Stress in the Ground - Sigra Australia€¦ · • Horizontal stress under zero lateral strain ......
Transcript of Stress in the Ground - Sigra Australia€¦ · • Horizontal stress under zero lateral strain ......
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Stress in the Ground
Ian Gray
Eurock 2018 - 22 May
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What is stress?
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Stress is not Pressure
• Stress acts in directions and is described
by a tensor
• Pressure acts in all directions and is a
scalar quantity
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Why does stress matter?
• If stress exceeds strength failure occurs
• If stress is too low failure may also occur
Imagine a string of cotton reels
• Stress affects permeability of rock
Especially in coals
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Sedimentary Deposition
• Marine or lacustrine environment
• Usually low energy process
• Soil stress states exist
– Consolidation – greatly assisted by earthquakes
– Limits of active and passive states
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Effective Stress in a Soil
𝜎′=𝜎 − 𝑃
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Concept of effective stress holds with point contact – fluid acts on all surfaces
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Effective stress also applies to
clays
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Lithification Occurs
–Cementation and crystal growth take place
–The soil becomes a rock with remaining soil stresses?
–With stiffer and more elastic properties
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What happens if there is little
void space?
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In rock the effective stress
equation changes
𝜎′𝑖𝑗 = 𝜎𝑖𝑗 − 𝛿𝑖𝑗𝛼𝑖𝑃
𝜎′𝑖𝑖 = Principal effective stress
𝛿𝑖𝑗 = Kroneker delta
𝛼𝑖= Biot’s coefficient
𝑃= Fluid pressure
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휀 = 0
Z
𝜎𝑣 = 𝜌 ∙ 𝑔 ∙ 𝑧
Using this
model
(휀 =∆𝑙
𝑙)
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Rock as an Elastic Solid
- A big jump
• Vertical Effective Stress
𝜎′𝑣 = 𝜌𝑔𝑧 − 𝛼𝑣𝑃
• Horizontal stress under zero lateral strain
𝜎′ℎ = 𝜎′𝑣
𝑣
1−𝑣
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The Importance of Poisson’s
Ratio 𝜈• Lithostatic Horizontal stress
• 𝜎ℎ = 𝜎𝑣 (𝜈
1−𝜈)
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.1 0.2 0.3 0.4 0.5
v/(
1-v
)
v
Lateral stress due to Poisson's Ratio
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The fluid pressure is very
important• Fluid pressure can be at the minimum
principal stress level
• More usually at hydrostatic level where it
is related to groundwater
This indicates vertical connection
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Let us consider stress in an
environment where the lateral
strain is not zero• Lateral moment due to tectonic action
• Due to folds
– Anticlines Synclines Monoclines
• The effects of overconsolidation,
lithification, diagenesis, cooling and
anything else!
• Call it TECTONIC STRAIN
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Tectonic Strain – when the
lateral strain is not zero• Sedimentary rocks have stiffness that
varies layer by layer (or intrusion)
• Stresses vary with stiffness
• Tectonic strain is the strain that is required
to develop the measured stress
• The general rule is that tectonic strains are
fairly even through sedimentary
sequences. There are exceptions!
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Layered Sedimentary Strata with Varying
Stiffness and Poisson’s Ratio
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Queensland
site
Stress ___
Tectonic
strain_____
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Queensland Site
Stress ___
Tectonic
strain_____
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Igneous Rocks
• Dykes and sills are indication of the
stresses existing at the time of injection
they are giant hydrofractures
• Igneous bodies cool and loose strain but
may gain high surface stress with uneven
cooling
• Stiff rocks attract much more stress with
strain hence a stiff, strong rock may be
highly stressed or de-stressed.
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Poroelasitic Behaviour
• Strain in terms of effective stress
• ∆휀𝑖=1
𝐸𝑖∆𝜎′𝑖 −
𝑣𝑗𝑖
𝐸𝑗∆𝜎′
𝑗 −𝑣𝑘𝑖
𝐸𝑘∆𝜎′
𝑘
• 휀𝑖 =1
𝐸𝑖(∆𝜎𝑖−
𝑣𝑗𝑖
𝐸𝑗∆𝜎𝑗 −
𝑣𝑘𝑖
𝐸𝑘∆𝜎𝑘)
−∆𝑃(1
𝐸𝑖𝛼𝑖 −
𝑣𝑗𝑖
𝐸𝑗𝛼𝑗 −
𝑣𝑘𝑖𝐸𝑘
𝛼𝑘)
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The Big Sponge
• The equations describe a block of rock
that behaves as a sponge which expands
with internal fluid pressure.
• Determining poroelastic coefficients
requires measuring the elastic parameters
and then injecting fluid into the specimen
while monitoring strain.
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Deformation under a unit stress
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Recovery under unit fluid pressure
Biot’s? Tensor
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Effective Stress and a fracture
• The concept of effective stress only
applies to the direction normal to the
surface in question.
𝜎𝑛′ = 𝜎𝑛 − 𝛼𝑃
• Where 𝜎𝑛′ is the effective normal stress across a
specific plane
• 𝜎𝑛 is the total stress across the plane
• 𝛼 is Fracture area ratio
• 𝑃 is the fluid pressure
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Effective stress in fractures
• Use the term fracture area ratio rather
than Biot’s or poroelastic Coefficient
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𝛼= Blue Fractured Area/Total Area
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Consider a block of obsidian
with a single fracture
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En Echelon Fractures
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Slope with not fully connected
fractures
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Slope with fully developed
fracture set
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Fracture Area Ratio
• This is highly variable
• Is the fracture open or closed?
• What is the infill?
• Crystalline infill has a true poroelastic
Biot’s coefficient = 0
• Consider talc in its original state Biot=0
• If disturbed does Biot =1?
• It is dependen
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Failure due to changing
effective stress• Effective stress changes because of
• Varying fluid pressure
• Changing fracture system leads to
changing Fracture Area Ratio
- This really means the fractures are
developing and linking up
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Stress Measurement in Rock
• Hydrofracture
• Borehole Breakout
• Overcoring
• Slot jacking
• Anelastic Recovery
– Uncertain and difficult to achieve
• Kaiser Effect
– Developed for metal does not work in rock
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Hydrofracture
• If minor stress is not coincident with hole axis
then there is a problem
• Opening pressure flow rate dependent and
dependent on openness of fracture – joint.
• By definition the pressure between the
packer and borehole wall must be higher than
the fluid pressure – hence the packers
frequently induce failure.
• Requires highly skilled interpretation
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Pre-Closure
analysis :
fracture closure
pressure
After-Closure
analysis :
kh/ μ, and Pi
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G-Function plot
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Hydrofracturing can be useful• To establish a minimum stress from
closure
• As part of diagnostic fracture injection
testing when the closure phase is also
used for permeability measurement
• To determine in-situ fluid pressure at the
end of leak-off period
• To determine normal stress across a joint
• To measure stress in a hole that has been
drilled40
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Acoustic scan showing breakout
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Borehole Breakout
• Primarily Biaxial stress direction indicator
• May be analysed for major stress value if
you know the UCS of the rock and the
minimum stress (from hydrofracture
closure)
• Relations between sonic logs and UCS not
adequately reliable for quantitative
analysis of breakout
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Surface Overcoring
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Overcoring
• Most convenient indirect system
• Requires, elastic though not necessarily
linearly elastic response for analysis
• This means that failure of the borehole
wall is a problem, i.e. if major stress is 1/3
to 1/2 of UCS breakout may occur
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Overcore devices
• Glued devices – all give poor reliability in
wet holes
– Doorstoppers
– Leeman Triaxial Cell - ANZSI cell
– CSIRO HI Cell - Borre Probe
– Cone cell
• Mechanical Devices
– USBM deformation gauge
– Sigra IST tool
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Doorstopper
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CSIRO HI Cell
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Cone Cell
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Sigra IST system
• Quick biaxial overcore system
– 100 m hole overcore in 1 ½ hours
– 500 m hole overcore in 3 hours
– 800 m hole overcore in 4 hours
• Used primarily with HQ wireline coring
system
• Mostly used in vertical holes
• Capable of operation to 1500 m vertical
depth50
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Sigra Stress
Measurement Tool
51 © 2010
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In-Situ Stress Measurement Tool
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IN-SITU STRESS MEASUREMENT
TOOL
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IST TOOL
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IST – TOTAL DEFORMATION
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IST – OVERCORE DEFORMATION
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IST RADIAL DEFORMATION &
BEST THEORETICAL FIT
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Combinations of Solution
• 1 x 6 pin pair set solution
• 6 x 5 pin pair set solution
• 15 x 4 pin pair set solution
• 20 x 3 pin pair set solution – do not use
as no redundancy
• Can handle minor breakout or fracture
problems – just ignore pin result
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Used in multiple rock types
• Very weak rock with UCS of 4 MPa –
Crinum Colliery
• Very hard rock with UCS of 280 MPa and
stiffness of 80 GPa at Burdekin Falls Dam
Spillway
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Sedimentary
sequence at
Grasstree Mine
German Creek
seam mined first
below Aquila
seam
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Grasstree mine
tectonic strains
of unmined area
in north and
above mined
area to south
where stresses
are aligned with
the longwall
block
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Grasstree Conclusions
• Even tectonic strains throughout sequence
in unmined areas
• Similar apparent tectonic strains 120 m
above goaf of German Creek seam but
re-orientated
• Mid panel – in line with longwall panel
• Over top of pillars - variable
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Illawarra
Site
Complex
stress
distribution
and
rotation of
direction
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Illawarra Site
• Original NE-SW stress orientation
• Very complex stress distribution
– Required IST, borehole breakout & seismic to
understand
• Principal stress directions rotated up to 90
degrees
– Due to reverse and slip strike faulting relieving
stress
– Frequent reverse faulting leading to in-seam
shears68