Structural modeling of casings in high temperature

geothermal wells

05/02/2023

#GGW2016

Gunnar Skúlason Kaldal (gsk@isor.is)PhD student at University of Iceland / Engineer at ÍSOR – Iceland GeoSurvey

Magnús Þór Jónsson, Halldór Pálsson and Sigrún Nanna KarlsdóttirUniversity of Iceland

Introduction• PhD project at University of Iceland• Structural models of casings for evaluating well integrity and casing

failure modes• Examples FEM analyses and results• Thermal expansion is one of the most severe structural concerns in

high temperature geothermal wells• Cemented steel casings are constrained by the cement and high forces

generate plastic (permanent) deformations as the casings warm up• Failure modes from thermal expansion (and contraction) include:

• Casing collapse (in the form of a bulge/pucker) • Tensile rupture where the casing (pin) is teared out of the coupling (box) by the

threads or the pipe body

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High temperature geothermal wells• Typical casing program includes 3 casings (API grades)

• Conditions sometimes call for more casings• Casings are cemented over their full length• Perforated liner supports the wellbore in the

production section of the well• Expansion spool is used to allow thermal expansion of

the production casing at the wellhead

Fig: Sigrún Nanna Karlsdóttir

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High temperature geothermal wells• The design of high temperature geothermal wells is

based on API materials and methods, and knowledge gained over the past decades in the geothermal industry

• The design procedure for typical high temperature geothermal wells is good and failures are not very common in conventional wells

• There are however several exceptions• Many parameters influence success of wells• “Structural success” is an important one, for usability

and overall reliability and safety

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Design challenges for the future• Most casing failures that occur in wells are directly related to

large temperature changes and high annular pressure

• Typical wellhead temperatures in high temperature geothermal wells is 200-300°C

• In IDDP-1, still the hottest recorded well to date, superheated steam was produced at the wellhead with temperatures of 450°C

• Future aim is to produce from supercritical source where temperatures could reach as high as 550°C – IDDP-2?

• This provides new challenges in casing design as design standards do not account for these high temperatures

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Failure modes• Collapse (bulge/pucker) – pressure difference/thermal expansion• Tensile rupture – thermal expansion (axial)

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HS Orka / ÍSOR

Adopted from a diagram by Rahman & Chilingarian, 1995

FEM Modeling• The nonlinear behavior of materials, displacements

and friction between contacting surfaces are solved with numerical methods.

• The (Nonlinear) Finite Element Method is used.• Thermal and structural models of the cased section of

the well.• The models are used to evaluate the structural

integrity of the casings when subjected to transient thermo-mechanical loads.

• Three models presented:– Cased section of the well (2D axi-symmetric)– Connection in concrete (2D axi-symmetric)– Section of the well (3D collapse analysis)

i.ii.

iii.

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FEM results1. Cooling due to drilling2. Warm-up3. Discharge (12 minutes)4. Discharge (3 months)

1 2

3 4

700

m

100 m

• Production history modeled.• T-P logs and wellhead data are used as load.• Transient thermal analysis is performed and

the results used as load in the structural analysis.

1. Cooling due to drilling.2. Thermal recovery.3. Discharge (12 min).4. Discharge (3 months).

◦C

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• The wellhead rises as the casing suddenly warms up during discharge.

• The production casing expands and slides inside the wellhead (expansion spool). Wellhead displacement.

Temperature distribution after 9 days of discharge.

Stress-strain curves are implemented for steel. Stress reduction at elevated temperatures is accounted for by scaling E, σy and σu (acc. to Snyder). Friction is defined between

casings and concrete.

Karlsdottir, S.N. and Thorbjornsson, I.O.,2009

FEM results #GGW2016

Wellhead displacement survey

Photographic series of the wellhead of HE-46 during discharge.

52 mm40 mm

Merged photographs of the wellhead of RN-32 after 9 days of discharge.

26 mm

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• Wellhead displacement measured during discharge.

Kaldal, G.S., Jonsson, M.T., Palsson, H., Karlsdottir, S.N., 2015. Structural modeling of the casings in high temperature geothermal wells. Geothermics 55, 126 – 137.

Wellhead displacement survey

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• Modeled wellhead displacement compared to data.

Model resultsKaldal, G.S., Jonsson, M.T., Palsson, H., Karlsdottir, S.N., 2015. Structural modeling of the casings in high temperature geothermal wells. Geothermics 55, 126 – 137.

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• Anchoring of couplings in concrete.• Large stresses are produced near couplings.

Concrete failure

Model i. Cased section of the well Model ii. Coupling in concrete

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• Connection displacement in concrete.• Upward displacement of 5 mm.

Damaged concreteConcrete failure

• Case study: structural analysis of IDDP-1.• Operation history modeled.

• Initial conditions.• Warm-up.• Discharge.• Shut-in and quenching.

Kaldal, G.S., Jonsson, M.T., Palsson, H., Karlsdottir, S.N., 2015. Structural modeling of the casings in high temperature geothermal wells. Geothermics 55, 126 – 137.

Discharge history of IDDP-1

FEM results #GGW2016

Ingason et.al, Geothermics 2013:

• Stress and strain analysis. Discharge phase V of IDDP-1

Anchor casingProduction casing

FEM results #GGW2016

• Permanent strain is generated in the casings during the operation history.

Anchor casingProduction casing

Discharges: Cyclic stress-strain and temperature of the production casing at 50 m depth.

FEM results #GGW2016

• Collapse analysis of the production casing.• Some instability needs to be introduced.

Collapse analysis

0 5 10 15 20 25 30 35 40 450

10

20

30

40

50

60

70

80

90

100

D/t ratio

K55

Col

laps

e pr

essu

re [M

Pa]

Yield strength collapsePlastic collapseTransition collapseElastic collapse9 5/8 (47.0 lb/ft)13 3/8 (68.0 lb/ft)

ISO/TR 10400:

Eigenvalue buckling analysis (theoretical collapse strength).

1

5

2

6

3

7

4

8

Casing: OD = 13 3/8 in, t = 12.2 mmAPI collapse resistance: 13.4 MPa

Eigenvalue buckling analysis (theoretical collapse strength). Nonlinear buckling analysis (includes nonlinearities). Effect of initial geometry; mode shape perturbation, effect of

ovality and external geometric defect. Collapse shape with and without external concrete support.

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Nonlinear buckling analysis.Other defects:

Mode shape perturbation Ovality External defect Water pocket in concrete

Casing: OD = 13 3/8 in, t = 12.2 mmAPI collapse resistance: 13.4 MPa

• Limit load for a perfectly round casing: 38.4 MPa• Limit load using mode shape perturbation: 21.6 MPa • API collapse resistance: 13.4 MPa

0 10 20 30 40 50 60 70 800

10

20

30

40

50

60

Load

, ext

erna

l pre

ssur

e [M

Pa]

UX displacement [mm]

Perfectly round casing1st mode shape perturbation (0.0005 scaling)1st mode shape perturbation (0.001 scaling)Collapse resistance, 13.4 MPa (API, ISO/TR)Elastic collapse (Timoshenko 1961)

Mode shape perturbation

Dmax

Dmin

Effect of ovality

0 10 20 30 40 50 60 70 800

10

20

30

40

50

60

Load

, ext

erna

l pre

ssur

e [M

Pa]

UX displacement [mm]

Perfectly roundOvality (0.1%)Ovality (0.5%)Ovality (1.0%)Ovality (2.0%)Ovality (3.0%)Collapse resistanceElastic collapse

Von Mises stress at collapse: 440 MPaCollapse at 300°C and 20 bar (wall pressure)

Water pocket in concrete

Collapse analysis

Nonlinear buckling analysis

0 50 100 150 200 250 3000

10

20

30

40

50

60

Displacement [mm]

Loa

d, e

xter

nal p

ress

ure

[MPa

]

Concrete support (linear MP)Without concrete support (linear MP)Concrete support (non-linear MP)Without concrete support (non-linear MP)Collapse resistance, 13.4 MPa (API, ISO/TR)Elastic collapse (Timoshenko 1961)

Casing: OD = 13 3/8 in, t = 12.2 mmAPI collapse resistance: 13.4 MPa

Effect of external defect and concrete support

Collapse analysis #GGW2016

Summary and conclusions• Analyses of the casings in high temperature geothermal wells were presented.• High temperature and pressure differences generate many challenges in geothermal wells.• Thermal expansion is one of the major cause of casing failures.• Three models were presented here:

• (i) the cased well, (ii) detailed coupling in concrete and (iii) 3D model of a section (collapse analysis).

• The models are used to evaluate the structural integrity of casings.• Can be used to analyze various load scenarios and material selections.• Conclusions…

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Conclusions - Collapse• Caused by excessive net external pressure• Impurities and geometry (ovality, eccentricity, material..)• Cement integrity and casing roundness (and other defects) have great

effect on collapse resistance of casings• Cement is very important for lateral and radial support• Collapse resistance can be increased by selecting proper materials (HC)

under strict quality control (casing roundness important)• Biaxial loads affect collapse resistance HS Orka / ÍSOR

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Dall‘Acqua et.al 2012 Burst and Collapse Responses of Production Casing in Thermal Applications

Wu et.al 2008 Casing Failures in Cyclic Steam Injection Wells

• Collapse strength reduction due to axial tension is incorporated into API standards (compression not)

• Axial compression plus net external pressure probably also leads to reduction in collapse resistance (is not well known or standardized)

Conclusions – Tensile rupture• Caused by excessive axial tensile stress when casings cool down

after production, i.e. due to long period shut-in or killing operations

• The phenomenon is well understood, but limits of cooling rate or limit in temperature variations ∆T remains unresolved

• The failure mode has occurred in several wells in Iceland in connection to fast cooling while pumping cold water into a hot well

• FEM analyses indicate that:• Failures are more likely to form near changes in outer casings, e.g. at

material grade changes (T95-K55) and near casing shoes

• Thermal gradient between casing layers leads to thermal expansion mismatch which generates stress/strain

• Slow temperature changes have less consequences than fast ones

• The thermal load is more severe for the innermost casing which is in direct contact to the geothermal fluid than external casings (provided that cementing in between is good)

HS Orka / ÍSOR

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Adopted from a diagram by Rahman & Chilingarian, 1995

What further information is needed?• Material integrity at high temperatures >350°C

• Strength reduction• Corrosion• Creep and stress relaxation

• Can wells for 300°C+ be designed within the elastic region of materials?

• Is it possible to quench/cool a >550°C hot well without causing casing failures?

• What can we do to mitigate thermal expansion?

A well known problem of thermal expansion (ΔT day/night)

Conclusions – Looking ahead #GGW2016

Acknowledgements• The University of Iceland research fund• The Technology Development Fund at RANNIS –The Icelandic Centre

for Research• Landsvirkjun – Energy Research Fund• GEORG – Geothermal Research Group

• Reykjavik Energy, ON, HS Orka, Landsvirkjun, Iceland Drilling, Iceland GeoSurvey (ÍSOR), Mannvit and the Innovation Center Iceland.

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Thank you

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