In-Situ Stress and Natural Fractures in Australian Basins
Transcript of In-Situ Stress and Natural Fractures in Australian Basins
School of Earth and Environmental Sciences
In-Situ Stress and Natural Fractures in Australian Basins
Ros King
Hani Abul Khair, Khalid Amrouch, Guillaume Backé, Adam Bailey, Bradley Grosser, Richard Hillis, Simon Holford, Marie Neubauer,
Mojtaba Rajabi, Josh Sage, Ernest Swierczek, David Tassone and Mark Tingay
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S3 Stress Structure Seismic
S3 Stress Structure Seismic
In-Situ Stress and Natural Fractures
“A fracture is considered open to fluid flow when it is optimally aligned with the in-situ stress field”
• Provide secondary permeability and pathways for fluid flow
• Behave as baffles to fluid flow, if sealing
• Hence, understanding natural fracture networks and the character of different fracture sets is crucial to exploration (and production) of unconventional resources
In-Situ Stress Natural Fractures Prediction Conclusions Background
S3 Stress Structure Seismic
In-Situ Stress and Natural Fractures
Three principals stresses within the Earth’s Crust.
Three types of fractures
• Andersonian Faulting Theory (1951) states that:
o Shear Fractures (red) form ~30° to σ1
o Tensile Fractures (green) form parallel to σ1
o Stylolites (compaction bands) form perpendicular to σ1
In-Situ Stress Natural Fractures Prediction Conclusions Background
S3 Stress Structure Seismic
In-Situ Stress and Natural Fractures
Three principals stresses within the Earth’s Crust.
Three types of fractures
• Andersonian Faulting Theory (1951) states that:
o Shear Fractures (red) form ~30° to σ1
o Tensile Fractures (green) form parallel to σ1
o Stylolites (compaction bands) form perpendicular to σ1
In-Situ Stress Natural Fractures Prediction Conclusions Background
σ1
σ3 σ2
σ2
σ3 σ1
σ3
σ1 σ2
Normal Fault Stress Regime
Thrust Fault Stress Regime
Strike-slip Fault Stress Regime
S3 Stress Structure Seismic
In-Situ Stress and Natural Fractures
“A fracture is considered open to fluid flow when it is optimally aligned with the in-situ stress field”
In-Situ Stress Natural Fractures Prediction Conclusions Background
Carnarvon Basin (Bailey in progress)
S3 Stress Structure Seismic
In-Situ Stress and Natural Fractures
“A fracture is considered open to fluid flow when it is ~30° or parallel to the maximum principal stress”
• Mapping the in-situ stresses allows to predict:
– The permeability of existing fracture networks
– The orientations of fracture open to fluid flow
– The orientations of new fractures
In-Situ Stress Natural Fractures Prediction Conclusions Background
Carnarvon Basin (Bailey in progress)
S3 Stress Structure Seismic
In-Situ Stress in Australian Basins
• The Australian Stress Map is well controlled by data
• The pattern of stresses observed are coincident with the complex plate boundaries of the Indo-Australian Plate
In-Situ Stress Natural Fractures Prediction Conclusions Background
Courtesy: The World Stress Map Project
Australian Stress Map (Hillis & Reynolds, 2000
S3 Stress Structure Seismic
In-Situ Stress in Australian Basins
• The Australian Stress Map, indeed the World Stress Map, is composed of multiple data types:
o Earthquake Focal Mechanisms
o Boreholes Breakouts and Drilling-Induced Tensile Fractures (petroleum well data)
o Borehole Slotter
o Overcoring
o Hydraulic Fractures
o Geological Indicators
EQ Mags above 3.0M Yellow 5.0M Orange 4.0-4.9M Pink 3.0-3.9M Jan 1990 to August 2005 GA
SE Australia (King et al., 2012)
In-Situ Stress Natural Fractures Prediction Conclusions Background
S3 Stress Structure Seismic
In-Situ Stress in Australian Basins
• Vertical Stress (σv) – Assumed to be vertical
– Magnitude calculated from density logs
• Maximum Horizontal Stress (σH) – Orientation from drilling-induced
tensile fractures (DITFs)
– Magnitude calculated from relationship of circumferential stress and minimum horizontal stress
• Minimum Horizontal Stress (σh) – Orientation from borehole
breakouts
– Magnitude calculated from leak-off tests (LOTs)
σh
σH
σv
In-Situ Stress Natural Fractures Prediction Conclusions Background
S3 Stress Structure Seismic
In-Situ Stress in Australian Basins: Orientations
In-Situ Stress Natural Fractures Prediction Conclusions Background
Carnarvon Basin (Bailey in progress)
S3 Stress Structure Seismic
In-Situ Stress in Australian Basins: Magnitudes
Carnarvon Basin (Bailey in progress)
σh
σH
σv
Normal σv > σH > σh
Strike-Slip σH > σv > σh
In-Situ Stress Natural Fractures Prediction Conclusions Background
S3 Stress Structure Seismic
In-Situ Stress in Australian Basins: Magnitudes
σh
σH
σv
In-Situ Stress Natural Fractures Prediction Conclusions Background
Normal σv > σH > σh
Carnarvon Basin (Bailey in progress)
S3 Stress Structure Seismic
In-Situ Stress and Natural Fractures
“A fracture is considered open to fluid flow when it is ~30° or parallel to the maximum principal stress”
• Mapping the in-situ stresses allows to predict:
The permeability of existing fracture networks
The orientations of fracture open to fluid flow
The orientations of new fractures
In-Situ Stress Natural Fractures Prediction Conclusions Background
Otway Basin (Sage, 2013)
Otway Basin (Bailey et al., 2013)
Perth Basin (King et al., 2008)
S3 Stress Structure Seismic
Natural Fractures
• Most basins have been subject to multiple tectonic and uplift events that have resulted in several generations and sets of fractures.
• BUT, which of these behave as fluid pathways?
In-Situ Stress Natural Fractures Prediction Conclusions Background
Otway Basin (Sage, 2013) Otway Basin (Sage, 2013)
S3 Stress Structure Seismic
Characterising Natural Fractures
• Fractures observed in image logs: Open vs. Closed
o Open fractures, dark and conductive (filled with drilling mud)
o Closed fractures, light and resistive (filled with cement)
In-Situ Stress Natural Fractures Prediction Conclusions Background
Closed Fractures Open Fractures
S3 Stress Structure Seismic
Characterising Natural Fractures
• Are open fractures really open?
In-Situ Stress Natural Fractures Prediction Conclusions Background
Well Senecio-1, North Perth Basin (Bailey, 2011)
S3 Stress Structure Seismic
Characterising Natural Fractures
• Are open fractures really open?
In-Situ Stress Natural Fractures Prediction Conclusions Background
Otway Basin (Tassone et al., 2012)
S3 Stress Structure Seismic
Characterising Natural Fractures
• Some common cements may be electrically conductive (e.g. siderite) in image logs
In-Situ Stress Natural Fractures Prediction Conclusions Background
Well Jacaranda Ridge-1, Otway Basin (Bailey et al., in review)
S3 Stress Structure Seismic
Relationship Between In-Situ Stress and Fractures
• Intimate relationship between stress and fractures
In-Situ Stress Natural Fractures Prediction Conclusions Background
Perth Basin (Bailey, 2011)
3D Mohr Circle used for Fault Reactivation Diagrams (Courtesy M. Tingay)
S3 Stress Structure Seismic
Relationship Between In-Situ Stress and Fractures
In-Situ Stress Natural Fractures Prediction Conclusions Background
Perth Basin (King et al., 2008)
Fracture
S3 Stress Structure Seismic
Conclusions
• Fractures required for production of unconventionals, so an important factor in exploration
• The in-situ stress field is a major control on fracture formation and reactivation
• A fracture is considered open to fluid flow when it is ~30° or parallel to the maximum principal stress
o The orientations of fracture open to fluid flow
o The orientations of new fractures
• We map and characterise fractures using image logs, core and outcrop
• Open fractures can be both open and siderite filled
In-Situ Stress Natural Fractures Prediction Conclusions Background