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Stanford UniversityGlobal Climate & Energy Project
Integrated Experimental and Modeling Studies of CO2 Sequestration in Coal
Mark ZobackDepartment of Geophysics
Stanford University
GCEP SymposiumSeptember 20, 2006
Sequestration in Coalbeds
– CO2 is sequestered as an adsorbed phase.– CO2 injection can result in enhanced coalbed methane (ECBM) recovery,
off-setting costs for sequestration.– Potential to store large volumes of CO2 (U.S./Canada/China/India/Russia) – Critically important in rapidly growing economies of China and India
http://www.ieagreen.org.uk/8.pdf
Transport Modeling of CO2 Storage and ECBMCarolyn Seto, Kristian Jessen and Lynn Orr
Sorption and Transport in Coalbeds: A Laboratory and Simulation Investigation
T. Chaturvedi, G.-Q. Tang, W. Lin, K. Jessen and Tony Kovscek
Seismic Monitoring of CO2 Storage in Coal BedsChuntang Xu, Jolene Robin-McCaskill, Youli Quan and Jerry Harris
Simulation Studies of CO2 Sequestration and ECBM in Coalbeds of the Powder River Basin
Hannah Ross, Paul Hagin and Mark Zoback
_______________
Using CO2 Injection to Quench Coal Bed Fires – Lynn Orr
GCEP-Supported Coal Sequestration Project Studies at Stanford
Motivating Questions
1. Is it feasible to sequester CO2 in unmineable coalbeds? Are there potential show-stoppers such as a severe reduction of perm with CO2 saturation or the plasticization of coal in the presence of CO2?
2. If so, what volumes of CO2 can be sequestered? Are we accurately simulating injection into a heterogeneous, dual-porosity medium in which transport is controlled by complex physical and chemical processes?
3. To what degree could CO2 injection result in significant improvements in CH4 recovery (ECBM)? Can we can we accurately simulate cost recovery in commercial applications?
4. How do we effectively monitor CO2 storage in coal? Can we distinguish among gases present in situ (CO2, CH4, N2, etc.)?
Transport In Coal
increasing size
increasing sizeincreasing
sizeincreasing
size
adsorption on internal coal surfaces
diffusion through matrix and micropores
bulk flow in the fracture network
If diffusion times are small, matrix is in equilibrium with fracture system
Asorption/Desorption During Transport
Kovscek and Tang, 2004
Fluid Flow SimulationCO2 Sequestration and ECBM
5-spot, 80-acre well spacing using the simulator GEM.
1. Primary production 5 years:– 4 production wells
2. Base case: – One injector and 4 producers – Years 6-11.– Injector BHP constraint of 4 MPa (~600 psi).
3. Hydraulic fracture case: – Hydraulic fracture placed at base of injection well
• 60m in radius, permeability of 1000 md, and porosity of 30%.
Powder River Basin
Currently ~15,000 active CBM wells.
~50,000 more to be drilled in next decade.
History-Matching to Constrain Model Parameters
0.050.011-0.1Matrix porosity
0.5 mD0.04-0.7 mDMatrix permeability
100 mD10-160 mDVertical face cleat permeability
100 mD10-160 mDHorizontal butt cleat permeability
300 mD100-500 mDHorizontal face cleat permeability
ModeMinimum and Maximum ValueProperty
930937Well 531513153Well 416951696Well 328542844Well 217621768Well 1
History-matched Average Water Production per
Month (bbl/month)
Average Water Production per Month (bbl/month)
(WOGCC, 2006)Production Wells
History-matching water production
Geostatistics
3D Model and Cleat Permeabilities
Depth to the top: 315-361 m (1030-1180 ft)Pressure grad: 7.25 kPa/m (0.315 psi/ft)
Horizontal face cleat permeability
Horizontal butt cleat permeability
Vertical permeability
CO2 and CH4 Adsorption/DesorptionAfter 11 Years
CO2 Adsorption CH4 Desorption
Mole/m3 Mole/m3
Volumes of CO2 Injected and CH4 Produced after 6 Years of Injection
• One injection well can inject ~23 kt of CO2 per year.
• Sequester ~95% of total CO2 injected.
• With ECBM, CH4production increased by ~7 fold.
• To first order, need ~2,300 injection wells to sequester the annual CO2 emissions for the state of Wyoming.
• Hydraulic fracture increased injection by ~30%.
Importance of CH4 Adsorption
CO2, CH4, N2 Adsorption/Desorption
• Pure components are well fit by Langmuir isotherms
• CO2 adsorbs preferentially to CH4 and N2
• Adsorption hysteresis for all gases (small for N2)
Powder River Basin (WY) Coal
Kovscek and Tang, 2004
Kovscek and Tang, 2004
Total Volumes of CO2 Injected and CH4 Produced after 6 Years of Injection
• One injection well can inject ~23 kt of CO2 a year.
• Sequester ~95% of total CO2 injected.
• With ECBM, CH4production increased by ~7 fold.
• To first order, need ~2,300 injection wells to sequester the annual CO2 emissions for the state of Wyoming.
• Hydraulic fracture increased injection by ~30%.
Coal Matrix Shrinkage and Swelling
• Shrinkage of coal matrix• Increase in permeability with CH4 desorption.
• Swelling of coal matrix• Decrease in permeability with CO2 adsorption
• Stress dependent permeability• Decrease in permeability with decrease in pore
pressure.
Apparatus for sorption/k-reduction/displacement
• Pore pressure: 60~1100 psi• Gas mixtures made in the lab by weight
Permeability Reduction with CO2 AdsorptionCrushed Coal Pack
Severe decrease in permeability due to CO2 adsorption (matrix swelling).
How do changes in porosity and permeability with adsorption or desorption affect flow processes? Overburden Pressure=400 psi
N2
CO2
50% CO2, 50% N2
25% CO2, 75% N2
85% CO2, 15% N2
75% CO2, 25% N2
0
2
4
6
8
10
12
14
16
18
20
0 100 200 300 400 500 600 700 800 900 1000Pore Pressure (psi)
Per
mea
bilit
y (m
D)
5x
Constant Confining Pressure of 400 psi
100x
Net Permeability ReductionPure and Mixed Gases (Crushed Coal Pack)
• Small fraction of N2prevents severe perm reduction
• Overall perm reduction may be smaller at elevated temperature
• Overall perm reduction may be smaller in whole cores
Pure CO2Mixed Gases
Simulation of CO2 Selectivity During Flowco
al &
CH
4
CH4 + CO2 + N2
gas analyzer
CO2 /N2
p = 600 psiSw = 0
Flue Gas
Post Separation
CH4
DataSimulation
N2
85/15 CO2 /N2
CO2
24/76 CO2 /N2
N2
CH4
CO2
Sensitivity to Hysteresis
CH4 + N2 + CO2
54%N2+46% CO2
• Case 1: Base case• Case 2: Ignoring hysteresis of N2 adsorp/desorp
N2
CH4CO2
12
Advanced Modeling Questions
Analytical Solutions for Simple Flow Paths • Can multiphase, multicomponent flow in coal be
described with analytical 1D solutions?
Streamlines for Computational Efficiency• Can 1D analytical solutions be used for rapid
and efficient streamline calculations or are numerical solutions along streamlines required?
Analytical Solutions by the Method of Characteristics (and Finite Differences)
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2
Sg, u
D
SgSg (fd 1000)uD
0
0.2
0.4
0.6
0.8
1
0 0.5 1 1.5 2
zCH
4 , zC
O2 ,
zH2O
zCH4zCO2zH2O
λ
I
A B
CC
O
CH4
CO2H2O
IA
C
BO
• Compositional mechanisms are revealed by the analytical theory.
• Numerical solutions will likely be required for streamline simulations with gravity.
• 1D solutions will be useful for evaluation of adsorption models and comparison with experiments.
Large CH4 bank propagates through displacement
Single PhaseThree Component
Dimensionless Flow Velocityinlet outlet
Analytic/Finite Difference
Extension of Analytic Solutionsto 4 Component/2 Phase Flow
H2O
CH4
N2 CO2
crossover tieline
AB
O
CD
AFE
I
0
0.4
0.8
1.2
0 0.5 1 1.5 2 2.5 3
zN2,
z CH
4, z C
O2,
z H2O
zN2zCH4zCO2zH2O
0
0.4
0.8
1.2
1.6
0 0.5 1 1.5 2 2.5 3
Sg, u
D
SgSg (fd 1000)uD
λ
A
OB
CCF
D D
E
I
Relatively diffuse displacement front
Analytic/Finite Difference
Dimensionless Flow Velocity
Streamlines for 3D Coal Bed Simulation?
• Streamlines can capture flow in heterogeneous systems.
• If they evolve slowly, then 1D solutions along streamlines can be used to model the combination of compositional effects and flow.
• Streamline computations are fast and affected less by numerical dispersion.
• Compare with conventional coal bed simulation approaches.
Adaptive pressure solveAdaptive pressure solve Move compositionsMove compositionsMOC (analytic) or MOC (analytic) or
HO (numerical) upwindHO (numerical) upwind
Accurate streamline tracingAccurate streamline tracing
Gas injection: streamlines for transport
Improved mapping to streamlinesImproved mapping to streamlinesOr, Improved mapping to pressure gridOr, Improved mapping to pressure grid
Stop
Seismic Monitoring of CO2 Storage in Coal Beds
• Acoustic measurements of saturated coals- Differential Acoustical Resonance Spectroscopy- Changes in compressibility and attenuation with saturation
• Seismic monitoring of CO2 Storage- Time-lapse simulation of seismic attributes- Integration of active and passive monitoring strategies
Time Lapse:AcousticalProperties
vs.Saturation:
CH4, CO2, H2O
DARSAcoustical
Spectroscopy
Low FrequencyCompressibilityAttenuation
3-D Seismic Simulation
Active/Passive SeismicSeismic attributesDynamic aperture geometryTime regularized inversion
DARS: Differential Acoustical Resonance Spectroscopy
Phase-lockAmplifier
Power Amp
SignalGenerator
Charge Amp
Detector
Xducer
Tank
Cavity
PC ControllerSamplePositioner
Resonance Profile
1000 1020 1040 1060 1080 1100 1120 1140 1160 1180 12000
0.05
0.1
0.15
0.2
0.25
Frequency (Hz)
Acoustic Resonance Spectrocopy
d31:c15.85Td31:c46.85T
Boise SSEmpty
Q= f / W
ws
AAs
w
f fs
Ener
gy D
ensi
ty
Frequency (Hz)
Posi
tion
AcousticVelocity(1st mode)
Acousticpressure
DARS Calibration with Gas Permeability
k - gas injection (mDarcy)
κe = κ∞ +
2ϕκ f
l0
Diω
1− exp(− iω / D l0
2)
⎡
⎣ ⎢
⎤
⎦ ⎥
D = k /ϕη(κ f + κ p ) l0 = samplelength
Dynamic Diffusion
Permeability (gas injection), mD
Per
mea
bilit
y (d
iffus
ion
mod
el),
mD
Resonance Profiles
DARS Measurements of Coal Samples
2.42+/-0.3240*87+/-0.5Coal
Sample 5
2.31+/-0.2770*12.111+/-0.5Coal
Sample 4
2.51+/-0.2325*13.915+/-0.5Coal
Sample 3
2.62+/-0.15218*7.1418+/-0.5Coal
Sample 2
2.73+/-0.150.05 – 0.11.919+/-0.5Coal
Sample 1
DARS Bulk
Modulus (GPa)
EstimatedPermeability
(md)
MeasuredPorosity
( %)
Measured Volume
(cc)
Coal Samples
* Dynamic diffusion estimates of permeability
High Interest: Attenuation
Berea-1 Berea -2 Boise Chalk CoalDensity (g/cc) 2.1 2.1 2.3 1.8 1.1Porosity (%) 22.0 19.2 12.0 34.0 1.9Permeability (mD) 367.7 127.1 0.9 2.1 0.1V dry (km/s) 2.5 2.8 2.6 3.0 2.2
3.00
3.20
3.40
3.60
3.80
4.00
4.20
4.40
1076 1078 1080 1082 1084 1086 1088 1090 1092
Frequency (Hz
Berea-1
Berea-2Chalk
BoiseCoal
Line
wid
th(H
z)
Frequency (Hz)
Q = f /W
1000 1020 1040 1060 1080 1100 1120 1140 1160 1180 12000
0.05
0.1
0.15
0.2
0.25
Frequency (Hz)
Acoustic Resonance Spectrocopy
d31:c15.85Td31:c46.85T
Boise SSEmpty
Q= f / W
ws
A
As
w
f fs
Ener
gy D
ensi
ty
Frequency (Hz)
DARS: Flow-induced Attenuation(Drained vs Undrained Sample)
Berea Sandstone Closed
Open
250
260
270
280
290
300
310
320
330
340
350
-50 -40 -30 -20 -10 0 10 20 30 40 50
Sample positioSample position
Qsys
Seismic Monitoring of CO2 Storage in Coal Beds
• Acoustic measurements of saturated coals- Differential Acoustical Resonance Spectroscopy- Changes in compressibility and attenuation with saturation
• Seismic monitoring of CO2 Storage- Time-lapse simulation of seismic attributes- Integration of active and passive monitoring strategies
Time Lapse:AcousticalProperties
vs.Saturation:
CH4, CO2, H2O
DARSAcoustical
Spectroscopy
Low FrequencyCompressibilityAttenuation
3-D Seismic Simulation
Active/Passive SeismicSeismic attributesDynamic aperture geometryTime regularized inversion
Triaxial Deformation Experiments
• Permeability as a function of effective stress, differential stress, CO2 saturation and temperature
• Static and dynamic elastic moduli
• Coal plasticization
Conventional Triaxial PressMax. Confining Pressure = 100 MPa
Pressure Vesselwith Heater Coil
CoreholderSample assembly
Modification of Triaxial Testing Systemfor Coal
Pore Pressure Inlet
Axial and Radial Strain Gauges
Axial Load Cell
Pore Pressure Outlet
Ultrasonic P/S2Transducers
Core Holder Assembly
Permeability - Effective Stress
0.1
1
10
100
1000
0 200 400 600 800 1000 1200 1400 1600
Effective Stress (psi)
Pe
rmeab
ilit
y (
mD
)
Field Measurements(San Juan Basin)
Lab Measurements(Appalachian Basin)
McKee et al., 1988
Permeability and Effective Stress
Laboratory Samples(Appalachian Basin)
Field Measurements(San Juan Basin)
Per
mea
bilit
y, m
D
Effective Stress (psi)
Effect of CO2 on Coal Plasticity
0
50
100
150
200
250
300
350
400
450
0 1 2 3 4 5 6 7
Effective Stress (MPa)
So
fte
nin
g T
em
pe
ratu
re (
Ce
lsiu
s)
CO2Helium
Khan and Jenkins (1985)
Effect of CO2 on Coal Plasticity
Pla
stic
izat
ion
Tem
pera
ture
of B
itum
inou
s C
oal,
ºC
400
300
200
100
0
Effective Stress (MPa)0 1 2 3 4 5 6 7
CO2
He
Transport Modeling of CO2 Storage and ECBMCarolyn Seto, Kristian Jessen and Lynn Orr
Sorption and Transport in Coalbeds: A Laboratory and Simulation Investigation
T. Chaturvedi, G.-Q. Tang, W. Lin, K. Jessen and Tony Kovscek
Seismic Monitoring of CO2 Storage in Coal BedsChuntang Xu, Jolene Robin-McCaskill, Youli Quan and Jerry Harris
Simulation Studies of CO2 Sequestration and ECBM in Coalbeds of the Powder River Basin
Hannah Ross, Paul Hagin and Mark Zoback
_______________
For more information, see 5 posters this afternoon
GCEP-Supported Coal Sequestration Project Studies at Stanford
Permeability ReductionDue to Surface Coverage?
Pure Gases
N2
CO2
Crushed Coal Pack
Sensitivity Analyses
Recommended