Page 87 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 63

1.4.9
Casing Design [ ]

1.4.9. Factors Affecting Pipe Yield Strengths

Corrosion. Corrosion may occur during production due to elec-
trochemical reaction with corrosive agents. It may also occur as uni-
form reduction in wall thickness, localized patterns of metal loss, or
pitting. Of the three, pitting causes the greatest problem.
Corrosion rates increase with higher temperatures (rates approxi-
mately double for every 31˚C increase in temperature), higher fluid
velocities and/or abrasive solids (eroding away films forming on the
metal surface), and higher concentrations of corrosive agents such as
oxygen, carbon dioxide, and hydrogen sulfide.
Collapse and burst pressures are determined by the thinnest part of
the wall, while tensile strength is determined by the remaining cross-
sectional area.
Corrosion in a particular fluid can be measured using a corrosion
coupon. This is usually in the form of a ring of metal that is weighed
accurately, exposed to the fluid under realistic conditions of tempera-
ture, etc. and re-weighed after a period of time. The metal loss is
recorded in weight loss (lbs) per square foot exposed per year.
If corrosion is considered a potential problem, then various treat-
ments of the completion fluid can be made to reduce or eliminate it.
Wear. If significant wear is expected during drilling (monitoring
metal in the returns; using tooljoints with rough hardfacing; high/shal-
low doglegs, etc.) then a Kinley caliper, ultrasonic imaging tool (USIT)
log, or other tools can be run to measure wall thickness reduction.
Collapse and burst pressures will be determined by the thinnest part of
the wall while tensile strength will be determined by the remaining
cross-sectional area.
Wear is influenced by side forces (in turn influenced by dogleg
severity, inclination, and tension), number of rotating hours, rough-
ness and hardness of the rotating surface, and number of round trips
carried out.
Wear reduction measures may include protectors (standard or
those that rotate on the pipe), using downhole motors, ensuring hard-
facing is smooth and has been run in open hole first, and restricting
dogleg severities and inclinations.
Wear mitigation measures may include using thicker wall sections
and/or higher grade steels over the areas of potential wear. Experience

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