B3 Magnús Þór Jónsson

Principles of limit state design for geothermal wells

Magnús Þór Jónsson and Lilja Magnúsdóttir

GEORG Geothermal Workshop 2016 – Reykjavik, Iceland 1

GEORG Geothermal Workshop 2016 – Reykjavik, Iceland

Outline:- Introduction- Limit State Design (LSD)- Failures and Failure Reasoning- Improvements based on LSD- Conclusion

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Introduction

Motivation:A review of the literature shows that the cost of geothermal wells is in the range of 20-50% of the total investment cost of a geothermal power plant. Therefore, the financial feasibility of the plant is sensitive to the success rate of the wells.

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Introduction

Objective:Geothermal wells are subjected to various types of loads and deformations arising from service requirements that may range from standard to extreme values during the construction, operation or discharging process.

The objective is to design a casing and a wellhead that can withstand such demands throughout it expected lifetime.

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Introduction

Traditional Design:In the traditional allowable stress design, the design is based on the criteria that the stresses resulting from the design loads are lower than the allowable stresses based on material strength and safety factors.

As the high temperature wells are more likely to collapse, a new methodology is presented to reduce that risk.

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Limit State Design

Design Philosophies:The limit state design is based on explicit consideration of various conditions where the structure may not fulfill its intended function. For these conditions, the strength is estimated and used as a limit for such behavior.For types of limit states:

Serviceability limit stateUltimate limit stateFatigue limit stateAccidental limit state

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Limit State Design

Limit State of Geothermal Well:A limit state of a geothermal well is defined by the characterization of conditions for which a part or an entire well fails to perform the function that is expected of it.

Only two limit states are studied, ultimate limit state and accidental limit state.

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Limit State Design

Failure and Failure Reasoning:Wells with high temperature and high pressure are more likely to fail or reach the ultimate limit state.

Failure modes:Casing or coupling rupture Collapse or bucklingduring installation or quenching during discharging

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Limit State Design

Failure and Failure Reasoning:Principles of limit state design:

Consider:Load conditionsMaterial selectionDimensions

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Limit State Design

Failure and Failure Reasoning:Load conditions: Axial and bending loads before and during cementing.Axial and bending loads when drilling. Axial thermal loads during discharging.

Radial loads applied by internal and/or external pressure.Radial thermal loads during discharging.Radial loads applied by thermal expansion of trapped water.

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Limit State Design

Failure and Failure Reasoning:Material selection:

Strength Thermal expansion BauschingerTemperature dep. effectstrength reduction

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Limit State Design

Failure and Failure Reasoning:Dimensions:

Thickness Integrity

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Limit State Design

Improvements based on LSD:

Decrease the loads. (Construction, Operation and Quenching)

Select material that has optimum performance.

Increase the wall thickness of production casing.

Reduce integrity of the production casing, use welding connections instead of couplings.

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Conclustion

Summary:

For the ultimate limit state design, the temperature dependent yield conditions are considered for ultimate stresses and strains including the Bauschinger effect for cyclic loading.

At high temperatures, the stiffness and the strength of steel are reduced. That is, however, usually not the only reason of a rupture failure which occurs by a combination of expansion, material degradation and geometrical discontinuity.

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Conclustion

Thank you!

Questions?

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Engineering

B3 Magnús Þór Jónsson