geomechanical well damage in conventional and ......and unconventional reservoirs 53 rd us rock...

© 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




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




Outline








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




Compaction and Subsidence

• Reservoir ‘shortens’ vertically• Overburden ‘bends’




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




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




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




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




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’




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




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




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




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)




Outline









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!


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


Caliper through a half-wave buckle (three eccenterings)


calipers become eccentered

Calipers become eccentered in the opposite direction

Calipers are no longer eccenteredCalipers become

eccentered again but in the original direction


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


3D shape interpretation (requires special software)

Lateral displacement = 1.6 inches


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.




Outline









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




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




Outline









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




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




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




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)




Extras




Progression of ultrasonic tool eccentering (shear)


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


The ultrasonic tool acts like a pendulum and is very sensitive to minor casing deformations




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


geomechanical well damage in conventional and ......and unconventional reservoirs 53 rd us rock...

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