5/10/2016 Structural modeling of casings in high temperature geothermal wells Gunnar Skúlason Kaldal (PhD student at the UI) ([email protected], [email protected]) Materials Challenges in Geothermal Utilization Seminar on Materials Challenges in Geothermal Utilization Wednesday the 11th of May 2016
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5/10/2016
Structural modeling of casings in high temperature geothermal wells
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).
• 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.