CSPG Luncheon Calgary February 5 th 2015 Geomechanical controls on fault and fracture distribution with application to structural permeability and hydraulic stimulation Scott Mildren - Ikon Science
Australian Perspective Source: Gloucester Coal Seam Gas Project, Community Information Fact Sheet, no. 8, October 2008, AGL website, viewed 9 February 2011, http://www.agl.com.au/downloads/gloucester_csg_facts.pdf
Australian Perspective Source: US Department of Energy
Contents 1. Introduction 2. Stresses On Planes 3. Stress Determination 4. Mechanical Stratigraphy (Part 1) 5. A Cheesy Interlude 6. Mechanical Stratigraphy (Part 2) 7. Natural or Drilling Related Fractures? 8. Conclusions
Introduction
Why Do We Bother? Oilprice.net
Why Do We Bother? Effects of oil price drop Reduction in company value ExxonMobil and Chevron both down 6% in last 6 months Halliburton down 40% Smaller companies with % reductions ranging between the two Companies need to secure loans Secure loans by increasing reserves Shift to development over exploration
How Do We Do It? Geomechanics can play an important role Increase reserves by. Increase volume Mapping migration pathways, seal integrity, pore pressure distribution Increase recoverable % Optimising hydraulic fracturing, sweep efficiency, reservoir characterisation Lower development costs Lower NPT, better well locations and minimise hydraulic fracturing
What does stress have to do with it? s Hmax Non-Sealing Faults (423 North Sea faults) Flood Directionality (>80 field cases) s Hmax Rate Correlations (8 field areas, >0.5 million well pairs)
What does stress have to do with it? 30 cm Fracture Aperture 0.25 mm 30 cm 30 cm Matrix Permeability 1 md Ave. Permeability 1 md Matrix Permeability 1 md Ave. Permeability 13 510 md
What does stress have to do with it? Structural permeability (faults or fractures) can significantly impact on permeability. Permeability of a fault or fracture is related to the stresses that are acting on it and the strength of that feature. If we can determine the stresses and their relationship to the strength we can better predict permeability.
Stresses On Planes
The Stress Components Earth s surface is a free surface (no shear stresses) and therefore the vertical stress is a principal stress constraining the other two principal stresses to the horizontal plane. Sv Shmin SHmax
Stresses on a Surface Sv SHmax Shmin
Stresses on a Surface Normal Stress (s n ) Shear Stress ( )
Stress Determination
The Stress Components Sv Weight of the overburden SHmax Shmin
Borehole Stresses s H s H
Borehole Stresses Borehole Breakouts (Compressional) Drilling Induced Tensile Fractures (Tensional)
Mechanical Stratigraphy AZIMUTH 0-360 AZIMUTH 0-360
Horizontal Stress Magnitudes Component from vertical load Tectonic component E
Stress Determination There exists an interdependent link between pore pressure, rock properties and geomechanics. Each of these elements needs to be understood to determine stress magnitudes.
Mechanical Stratigraphy (Part 1)
Mechanical Stratigraphy Low Tectonic Strain e h = (x-dx)/x Dominant term s hmin s V Low Tectonic Strain Controlled by Poisson s ratio Sand Shale Sandstone (or steel) = High E, Low Shale, Coal (or rubber) = Low E, high
High Tectonic Stress Australian Plate High Tectonic Stress CSG High stress sands Low stress coals Breakout preferentially occurs within sands
Mechanical Stratigraphy High Tectonic Strain e h = (x-dx)/x Dominant term s hmin s V High Tectonic Strain Controlled by Young s Modulus Sand Shale Sandstone (or steel) = High E, Low Shale, Coal (or rubber) = Low E, high
Low Tectonic Stress Lower tectonic stress within North American Plate Shale gas Low stress sands Higher stressed shales Breakout preferentially occurs within shales
Mechanical Stratigraphy Ainsa Quarry turbidites, North Spain Shale 2m Sand Centre for Integrated Petroleum Research, Norway & University of Aberdeen http://www.safaridb.com/page/show/example_browse Note the stiff sand layers, intercalated with the soft shale layers
Gamma Ray Lithology Young s modulus E Poisson ratio Mechanical Stratigraphy Ainsa Quarry turbidites, North Spain 2m 2m Centre for Integrated Petroleum Research, Norway & University of Aberdeen http://www.safaridb.com/page/show/example_browse Note the stiff sand layers, intercalated with the soft shale layers How does the mechanical stratigraphy affect stress, strength and rock failure?
Mechanical Stratigraphy
Hydraulic Fracture Propagation A classic kitchen sink experiment: Hubbert and Willis, 1957 Wellbore Fracture Gelatin filled Perspex tube Completed Interval Least Principal Stress Gelatin Least principal stress determines orientation of tensile fracture
Hydraulic Fracture Propagation Vertical fracture in stratified gelatin Fracture Wellbore Fractures can propagate along a plane-of-weakness. Here the gelatin detaches along a layer. Completed Interval Fractures propagate further in some strata than in others Fractures are planar are they?
A Cheesy Interlude
A Little Bit Of Cheese
A Little Bit Of Cheese
A Little Bit Of Cheese
A Little Bit Of Cheese
A Little Bit Of Cheese
A Little Bit Of Cheese
A Little Bit Of Cheese
Mechanical Stratigraphy (Part 2)
Infillings of calcite and bitumen within interbedded dolostones, limestones, siliceous shales and mudstones of the Monterey Fm (Sibson, 1996)
Permeability Network s 1 s 3
Structural Permeability
Stresses on a Surface Normal Stress (s n ) Shear Stress ( )
Assessing Fracture Permeability P shear = σ n + 2T τ μ P hybrid = σ n 4T2 τ 2 4T ΔP P tensile = σ 3 + T Mildren et al (2005)
Fracture Modes Relationship between stress and strength that describes the occurrence, mode and orientation of failure s n
Infillings of calcite and bitumen within interbedded dolostones, limestones, siliceous shales and mudstones of the Monterey Fm (Sibson, 1996)
Assessing Fracture Permeability DP [MPa] 8.3 2.7
Fault Risk and Migration Pathways
Natural or Drilling Related Fractures?
Borehole Stresses s H s H
Natural vs Drilling Related Fractures What is a natural feature? Any feature that exists prior to drilling What is an induced feature? Any feature created as a result of drilling or testing
Natural vs Drilling Related Fractures Borehole breakout DITF
Natural vs Drilling Related Fractures Breakouts Natural Fractures/Fault Compression Tension
Natural vs Drilling Related Fractures Natural Drilling Enhanced Natural Fractures/Fault Compression Tension
Natural vs Drilling Related Fractures Drilling Enhanced Fractures or Inclined DITF? Compression Tension
Fracture Classification
Structural Interpretation 100m Mostly continuous fractures and faults Mostly discontinuous fractures Low fault density
Conductive Fractures 0 20 0 19 0 8 Discontinuous wellbore Continuous around the wellbore Full Wellbore Dominant strike direction oriented approximately 150 N and secondary populations striking east-west, and approximately 060 N Discontinuous population has an east-west striking population not reflected in continuous and full wellbore fractures.
Fracture Permeability Upper Lynott Formation DP [MPa] DP [MPa] 42.5 8.3 Cohesive model 38.5 2.7 Non-cohesive model
Conclusions
Conclusions Mechanical properties play a large part in governing the way rocks behave under stress. High vs low tectonic stress environments. Dramatic geomechanical differences between formations within individual wells. Fracture distribution, orientation and mode are controlled by mechanical stratigraphy and stress distribution. Stresses can dramatically impact on structural interpretation of image logs.
Thank you Any Questions? Dr. Scott Mildren smildren@ikonscience.com