Structural modeling of casings in high temperature geothermal wells

Transcription

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

2 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

3 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 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

4 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 5/10/2016

5 Casing loads In general casing design is based on axial tension, burst and collapse Tension/ compression Burst 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

6 Casing loads and failure modes Axial tension/ compression Burst Collapse Bending Erosion / corrosion Compression: Yield of pipe body or coupling, Euler buckling Tension: Rupture of the pipe body or coupling Inner vs. outer pressure greater that the burst strength of the casing results in: Yield (plastic deformation) Pressure loss (failure) outer vs. inner pressure greater that the collapse resistance of the casing results in: Collapse (complete or partial) Bending loads are additional loads that have effect on axial, radial and lateral loads Many forms of corrosion: Uniform corrosion Pitting Embrittlement Cracking 5/10/2016

7 Stress Temperature effect Strength reduction at elevated temperatures Thermal expansion Casing length Temperature change Ref: Behavior of High Strength Structural Steel at Elevated Temperatures, Chen et.al 2006: Strain 5/10/2016

8 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

9 Coupling rupture 5/10/2016

10 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 twophase flow, water hammer, cavitation) Probably a combination of defects and loads

11 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 thermomechanical 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).

12 Nonlinearities R A ΔT L 0 R B Geometric large displacements and rotations Material stressstrain curves, temperature dependancy,... Status Closing gaps, contact, friction,...

13 FEM - Material properties Stress-strain curves: 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 Surface friction (contact elements): Strength reduction at elevated temperatures: Temperature dependency:

14 700 m 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) m

15 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.

16 Wellhead displacement measurements 26 mm 40 mm 52 mm Photographic series of the wellhead of HE-46 during discharge. Merged photographs of the wellhead of RN-32 after 9 days of discharge.

17 Wellhead displacement 9 days

18 Wellhead displacement 9 days

19 Collapse analysis Collapse analysis of the production casing. Some instability needs to be introduced. 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. Eigenvalue buckling analysis (theoretical collapse strength). K55 Collapse pressure [MPa] Yield strength collapse Plastic collapse Transition collapse Elastic collapse 9 5/8 (47.0 lb/ft) 13 3/8 (68.0 lb/ft) D/t ratio Casing: OD = 13 3/8 in, t = 12.2 mm API collapse resistance: 13.4 MPa

20 Collapse analysis Mode shape perturbation 60 Nonlinear buckling analysis. Other defects: Mode shape perturbation Ovality External defect Water pocket in concrete Water pocket in concrete Load, external pressure [MPa] 20 Perfectly round casing 1st mode shape perturbation ( scaling) 10 1st mode shape perturbation (0.001 scaling) Collapse resistance, 13.4 MPa (API, ISO/TR) Elastic collapse (Timoshenko 1961) UX displacement [mm] 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 Effect of ovality Von Mises stress at collapse: 440 MPa Collapse at 300 C and 20 bar (wall pressure) D min D max Load, external pressure [MPa] Perfectly round Ovality (0.1%) Ovality (0.5%) 20 Ovality (1.0%) Ovality (2.0%) 10 Ovality (3.0%) Collapse resistance Elastic collapse UX displacement [mm] Casing: OD = 13 3/8 in, t = 12.2 mm API collapse resistance: 13.4 MPa

21 Collapse analysis Nonlinear buckling analysis Effect of external defect and concrete support Load, external pressure [MPa] Concrete support (linear MP) Without concrete support (linear MP) Concrete support (non-linear MP) 10 Without concrete support (non-linear MP) Collapse resistance, 13.4 MPa (API, ISO/TR) Elastic collapse (Timoshenko 1961) Displacement [mm] Casing: OD = 13 3/8 in, t = 12.2 mm API collapse resistance: 13.4 MPa

22 Case study of IDDP-1 As-built drawing of IDDP-1 (Pálson et.al 2013, Drilling of the well IDDP-1)

23 Load history of IDDP-1 Load history: Ingason et.al, Geothermics 2013: Wellbore load in the model:

24 Transient thermal distribution Thermal recovery from drilling Discharge 2 months Discharge 11 months Quenching 8 hours

25 FEM Results IDDP-1 Wellhead displacement Effect of cyclic loads Production casing at 50 m depth. Max von Mises strain

26 FEM Results IDDP-1 Discharge phase I Production casing Anchor casing

27 FEM Results IDDP-1 Discharge phase V Production casing Anchor casing

28 FEM Results IDDP-1 Quenching Production casing Anchor casing

29 Anchoring of couplings in concrete Model of the whole well Detailed BTC coupling model 5/10/2016

30 Anchoring of couplings in concrete Coupling displacement in concrete 5/10/2016

31 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

32 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. 5/10/2016

33 5/10/2016 Thank you