Important Material Properties of Tubular and Completion String

It is imperative to understand mechanical properties of tubular because it involves the safety structure of the well. Failure in completion can cause major catastrophic problems in people safety, major loss of expenditure and loss of production from a well.

This article will describe the basic mechanical properties which are very essential to understand.

Stress

Stress is an applied force per unit area.

Figure 1 – Stress Diagram

Strain

When a tubular is subjected to a tensile load, the tubular becomes longer and the amount of elongation is called “strain.” Strain can be described as per the equation below;

Where;
ΔL = change in length of tubular
L = original length of tubular

Figure 2 – Strain Diagram

Hooke’s Law and the Modulus of Elasticity

Steel, which is a ductile material, exhibits elastic behavior. Elasticity is a material property which allows the material to come back to its original shape when the load is released. According to the Hooke’s Law, stress is proportional to strain up to an elastic limit. Therefore, stress and strain under the elastic limit can be described as the equation below;

σ = E × ɛ

Where;

σ = stress of material
ɛ = strain of material
E = Young’s modulus (Modulus of elasticity)

Note: Young’s modulus of elasticity of steel is about 30×106 psi.

Stress and Strain Curve

When forced is applied into a material, stress and strain can be plotted like this (Figure 3). It is important to understand the meanings of several points on the stress-strain plot.

Figure 3 – Stress and Strain Curve

Elastic region (green shaded area) – Under the elastic region, material will go back to its original shape once the forced is released.

Plastic region (red shaded area) – Under the plastic region, material will be plastically deformed therefore it will not be able to reverse back to its original shape.

Proportional Limit – Under the elastic limit, stress is a proportional limit to strain and the relationship between stress and strain is under the Hooke’s law.

Elastic Limit – The elastic limit is the maximum stress which material behaves under the Hooke’s law (stress and strain have a liner relationship). Beyond the elastic limit to the yield point, the material still behaves elastically.

Yield Point (Yield Stress) – This is the maximum stress that material can withstand before it is plastically deformed. Beyond the yield point, material will not be able to come back to its original shape. If the stress is applied over the elastic limit, but below the yield point, the material will be able to recover back to its original shape since it is still within the elastic limit. However, the Hooke’s law does not apply.

API defines the yield stress as the minimum tensile stress required to elongate the pipe 0.5% or 0.65% depending on the tubing grade.

Ultimate Tensile Stress – This is the maximum stress that material can withstand and it is shown as the top of the engineering stress-strain curve. Beyond this point, the cross sectional area of material begins to reduce rapidly over a relatively small length of material and this is called “neck.”

Failure – This is the point where the material will be parted.

Poisson’s Ratio (μ)

Experiments have shown that when the material is under tension, both axial and radial strain will occur. In the elastic region, these two strains are proportional to each other. This is called poisson’s ratio (μ) and the relationship is shown below;

Figure 4 –Illustrate of Poisson’s Ratio

Ductile and Brittle Material

Ductile material as carbon steel is material which has a large degree of plastic deformation before being fractured. On the other hand, brittle material such as grass has a very low degree of plastic deformation. It indicates that after yield strength is exceeded, the brittle material will break apart very quickly, but the ductile material will be able to elongate further before parted.

Figure 5 – Comparison between Ductile and Brittle Material

References

Jonathan Bellarby, 2009. Well Completion Design, Volume 56 (Developments in Petroleum Science). 1 Edition. Elsevier Science.

Wan Renpu, 2011. Advanced Well Completion Engineering, Third Edition. 3 Edition. Gulf Professional Publishing.

Ted G. Byrom, 2014. Casing and Liners for Drilling and Completion, Second Edition: Design and Application (Gulf Drilling Guides). 2 Edition. Gulf Professional Publishing.

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