13 - The API Casing Steel Grades...How Are They Defined?
Updated: Aug 22, 2018
Casing is an integral part of an oil & gas well. It plays a crucial role in ensuring well integrity during the complete well life cycle, from drilling, to production, till abandonment. As well as after abandonment, as some casings deteriorate and begin to leak hydrocarbon to surface.
This extended period, where the casing is put to service, commands a thorough casing selection process. This process takes into consideration the environment where the casing is being placed in, and the application of which the casing will be used for.
What Criteria We Look At When Selecting Casing?
Casing selection comes after a Casing Design process, which takes into account well profile and all loads exerted on the casing during the complete well life cycle.
There are five casing aspects we look at when performing Casing Design:
Weight per unit of length
Joint Length
Steel grade
The first three criteria are considered dimensional and are largely influenced by operations (availability, completion equipment, casing seat depth, drill bit size, future intervention...etc).
The remaining two criteria are more crucial to well integrity. This article will touch on the Steel Grade criteria only.
What is a Casing Steel Grade?
A Casing Steel Grade is a category of steel that has its own characteristics and specifications.
Casing Steel Grades differ in three aspects:
Steel alloy formulation and composition
Manufacturing process in terms of the applied heat treatment
Mechanical properties such as minimum yield strength
What are the API Casing Steel Grades?
While there are dozens of Casing Steel Grades available in the market, API has specified the following as API grades:
H-40
J-55
K-55
M-65
N-80 (N-80.1, N-80.Q)
L-80 (N-80.1, L-80.9Cr, L-80.13Cr)
C-90
C-95
T-95
R-95
C-110
P-110
Q-125
A Casing Steel Grade is expressed by a Letter and a Number. While the Number represents the minimum yield strength, the Letter has no significance and has been arbitrarily chosen.
Is There Any Other Classification For Casing Steel Grades?
Yes, the above casing grades are further grouped under 4 groups (or what is commonly known as Service Levels). This categorization takes into consideration two factors:
resistance to sulfide stress cracking
working pressure
the API Casing Steel Grades groups are:
Group 1: H-40, J-55, K-55, N-80, R-95
Group 2: M-65, L-80, C-90, C-95, T-95, C-110
Group 3: P-110
Group 4: Q-125
Also, There is another independent categorization called Product Specification Level (PSL). This categorization divides the API Casing Steel Grades into three PSLs according to their application:
PSL-1: Common Application Casing (H-40, J-55, K-55, M-65, N-80, R-95)
PSL-2: Corrosion-Resistance Casing (L-80, C-90, T-95, C-110)
PSL-3: Deep Well Casing (P-110, Q-125)
Final Words
Despite the different categorizations of Casing Steel Grades, the casing working environment (sour and sweet, pressure ratings, temperatures) remains central to the casing selection process. Therefore, identifying the correct casing grade at the design phase is fundamental to ensure well integrity during its lifespan.
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