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© 2019 Chevron Corporation All rights reserved
Geomechanical Well Damage in Conventional
and Unconventional Reservoirs
53rd US Rock Mechanics / Geomechanics Symposium
Russ EwyJune 25, 2019 Robert Polzer, Peter Connoly, Bill Jenkins, Ryan Edwards, Matt
Paradeis, Rajesh Nair, Lisa Song, Vahid Tohidi, Mike Ash, Jim Baranowski, Exponent (Brun Hilbert, Nicoli Ames), Noetic, C-FER
With thanks to:
2© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Outline
• Quick compaction/subsidence overview• Types of well deformation/damage
– In a compacting reservoir– Above a reservoir
• Key controls on well damage risk• Diagnosis tools (determining the deformed casing shape)• Screening methods for damage risk in the reservoir and in the
overburden• Well deformation/damage during hydraulic fracturing in
unconventionals• Summary
3© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Key Messages
• Well damage within depleting reservoirs is due to formation vertical compaction strain being directly transferred to the well
• Well damage in the overburden is almost always due to lateral shear slip on weak bedding interfaces, caused by overburden bending
• In unconventionals (‘gas’ shales), damage is due to lateral shear slip on bedding or on natural fractures, but likely caused by high fluid pressure during hydraulic fracturing
• Multifinger calipers, with special processing, are the best tool for determining the deformed casing shape
• Different levels of screening methods are available, for mitigation and prevention
4© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Outline
• Quick compaction/subsidence overview• Types of well deformation/damage
– In a compacting reservoir– Above a reservoir
• Key controls on well damage risk• Diagnosis tools (determining the deformed casing shape)• Screening methods for damage risk in the reservoir and in the
overburden• Well deformation/damage during hydraulic fracturing in
unconventionals• Summary
5© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
For Company Use Only© 2019 Chevron All rights reserved
Compaction and Subsidence
When we produce reservoir fluids, this usually results in a reduction of reservoir pressure
This pressure reduction causes the effective vertical stress to increase, which in turn causes compaction
6© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
For Company Use Only© 2019 Chevron All rights reserved
Compaction and Subsidence
• Reservoir ‘shortens’ vertically• Overburden ‘bends’
7© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
For Company Use Only© 2019 Chevron All rights reserved
Compaction-related well damage occurs both within reservoirs and in the overburden above reservoirs
However, the mechanisms are very different
Within a compacting reservoir• Wells are directly loaded by the vertical
shortening of the formation• Deformation and damage are a function of
the reservoir compaction strain• Vertical and high-angle wells respond
differently
Above a compacting reservoir• Well deformation and damage are due to
shear slip along weak bedding planes• Greater risk of damage is generally
associated with greater total compaction of the reservoir
• More specifically, damage is associated with a high lateral gradient of total compaction
The next several slides will illustrate these deformations, and define compaction strain, total compaction, and lateral gradient
8© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
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Compaction Strain vs. Total Compaction
Sand 1Original thickness (100 feet)
Post-depletion thickness
Change in thickness (1 ft)
Sand 2 Post-depletion thickness
Change in thickness (1.5 ft)
Original thickness (50 feet)
Calculation ExamplesCompaction Strain• Sand 1
1 ft / 100 ft = 1%• Sand 2
1.5 ft / 50 ft = 3%
Total Compaction1 ft + 1.5 ft = 2.5 feet
Example reservoir with two stacked sands
9© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
For Company Use Only© 2019 Chevron All rights reserved
Within a compacting reservoirVertical and low-angle wells
• Casing is forced to be shortened• Deformation mode is generally buckling;
e.g. Euler-type buckle• Buckling can usually be prevented by
assuring complete lateral support throughout field lifetime– Complete cement placement– No solids (formation sand) production
Lateral displacement is magnified
Example, using special analysis of multifinger casing caliper
formationcement
steel casing
10© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
For Company Use Only© 2019 Chevron All rights reserved
Within a compacting reservoirHorizontal / high-angle, and intermediate angle
Horizontal / high-angle• Casing is squeezed vertically; lateral
crushing• Casing becomes ovalized, with a short-
axis in the ~vertical orientation• Diameter reduction can be significant
Intermediate angle• Simultaneous axial shortening and lateral
crushing• Often highest risk of failure due to these
combined deformation modes Lateral crushing is exagerrated
Horizontal / high-angle well
11© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
For Company Use Only© 2019 Chevron All rights reserved
Above a compacting reservoir
Lateral displacement is magnified
Example, using special analysis of multifinger casing caliper
• Variations in total compaction cause the overburden to flex and bend
• This creates shear stresses and shear strains, which can concentrate at layer boundaries
• Shear slip occurs at discrete locations, typically in weak formations (e.g. shales)
Lateral gradient of compaction is this ‘slope’
12© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
For Company Use Only© 2019 Chevron All rights reserved
Overburden Shear - pictures and models
High tensile stress
Casing rupture is usually due to the high tensile stress
High ovality
High ovality
Slight to high ovality;possible necking
Modeling by Exponent
13© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Shear often occurs at multiple depths in the overburden, typically in the first 100-200 m above the reservoir
100 ft(30 m)
reservoir top
And it can worsen with time
14© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Shear slip also can occur due to reservoir injection, especially steam injection
overburden shear
Example finite element model result, including pore pressure increase and heating
Project with R Nair
The overburden flexes and bends above an inflating
reservoir.This is essentially the
‘opposite’ of what happens above a compacting reservoir
Color = vertical strain
Mesh positions show displacement, exaggerated
15© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Slip on faults in the overburden is most likely to occur beyond the lateral boundaries of the reservoir
Vertical stress tends to decrease above the reservoir
Horizontal stress tends to increase above the reservoir
Overburden faults located beyond the reservoir boundaries will tend to have higher slip tendency
Project with R Polzer
(for a normal faulting environment)
16© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Outline
• Quick compaction/subsidence overview• Types of well deformation/damage
– In a compacting reservoir– Above a reservoir
• Key controls on well damage risk• Diagnosis tools (determining the deformed casing shape)• Screening methods for damage risk in the reservoir and in the
overburden• Well deformation/damage during hydraulic fracturing in
unconventionals• Summary
17© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Diagnosis tools/methods
• Sinker bars, drift tools– Essentially a ‘test’ to see what diameter and length tool can pass through the deformed
casing. – Does not reveal actual geometry
• Impression blocks (a hunk of lead)– Can reveal limited information about geometry
• Downhole camera (limited applications; requires clear fluid in hole)• Multifinger caliper
– With special processing, can reveal full 3D shape of deformed casing• Ultrasonic inspection tool
– Typically very sensitive to minor deformations. Good for early stages of deformation.
Deformed casing is usually discovered when a workover tool gets ‘hung up’
Casing caliper geometry
Centralizer
Centralizer
Caliper arms(typically 20 to 40)
Caliper arms are roughly midway between the two centralizers
Distance between centralizers is typically 5 - 7 feet
Additional parts of tool exist above the top centralizer (not shown)
DRAWING IS NOT TO SCALE!
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Progression of caliper eccentering through a shear deformation (two eccenterings)
DRAWINGS ARE NOT TO SCALE!
Calipers are eccentered
Calipers are briefly centered
Calipers become eccentered in the opposite direction
Calipers are no longer eccentered
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Caliper through a half-wave buckle (three eccenterings)
DRAWINGS ARE NOT TO SCALE!
calipers become eccentered
Calipers become eccentered in the opposite direction
Calipers are no longer eccenteredCalipers become
eccentered again but in the original direction
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Example shear deformation observed in casing caliper
Eccentering
Azimuth of eccentering(relative to tool zero)
Characterized by a ‘doublet’ of eccentering, with a 180-deg flip in eccentering direction
connection
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3D shape interpretation (requires special software)
Lateral displacement = 1.6 inches
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Beware: Most casing caliper software does not show the true 3D shape
Shear offset as seen in typical software
This is a series of stacked cross-sections.The well centerline is assumed straight.
23© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Outline
• Quick compaction/subsidence overview• Types of well deformation/damage
– In a compacting reservoir– Above a reservoir
• Key controls on well damage risk• Diagnosis tools (determining the deformed casing shape)• Screening methods for damage risk in the reservoir and in the
overburden• Well deformation/damage during hydraulic fracturing in
unconventionals• Summary
24© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
For Company Use Only© 2019 Chevron All rights reserved
Screening for damage in the reservoir: Calculate the compaction strain
Compaction strain = ∆P * Cbm∆P = amount of reservoir pressure reductionCbm = uniaxial-strain bulk volume compressibility (‘compaction coefficient’)
If compaction strain <3%, generally OK as long as complete cement
and no solids production
Cbm is often measured directly on core samples in the labOr, it can be calculated as:Cbm = [(1+ν)(1-2ν)] / [E(1-ν)]
Project with M Paradeis
Project with A DuToit (CUK)
Horizontal / high angle wells can be designed to resist high compaction strain (e.g. thick-wall casing). Shortening of vertical /
low-angle wells cannot be prevented.
‘Earth model’ examples
25© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
For Company Use Only© 2019 Chevron All rights reserved
Screening for damage above the reservoir: Calculate total compaction, and slip tendency (beds, faults)
Total Compaction = compaction strain * reservoir thickness
If total compaction ~3 feet (1m) or more, there is risk of well shear damage in the overburden (empirical, may not apply everywhere)
This calculation can be ‘back of the envelope’, or can be performed in an earth model, or in a finite element model
Project with A DuToit (CUK)
Mitigation techniques• Avoid areas with a high lateral gradient of compaction• Avoid areas with high slip tendency (beds, faults)• Leave the outer casing string uncemented if possible
Bedding-plane slip tendency calculated in an earth model (uses nucleus-of-strain solution to calculate stress changes in the overburden)
Slip tendency on overburden faults calculated in FEA
Project with P Connolly and T Buchmann
Project with R Polzer
26© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Outline
• Quick compaction/subsidence overview• Types of well deformation/damage
– In a compacting reservoir– Above a reservoir
• Key controls on well damage risk• Diagnosis tools (determining the deformed casing shape)• Screening methods for damage risk in the reservoir and in the
overburden• Well deformation/damage during hydraulic fracturing in
unconventionals• Summary
27© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Casing is nearly always deformed due to shear slip
17% - 35% ovality
900 m
In this example, all four wells on the pad were deformed, and all back near the heel
However, deformations can occur anywhere along the lateral well
These examples were due to slip on bedding, at a lithologic interface
Other cases have been
proved to be due to slip on natural
fractures
28© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Mechanism 1: Shear from accumulated lateral expansion
17% - 35% ovality
900 m
SIDE VIEW
Our models run so far suggest this creates quite low shear stress, except
very close to the hydraulic fractures
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Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Mechanism 2: Horizontal hydraulic fractures
17% - 35% ovality
900 m
SIDE VIEW
A horizontal frac does 2 things:- It reduces the effective normal stress to zero- It destroys any cohesion that is present on the bedding interfaceEven a small amount of shear stress will result in slip
30© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
© 2019 Chevron All rights reserved
Summary• Well damage within depleting reservoirs is due to formation vertical
compaction strain being directly transferred to the well– Buckling of vertical / low angle wells– Lateral crushing of high angle / horizontal wells
• Well damage in the overburden is almost always due to lateral shear slip on weak bedding interfaces, caused by overburden bending– Less commonly, slip on faults
• In unconventionals (‘gas’ shales), damage is due to lateral shear slip on bedding or on natural fractures, but likely caused by high fluid pressure during hydraulic fracturing
• Multifinger calipers, with special processing, are the best tool for determining the deformed casing shape
• Different levels of screening methods are available, for mitigation and prevention (back-of-envelope calcs, earth models, finite element)
31© 2018 Chevron | All rights reserved. This presentation may contain confidential information subject to contractual obligations and is not to be distributed or disclosed to others without the consent of the author.
Coordinated by Enterprise Technical Learning.
For Company Use Only© 2019 Chevron All rights reserved
Extras
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Coordinated by Enterprise Technical Learning.
For Company Use Only© 2019 Chevron All rights reserved
Progression of ultrasonic tool eccentering (shear)
DRAWINGS ARE NOT TO SCALE!
Sonde becomes eccentered
Sonde reaches maximum eccentering
Sonde briefly passes through zero eccentering
Sonde becomes highly eccentered in the opposite direction
Sonde is no longer eccentered
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The ultrasonic tool acts like a pendulum and is very sensitive to minor casing deformations
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Coordinated by Enterprise Technical Learning.
For Company Use Only© 2019 Chevron All rights reserved
Example ultrasonic tool response
Sonde eccentering
Azimuth of eccentering(relative to tool zero)
A ‘doublet’ of high eccentering, with 180 degree flip of direction
–Probably lateral shear offset
A ‘doublet’ of high eccentering, with 180 degree flip of direction
–Probably lateral shear offset
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