Download - ShaleTech 2011 Presenations
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A Study of Shale-Gas Recovery
Mechanisms
Dmitriy Silin and Timothy Kneafsey
Lawrence Berkeley National LaboratoryHouston, TX
August 25 2011
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Acknowledgments
Jonathan Ajo-Franklin, Peter Nico, Liviu Tomutsa, Earth Sciences Division, LBNL
Alastair MacDowell, Advanced Light Source, LBNL
Stefano Cabrini, Molecular Foundry, LBNL
Velimir Radmilovic, National Center for Electron Microscopy, LBNL
Andrew Mei, Engineering, LBNL
Iraj Salehi, GTI
Julia Gale, Texas BEG
BP, Chevron, Schlumberger, Whiting Petroleum
Funding: RPSEA Unconventional Gas Program
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The outline
• Objective: gain insights into physical mechanisms of
unconventional gas recovery
• Approach: Imaging the 3D pore structure of reservoir
rock samples at different scales
X-ray computed tomography (CT)
SEM/FIB/EDS
• What we observe
Diversity from nano- to mega-scale
Observations → shale gas flow model
− Bi-modal production rate decline
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Rock imagingCores X-ray CT
Fractures
Irregularities
Sample damage
Sample selection
for micro CT, FIB X-ray micro CT SEM/FIB/EDS
~3.4 mm0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
kr
Pc
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Unconventional resourceDomengine sandstone Barnett shale
Micro CT data: Jonathan Ajo Franklin, LBNL Micro CT data: Peter Nico, LBNL
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Micro-CTNon-destructive imaging
� Micro cracks
� Heterogeneity
� “Surprises”
100 µm
Micro-yeti footstep?Micro CT data: Peter Nico, LBNL
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FIB/SEM: micron-scale volume,
nanometer-scale resolution
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Reconstructing 3D pore structure
• Pros:
– Relatively simple sample preparation
– Automation of milling/imaging sequence
• Cons
– Cannot be repeated
– Depth of study: tens of microns
– Image segmentation might be difficult
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New Albany shale• Connected network of
“pores” filled with kerogen
• Approximately 18% of the bulk volume
• No visible porosity inside kerogen
Kerogen saturation
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Submicron fractures
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Barnett shale
Connected poresin kerogen
Intergranular pores
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Elemental mapping
FIB/SEM
SiCa
Fe, S
Al
C
SEM/EDS
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Common for different samples
Narrow pores
– Extremely low permeability
– Heterogeneous porosity
Variable presence of organic matter
– Thermal maturity
– Relative volume
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Gas flow model
ρ0: Gas density in standard conditions
Kerogen density
SK
Kerogen relative volume
Gas mobility
p: Gas pressure
φφφφ: porosity available for flow cf=f'(p): The slope of adsorption isotherm
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Geometry of flow
SPE 149489
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Model: bimodal recovery rate decline
“Late” (t > t*) recovery rate
“Early” (t < t*) recovery rate
SPE 149489
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Model testing
Data: http://www.rrc.state.tx.us/
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Model testing (2)D
imen
sio
nle
ss p
rod
ucti
on
rate
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Adsorption vs free gas storage
Kerogen relative volume
… if kerogen in reservoir conditions adsorbed methane like high-quality activated carbon
in laboratory at 40 Bar, 300 K ...
ASF ~ O(1)
Adsorption storage factor - ASF
Pores in activated carbon
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Summary and conclusions
� Imaging the pore-scale complexity of gas shale
requires nanometer-scale resolution
Small pores
Mineral diversity
Variable presence of organic matter
� Pore-scale analysis → critical parameters
Relative volume of kerogen
Adsorption storage factor - ASF
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Summary and conclusions (2)
� Pore-scale analysis + limited data → physics-
based model of fractured gas well production
Approximate analytical solution
Bimodal recovery rate decline
− Square-root early recovery decline
− Exponential late recovery decline
− Dividing point: limits of the stimulated reservoir zone
Relative contributions of free and adsorbed gas
storage, early and late production