A Study of Shale-Gas Recovery

Mechanisms

Dmitriy Silin and Timothy Kneafsey

Lawrence Berkeley National LaboratoryHouston, TX

August 25 2011

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

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

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

Unconventional resourceDomengine sandstone Barnett shale

Micro CT data: Jonathan Ajo Franklin, LBNL Micro CT data: Peter Nico, LBNL

Micro-CTNon-destructive imaging

� Micro cracks

� Heterogeneity

� “Surprises”

100 µm

Micro-yeti footstep?Micro CT data: Peter Nico, LBNL

FIB/SEM: micron-scale volume,

nanometer-scale resolution

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

New Albany shale• Connected network of

“pores” filled with kerogen

• Approximately 18% of the bulk volume

• No visible porosity inside kerogen

Kerogen saturation

Submicron fractures

Barnett shale

Connected poresin kerogen

Intergranular pores

Elemental mapping

FIB/SEM

SiCa

Fe, S

Al

C

SEM/EDS

Common for different samples

Narrow pores

– Extremely low permeability

– Heterogeneous porosity

Variable presence of organic matter

– Thermal maturity

– Relative volume

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

Geometry of flow

SPE 149489

Model: bimodal recovery rate decline

“Late” (t > t*) recovery rate

“Early” (t < t*) recovery rate

SPE 149489

Model testing

Data: http://www.rrc.state.tx.us/

Model testing (2)D

imen

sio

nle

ss p

rod

ucti

on

rate

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

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

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