plan view)

20 Cavity development Duration 1 day 1 day 5 day 5 day 10 day Cavity Maximum dimension 3 m x 1.5 m x 1.5 m Maximum dimension 7 m x 2.5 m x 2.5 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 10 day 15 day Maximum dimensions 12 m x 3.5 m x 4 m Maximum dimension 12m x 3.5m x 4m X Y Z 15 day Maximum dimensions 17 m x 4.5 m x 5 m Maximum dimension 17m x 4.5m x 5m X Y Z

21 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). (B) (A) z=91 Shale non-thermal Coal non-thermal Sandstone non-thermal Shale thermal Coal thermal Sandstone thermal Shale_nonthermal Coal_nonthermal Sandstone_nonthermal Shale_thermal Coal_thermal Sandstone_thermal Shale_nonthermal Z=94 Displacement at the roof (A) 90mm Displacement at the top of model (B) 23mm Y=60 X=200

22 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 Shale_nonthermal Coal_nonthermal Sandstone_nonthermal Shale_thermal Coal_thermal Sandstone_thermal

23 Geometry of the total coal removal Dimension of the excavated region 12(X) x 30(Y) x 6(Z) m Shale_nonthermal Coal_nonthermal Sandstone_nonthermal Shale_thermal Coal_thermal Sandstone_thermal Shale_nonthermal 395 m Shale_nonthermal Coal_nonthermal Sandstone_nonthermal Shale_thermal Coal_thermal Sandstone_thermal Shale_nonthermal Y Z X Arrangement of five burning panels Arrangement of seven burning panels

24 Subsidence measured at 395m below the surface(mm) Variation of ground subsidence at 395m level for 5 burning panels A B Shale_nonthermal Coal_nonthermal Sandstone_nonthermal Shale_thermal Coal_thermal Sandstone_thermal Shale_nonthermal Subsidence vertically above the gasification point Subsidence 100m from gasification point Minimum distance between two burning panels(m) Gasification cavity Subsidence at the central point (5m spacing, Point A) Original property 71mm Subsidence at 100m away(5m spacing, Point B) Original property 8mm d Measuring point

25 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 case Temperature affected area

26 Subsidence measured at 395m below the surface(mm) Variation of ground subsidence at 395m level for 5 burning panels A B 80 Shale_nonthermal Coal_nonthermal Sandstone_nonthermal Shale_thermal Coal_thermal Sandstone_thermal Shale_nonthermal 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 Minimum distance between two burning panels(m) 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

27 Subsidence measured at 395m below the surface(mm) Variation of ground subsidence at 395m level for 7 burning panels 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 Coal_nonthermal Sandstone_nonthermal Shale_thermal Coal_thermal Sandstone_thermal Shale_nonthermal Minimum distance between two burning panels(m) 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

28 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

29 Fracture properties(typical properties) Strength properties of joints/fractures Cohesion 5MPa Friction angle 15 o Fracture spacing 0.5 m Fractures are modelled using Coulomb Slip model Elastic properties of joints/fractures K n 10 GPa/m K s 5 GPa/m

30 Three different arrangements of fractures in the roof were considered Horizontal Fractures(dip 0 o ) Inclined fractures (dip 60 o ) Randomly oriented fractures

31 Arrangement of horizontal Fractures (three panels burning) Fracture spacing 0.5m Dip angle of the fractures 0 o

32 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

Subsidence measured at 395m below the surface(mm) Fractured roof (three panels burning) A B 100 80 60 40 20 0 Sub. at 0m -orginal property Sub. at 100m- orignal property Sub.

33 Subsidence measured at 395m below the surface(mm) Fractured roof (three panels burning) A B 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 Minimum distance between two burning panels(m) 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

34 Subsidence measured at 395m below the surface(mm) Fractured roof (Five panels) A B 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 Mininum distance between two burning panels(m) 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

$Inclined fractures(dip 60 o, spacing 0.$

35 Inclined fractures(dip 60 o, spacing 0.5m) Two different panel burning were considered Single panel 3 Panel with 20m spacing

Shale_nonthermal Z=94 Displacement at the roof (A) 300mm Displacement at the top of

36 Single panel burning(displacement) discontinuum Vs continuum B A B Shale_nonthermal Coal_nonthermal Sandstone_nonthermal Shale_thermal Coal_thermal Sandstone_thermal Shale_nonthermal Z=94 Displacement at the roof (A) 300mm Displacement at the top of model (B) 50mm Y=60 X=200 Displacement at the roof (A) 90mm Displacement at the top of model (B) 23mm

37 3 Panel with 20m spacing (Displacement) Displacement at the roof (Point A) 420mm Displacement at the surface(point B) 100mm B A

$Generation of randomly oriented fractures Four variables are important to derive the fractures Fracture density Fracture length(minimum and Maximum) Fracture orientations Fisher coefficients L i = L$

38 Generation of randomly oriented fractures Four variables are important to derive the fractures Fracture density Fracture length(minimum and Maximum) Fracture orientations Fisher coefficients L i = L min -D - R i (L min -D - L max -D ) -1/D Where R i Random number L min Minimum Length L max Maximum Length D Length distribution exponent Parameter Joint Set 1 Joint Set 2 Joint Set 3 Joint Set 4 Joint Mean Angle ( o ) Standard Deviation in Angle ( o ) Joint Minimum Length (m) Joint Maximum Length (m) Length Distribution Exponent (D) Joint Density (m -2 )

39 Conclusions Maximum Strength increases up to 400 o C and then decreases upon further increasing the temperature (Approach 2). Young modulus decreases by increasing the temperature from 400 o C to 800 o C 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).

40 Conclusions Gradual reduction of Young s modulus were observed with the increment of temperature Maximum stress increased up to 800 o C 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 800 o C

41 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) At 100m away(mm) 8 15

42 Conclusions The Variation of the properties was considered 5 Panels 7 Panels Property reduction 0% 20% % 0% 20% % At the centre point(mm) % % At 100m away(mm) Three different fracture orientations in the roof were incorporated Horizontal fractures Slant fractures Randomly oriented fractures

43 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) % % At 100m away(mm)

44 Thank you very much

TGA analysis Coarse sandstone Fine sandstone 25-400 C: Release

400-800 C: Dehydration of kaolinite and formation of

45 TGA analysis Coarse sandstone Fine sandstone C: Release of physical absorbed water in pores and on the surface occurs C: Dehydration of kaolinite and formation of metakaolinite takes place > 800 C: Recrystallization to form Mullite takes place

46 XRD analysis C 3, 800 Orthoclase Quartz Illite Intensity (cps) , File: 3, 800.raw - Type: 2Th/Th locked - Start: End: Step: Step time: 2. s - Temp.: 25 C (Room) - Time Started: 10 s - 2-Theta: Theta: Chi: Phi: X: 0.0 mm - Y: 0.0 Operations: Import 2θ ( ) 2 Theta (º) (*) - Orthoclase - KAlSi3O8 - Y: 1.18 % - d x by: 1. - WL: Monoclinic - a b c alpha beta gamma Base-centered - C2/m (12) F30= 55(0.0148, (I) - Illite-2M2 - KAl2(Si3Al)O10(OH)2 - Y: 0.47 % - d x by: 1. - WL: Monoclinic - a b c alpha beta gamma Base-centered - C2/c (15) I/Ic PD 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 (*) - Quartz, syn - SiO2 - Y: % - d x by: 1. - WL: Hexagonal - a b c alpha beta gamma Primitive - P3221 (154) I/Ic PDF F30=539( Al 2 O 3 2SiO 2 2H 2 O (Kaolinite) Al 2 O 3 2SiO 2 (Metakaolinite) + 2H 2 O Amorphous structures can not be detected in XRD analysis

47 XRD analysis C 3, 1000 Orthoclase Quartz Mullite Intensity (cps) Theta (º) 10 3, File: 3, 1000.raw - Type: 2Th/Th 20 locked - Start: End: Step: Step time: 2. s - Temp.: 25 C (Room) - Time 40Started: 10 s - 2-Theta: Theta: Chi: Phi: X: 0.0 mm - Y: Operations: Import 2θ ( ) (*) - Orthoclase - KAlSi3O8 - Y: 1.18 % - d x by: 1. - WL: Monoclinic - a b c alpha beta gamma Base-centered - C2/m (12) F30= 55(0.0148, (*) - Quartz, syn - SiO2 - Y: % - d x by: 1. - WL: Hexagonal - a b c alpha beta gamma Primitive - P3221 (154) I/Ic PDF F30=539( (I) - Mullite, syn - Al6Si2O13 - Y: 0.32 % - d x by: 1. - WL: Orthorhombic - a b c alpha beta gamma Primitive - Pbam (55) F30= 60(0.0135,37) The metakaolinite transforms to a spinel structure and amorphous silica at a temperature around 800 C 900 C. Al 2 O 3 2SiO 2 (Metakaolinite) SiAl 2 O 4 (spinel) + SiO 2 (amorphous) Upon further heating up to 1000 C, recrystallization to form Mullite takes place SiAl 2 O 4 (spinel) + SiO 2 (amorphous) 1/3 (3Al 2 O 3 2SiO 2 ) (Mullite) + 4/3SiO 2 (amorphous)