Pulsed Arc Electrohydraulic Discharge Stimulation of Coal ...
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Pulsed Arc Electrohydraulic Discharge Stimulation of Coal Seam Interburden for Gas Development Research team: Fei Ren 1,2 , Lei Ge 2 , Huilin Xing 1 ,Tom Rufford 2 , Victor Rudolph 2 1 School of Earth Sciences, 2 School of Chemical Engineering Background & Objectives Interburden is the mixture layer located between the coal measures. Many existing coalbed methane (CBM) wells have already drilled through these undeveloped layers (Fig. 1). However, the potential of coal seam interburden reservoirs (Fig. 2), mainly consisting of mud, clay and organic matter, has not yet been well researched or developed, when compared to that of coal or shale. This project aims at developing and validating an alternative stimulation method to replace traditional fracturing techniques, such as hydraulic fracturing, to effectively crack the thick but malleable mudstone layers without importing any outside chemical fluids into the subsurface or causing clay swelling, to improve the gas recovery from CBM wells. Fig. 3 Schematic of PAED setup for interburden stimulation Results Experimental Setup Develop and employ PAED stimulation technique to crack the interburden specimens at the labscale. The schematic of PAED setup is shown in Fig. 3 Fig. 2 A typical layout of coal formation Testing Sample Information The rationale of PAED stimulation is that two underwater copper electrodes are connected with a set of capacitor banks charged with a high voltage. When the circuit is switched on, a plasma is generated between the electrodes reaching temperatures of thousands of K, resulting in a compressive pressure pulse, the strong pulse/shockwave will propagate in the water and crack the interburden specimen nearby. Coaxial- cable D Control Board 50HZ 240V High voltage transformer Tap wate r Electrodes Core flooding rig with rock sample Spark gap R L C 96.7 mm 63.5mm Shock waves Protectio n device The funding and assistance from ARC , the School of ITEE, and CCSG at UQ is highly appreciated. Acknowledgement & References After the stimulation on C2 by PAED (Fig. 5), the permeability (Fig. 8) and porosity (Fig. 9) of testing C2 mortar specimen have both increased due to the impact of shock waves compared to the properties before PAED. Shockwave generation: • Charging voltage: 20 ~60 KV • Capacitance: 2 ~ 12 uF • Discharging period: nanoseconds ~ 70 us • Pulse number: adjustable • Electrodes gap: 5 mm Permeability measurement: • Confining pressure: 20 bar • Inlet pressure : 2 bar • Outlet pressure : atmosphere Fig. 4 Photograph of PAED testing setup Time (s) 0 500 1000 1500 2000 2500 Permeability (mD) 0 5 10 15 20 25 C2 before PAED stimulation C2 after PAED stimulation K= 16.01 ± 0.01 mD K= 2.68 ±0.01 mD Increase Fig. 8 C2 permeability before and after PAED stimulation Fig. 9 C2 porosity before and after PAED stimulation based on CT image Parameter settings for testing Depth (um) 0 2000 4000 6000 8000 10000 12000 14000 16000 Fracture fraction 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 C2 before PAED stimulation C2 after PAED stimulation Before PAED C2 porosity= 7.87% After PAED C2 porosity= 9.26 % Increase The CT scanning was utilized to analyse the fracture and void evolution before and after PAED stimulation on Gluluguba-2 coal (Fig. 7). 0 10000 20000 30000 40000 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Fracture fraction Depth(um) Before stimulation After stimulation Fig. 10 CT image thresholding at the stimulation side of the coal core sample before (a) and after (b) stimulation (white: fracture and pore) a b Fig. 11 Fracture fraction change with depth from stimulation side Both fracture and void of coal specimen after PAED stimulation have increased (Fig. 10), particularly the front part which is close to the shockwave source (Fig. 11). The specimen section close to wave source 1.RUDOLPH V. et al. 2014. Mudstones as methane sources: stimulation and gas production from coal seam interburden. Project Proposal. 2. SAGHAFI A. et al. 2010. Parameters affecting coal seam gas escape through floor and roof strata. 2010 Underground Coal Operators' Conference: 210-216. Fig. 5 Testing mortar samples made in lab Interburden Interburden Fig. 1 Schematic of a coalbed methane well The project is structured around the following objectives: • Experimentally measure the coal seam interburden properties relevant to gas development • Develop pulsed arc electrohydraulic discharge (PAED) and employ it to enhance/improve interburden permeability • Understand the mechanism of interburden breakage by PAED and simulate the multiphysics coupling using finite element method In the current stage, to explore and summarize the most efficient discharging circuit and parameter settings for strong shock wave fracking, preliminary tests on homogeneous mortar sample (Fig. 5) and identified coal sample (Fig. 7) were carried out. The testing specimens here are homogenous mortars with the compressive strength of approx. 6 MPa (Fig. 6). Fig.7 Gluluguba-2 coal from Surat Basin Sample info. : Surat Basin Gluluguba-2 GD017 Depth: 284 m Wrapped-up by epoxy Before PAED stimulation : porosity =3.11% After PAED stimulation : porosity =3.34% 0 1 2 3 4 5 6 7 0 0.5 1 1.5 2 2.5 Compressive strength (MPa) Extension (mm) Compressive Fig. 6 Compressive strength of mortar sample Another testing coal specimen from the Surat basin was covered in epoxy on the outside to make a cylindrical shape for PAED testing. All the testing specimens are with the same diameter of 63.5 mm, the core flooding testing and X-ray CT scanning were conducted on the samples before PAED to obtain their original structure and permeability.
Transcript of Pulsed Arc Electrohydraulic Discharge Stimulation of Coal ...
Pulsed Arc Electrohydraulic Discharge Stimulation of Coal Seam Interburden for Gas Development
Research team: Fei Ren1,2, Lei Ge2 , Huilin Xing1,Tom Rufford2, Victor Rudolph2 1 School of Earth Sciences, 2 School of Chemical Engineering
Background & ObjectivesInterburden is the mixture layer located between the coal measures.Many existing coalbed methane (CBM) wells have already drilledthrough these undeveloped layers (Fig. 1). However, the potential ofcoal seam interburden reservoirs (Fig. 2), mainly consisting of mud,clay and organic matter, has not yet been well researched ordeveloped, when compared to that of coal or shale. This project aimsat developing and validating an alternative stimulation method toreplace traditional fracturing techniques, such as hydraulic fracturing,to effectively crack the thick but malleable mudstone layers withoutimporting any outside chemical fluids into the subsurface or causingclay swelling, to improve the gas recovery from CBM wells.
Fig. 3 Schematic of PAED setup for interburden stimulation
Results
Experimental Setup
Develop and employ PAED stimulation technique to crack the interburdenspecimens at the labscale. The schematic of PAED setup is shown in Fig. 3
Fig. 2 A typical layout of coal formation
Testing Sample Information
The rationale of PAED stimulation is that two underwater copper electrodes are connected with a set of capacitor banks charged with a high voltage. When the circuit is switched on, a plasma is generated between the electrodes reaching temperatures of thousands of K, resulting in a compressive pressure pulse, the strong pulse/shockwave will propagate in the water and crack the interburden specimen nearby.
Coaxial-
cable
D
Control
Board50HZ
240V
High voltage
transformer
Tap
wate
r
Electrodes
Core flooding rig
with rock sample
Spark
gap
R
L
C96.7
mm
63.5mm
Shock
waves
Protection device
The funding and assistance from ARC , the School of ITEE, and CCSG at UQ is highly appreciated.
Acknowledgement & References
After the stimulation on C2 by PAED (Fig. 5), the permeability (Fig. 8) and porosity (Fig. 9) of testing C2 mortar specimen have both increased due to the impact of shock waves compared to the properties before PAED.
Shockwave generation:
• Charging voltage: 20 ~60 KV
• Capacitance: 2 ~ 12 uF
• Discharging period: nanoseconds
~ 70 us
• Pulse number: adjustable
• Electrodes gap: 5 mm
Permeability measurement:
• Confining pressure: 20 bar
• Inlet pressure : 2 bar
• Outlet pressure : atmosphereFig. 4 Photograph of PAED testing setup
Time (s)
0 500 1000 1500 2000 2500
Pe
rme
ab
ilit
y (
mD
)
0
5
10
15
20
25
C2 before PAED stimulationC2 after PAED stimulation
K= 16.01 ± 0.01 mD
K= 2.68 ±0.01 mD
Increase
Fig. 8 C2 permeability before and after PAED stimulation
Fig. 9 C2 porosity before and after PAED stimulation based on CT image
Parameter settings for testing
Depth (um)
0 2000 4000 6000 8000 10000 12000 14000 16000
Fra
ctu
re f
rac
tio
n
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
C2 before PAED stimulationC2 after PAED stimulation
Before PAED C2 porosity= 7.87%After PAED C2 porosity= 9.26 %
Increase
The CT scanning was utilized to analyse the fracture and void evolution before and after PAED stimulation on Gluluguba-2 coal (Fig. 7).
0 10000 20000 30000 40000
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Fra
cture
fra
ctio
n
Depth(um)
Before stimulation
After stimulation
Fig. 10 CT image thresholding at the stimulation side of the coal core sample before (a) and after (b) stimulation (white: fracture and pore)
a
b
Fig. 11 Fracture fraction change with depth
from stimulation side
Both fracture and void of coal specimen after PAED stimulation have increased (Fig. 10), particularly the front part which is close to the shockwave source (Fig. 11).
The specimen section close to wave source
1.RUDOLPH V. et al. 2014. Mudstones as methane sources: stimulation and gas
production from coal seam interburden. Project Proposal.
2. SAGHAFI A. et al. 2010. Parameters affecting coal seam gas escape through floor
and roof strata. 2010 Underground Coal Operators' Conference: 210-216.
Fig. 5 Testing mortar samples made in lab
Interburden
Interburden
Fig. 1 Schematic of a coalbed methane well
The project is structured around the following objectives:• Experimentally measure the coal seam interburden properties
relevant to gas development• Develop pulsed arc electrohydraulic discharge (PAED) and employ
it to enhance/improve interburden permeability• Understand the mechanism of interburden breakage by PAED and
simulate the multiphysics coupling using finite element method
In the current stage, to explore and summarize the most efficient discharging circuit and parameter settings for strong shock wave fracking, preliminary tests on homogeneous mortar sample (Fig. 5) and identified coal sample (Fig. 7) were carried out.The testing specimens here are homogenous mortars with the compressive strength of approx. 6 MPa (Fig. 6).
Fig.7 Gluluguba-2 coal from Surat Basin
Sample info. : Surat BasinGluluguba-2 GD017Depth: 284 mWrapped-up by epoxy
Before PAED stimulation : porosity =3.11%
After PAED stimulation : porosity =3.34%
0
1
2
3
4
5
6
7
0 0.5 1 1.5 2 2.5
Co
mp
ress
ive
stre
ngt
h
(MP
a)
Extension (mm)
Compressive
Fig. 6 Compressive strength of mortar sample
Another testing coal specimen from the Surat basin was covered in epoxy on the outside to make a cylindrical shape for PAED testing.
All the testing specimens are with the same diameter of 63.5 mm, the core flooding testing and X-ray CT scanning were conducted on the samples before PAED to obtain their original structure and permeability.
