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Structural modeling of casings in high temperature geothermal wells

Gunnar Skúlason Kaldal (PhD student at the UI) (gunnarsk@hi.is, gsk@isor.is)

Materials Challenges in

Geothermal UtilizationSeminar on Materials Challenges in Geothermal Utilization

Wednesday the 11th of May 2016

Introduction

• During the lifetime of high temperature geothermal wells, casings are subjected to multiple thermo-mechanical loads

• Casing failures (although rare) can occur due to large wellbore temperature and pressure changes

• For safety reasons, the structural integrity of casings is essential for the utilization of wells

• Casing failure modes and nonlinear finite-element (FEM) models of casings are presented and discussed here

Design challenges

• Casings in HT geothermal wells are constrained by cement and high forces generate plastic (permanent) deformations as the casings warm up.

• Most casing failures that occur in wells are directly related to large temperature changes

• Typical wellhead temperature in high temperature geothermal wells is 200-250°C

• In IDDP-1, the hottest 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

• This provides new challages in casing design

Well design – casing program

• Typical casing program: 3 casings

• Casings need to be deep enough in order to seal off unwanted feedzones and to be able to kill the well with water

• Casings are cemented externally from bottom to the surface

• Wellhead is attached to the anchor casing

• The production casing is allowed to expand inside an expansion spool below the master valve

• API casing grades are normally used. Most common: K55, L80, T95 and X-grades (line pipes).

• Casing components are joined with threaded couplings or welded

• Perforated liner is used to prevent collapse of the hole below the production casing

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Casing loads• In general casing design is based on axial

tension, burst and collapse

• In geothermal wells, high temperature generates most problems

• Thermal expansion mismatch between casing and concrete layers generates large forces

• Thermal gradient can be high (during discharge)

• Fast temperature changes have greater effect

• Maintainance stops or other shut-in periods where wells cool down generate further risk of failures due to cyclic loading

Tension/

compressionBurst Collapse

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Casing loads and failure modes

Axial tension/

compression

Compression:

Yield of pipe body or

coupling,

Euler buckling

Tension:

Rupture of the pipe

body or coupling

Burst

Inner vs. outer

pressure greater

that the burst

strength of the

casing results in:

Yield (plastic

deformation)

Pressure loss

(failure)

Collapse

outer vs. inner

pressure greater that

the collapse

resistance of the

casing results in:

Collapse (complete

or partial)

BendingErosion /

corrosion

Many forms of

corrosion:

Uniform corrosion

Pitting

Embrittlement

Cracking

Bending loads are

additional loads that

have effect on axial,

radial and lateral

loads

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Temperature effect

Ref: Behavior of High Strength Structural Steel at Elevated

Temperatures, Chen et.al 2006:

Strength reduction at

elevated temperaturesThermal expansion

Strain →

Str

es

s →

Casing length →

Te

mp

era

ture

ch

an

ge

→

Thermal expansion

• During first warm-up of wells compressive stress is generated in the casing

• This is due to thermal expansion

• The stress reaches the yield point and plastic strain is generated

• If the casing cools down again, high axial tensile forces are present (even though no forces were present before warm-up)

• Contraction of the casing can cause casing failures

• Failure modes:

- Rupture of the pipe body

- Coupling rupture

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Coupling rupture

Casing collapse

• During installation, complete collapse occurs if external cementing pressure exceeds the collapse resistance of the casing

• Casing collapse can also occur during the operation of wells

• Partial collapse is seen in operating wells

• Caused by expansion of trapped water in annulus between casings or high water content in concrete

• But, absolute reason not clear

• Could also be caused by local pressure fluctuations (vigorous two-phase flow, water hammer, cavitation)

• Probably a combination of defects and loads

FEM modeling

• The nonlinear behavior of materials, displacements and friction between contacting surfaces are solved with numerical methods.

• The Nonlinear Finite Element Method (FEM) is used.

• Thermal and structural models of the cased section of the well.

• The models are used to evaluate the sturctural integrity of the casings when subjected to transient thermo-mechanical loads.

• Applications

Cased section of the well (load history of global structure). Coupling in concrete (details modeled further). 3D section of the well (for collapse analysis).

Nonlinearities

Geometric – large displacements and

rotations

Material – stress-strain curves, temperature

dependancy, ...

Status – Closing gaps, contact,

friction, ...

L0

ΔT

RA

RB

FEM - Material propertiesStress-strain curves:

Strength reduction at elevated temperatures:

Temperature dependency:

Surface friction (contact elements):

• MP for casings, concrete and formation

• Thermal properties: Th.conductivity, specific

heat…

• Structural properties: Young‘s modulus,

Poissons ratio, density, thermal expansion

coefficient, coefficient of friction, tensile

curves…

FEM results

• The production history of the well is modeled

• T-P logs and wellhead measurements 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).

1. Cooling due to drilling

2. Warm-up

3. Discharge (12 minutes)

4. Discharge (3 months)

1 2

3 4

70

0 m

100 m

FEM results

• Upward wellhead displacement 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.

Friction is defined between

casings and concrete.

Wellhead displacement measurements

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

Wellhead displacement

9 days

Wellhead displacement

9 days

• 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

K5

5 C

oll

apse

pre

ssu

re [

MP

a]

Yield strength collapse

Plastic collapse

Transition collapse

Elastic collapse

9 5/8 (47.0 lb/ft)

13 3/8 (68.0 lb/ft)

Eigenvalue buckling analysis (theoretical collapse strength).

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

1

5

2

6

3

7

4

8

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

Lo

ad,

exte

rnal

pre

ssu

re [

MP

a]

UX displacement [mm]

Perfectly round casing

1st 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

, ex

tern

al p

ress

ure

[M

Pa]

UX displacement [mm]

Perfectly round

Ovality (0.1%)

Ovality (0.5%)

Ovality (1.0%)

Ovality (2.0%)

Ovality (3.0%)

Collapse resistance

Elastic collapse

Von Mises stress at collapse: 440 MPa

Collapse 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]

Load

, ex

tern

al p

ress

ure

[M

Pa]

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

“As-built” drawing of IDDP-1 (Pálson et.al 2013, Drilling of the well IDDP-1)

Case study of IDDP-1

Load history of IDDP-1

Wellbore load in the model:

Load history:Ingason et.al, Geothermics 2013:

Transient thermal distribution

Thermal recovery from

drillingDischarge – 2 months Discharge – 11 months Quenching – 8 hours

FEM Results IDDP-1Wellhead displacement

Max von Mises strain

Effect of cyclic loads

Production casing at 50 m depth.

Discharge phase I

Production casing Anchor casing

FEM Results IDDP-1

Discharge phase V

Production casing Anchor casing

FEM Results IDDP-1

Quenching

FEM Results IDDP-1

Production casing Anchor casing

Anchoring of couplings in concrete

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Model of the

whole well

Detailed BTC

coupling model

Anchoring of couplings in concrete

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Coupling displacement in concrete

Conclusions

FEM modeling results indicate that:

• Thermal expansion generates large forces in casings

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

• 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)

• The location of casing shoes and changes in casing thickness and/or material generates local strains in neighboring casings

• Couplings are anchored in the cement and due to this generate high stresses in the cement (near the couplings)

• Cement integrity and casing roundness (and other defects) have great effect on collapse resistance of casings

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Acknowledgements

• The University of Iceland research fund

• The Technology Development Fund at RANNIS –The Icelandic Centre for Research

• GEORG – Geothermal Research Group

• Landsvirkjun Energy Research Fund

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

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Thank you