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.

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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:

• OD and ID
• Weight per unit of length
• Joint Length
• Steel grade
• Connection type

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:

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• 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.

API Steel Grade &#; K55, N80, L80, C90, T95, P110, Q125

API, shorted for American Petroleum Institute, is considered as the most authoritative and comprehensive principles for oil and gas industry. API 5B and API 5CT are mainly used for tubing and casing, which standard the chemical properties, steel grades, manufacturing methods, color coding, heat treatment, inspection and other information.

The API steel grade is also used in the floating equipment production, such as the K-55 steel grade floating shoes.

Here we will introduce the classification and properties of different steel grade in API standard.

Steel grade refers to the yield strength of the products. It can be divided into nine categories of steel grade: H-40, J-55, K-55, N-80, L-80, C-90, T-95, P-110, Q-125.

In it, the letters are stand for the steel grade, which has no relation with the properties and the number refers to the minimum yield strength. The bigger the number is, the higher yield strength will be.

Besides, the API standard divide the nine categories into four groups, H, J, K, N in the first ground, L, C, T in the second ground, P in the third group and Q in the fourth group.

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• The first group steel grade has relatively low strength
  • H-40 is generally not used in tubing sizes becsuse the yield strength is relatively low and the cost saving over J-55 is minimal.
  • The J-55 are used for most wells when it meets the design criteria. J-55 has been the standard grade for tubing in most relatively shallow (< 9,000 ft) and low pressure (< 4,000 psi) wells on land.
  • Grade K-55 is similar to the J-55 except that the minimum tensile strength of K-55 is higher.
  • Grade N-80 is a relatively old grade with essentially open chemical requirements. It is acceptable for sweet oil and gas wells when it meets design condition. is further divided into two types: N-80-1 and N-80-Q. Both types share the same tensile strength but differentiate in delivery conditions.
    • Grade N80-1 can be either normalized or hot rolling.
    • Grade N80-Q must be through quenching-and-tempering heat treatment before delivery.
• Grade L, C, T represent restricted-yield casing and tubing which has normally anti-sulfur and an-corrosive performance
  • L-80 is a restricted yield-tubing grade which is divided into three types. L80-1, L80-9CR abd L80-13Cr.
    • L80-1 is less expensive but more subject to weight-loss corrosion. It is used commonly in many oil and gas fields because of higher strength than J-55.
    • L80-9CR and L80-13Cr is expensive and difficult to manufacture but they have strong corrosion resistance abilities. These two types are recommended to be used in heavily corrosive wells.
  • C-90 is a relatively new API grade, which is divided into two types: C90-1 and C90-2. only C90-1 is suggested being used in sour conditions. Typical, this grade must be special ordered and its use has been gradually replaced by T-95.
  • T-95 has relatively high strength and is SSC resisant but not weight-loss resistant. It is also have two types: T95-1 and T95-2.
• P-110 in the third ground is a high strength steel grade, which allows quenching-and-tempering heat treatment. It can be adopted in high pressure deep wells
• Grade Q-125 is high strength. If you need tubing or casing to bear quite high pressure, Q-125 is a good choice

Chemical Composition of API Steel Grade for Casing and Tubing, Mass Fraction (%) Group Grade Type C Mn Mo Cr Ni Cu P S Si min. max. min. max. min. max. min. max. max. max. max. max. max. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 H40 &#; &#; &#; &#; &#; &#; &#; &#; &#; &#; &#; 0.030 0.030 &#; J55 &#; &#; &#; &#; &#; &#; &#; &#; &#; &#; &#; 0.030 0.030 &#; K55 &#; &#; &#; &#; &#; &#; &#; &#; &#; &#; &#; 0.030 0.030 &#; N80 1 &#; &#; &#; &#; &#; &#; &#; &#; &#; &#; 0.030 0.030 &#; N80 Q &#; &#; &#; &#; &#; &#; &#; &#; &#; &#; 0.030 0.030 &#; 2 M65 &#; &#; &#; &#; &#; &#; &#; &#; &#; &#; 0.030 0.030 &#; L80 1 &#; 0.43 a 1.90 &#; &#; &#; &#; 0.25 0.35 0.030 0.030 0.45 L80 9Cr &#; 0.15 0.30 0.60 0.90 1.10 8.00 10.0 0.50 0.25 0.020 0.010 1.00 L80 13Cr 0.15 0.22 0.25 1.00 &#; &#; 12.0 14.0 0.50 0.25 0.020 0.010 1.00 C90 1 &#; 0.35 &#; 1.20 0.25 b 0.85 &#; 1.50 0.99 0.020 0.010 &#; C90 2 &#; 0.50 &#; 1.90 &#; NL &#; NL 0.99 &#; 0.030 0.010 &#; C95 &#; &#; 0.45 c &#; 1.90 &#; &#; &#; &#; &#; &#; 0.030 0.030 0.45 T95 1 &#; 0.35 &#; 1.20 025 d 0.85 0.40 1.50 0.99 0.020 0.010 &#; T95 2 &#; 0.50 &#; 1.90 &#; &#; &#; &#; 0.99 &#; 0.030 0.010 &#; 3 P110 e &#; &#; &#; &#; &#; &#; &#; &#; &#; &#; 0.030 e 0.030 e &#; 4 Q125 1 &#; 0.35 1.35 &#; 0.85 &#; 1.50 0.99 &#; 0.020 0.010 &#; Q125 2 &#; 0.35 &#; 1.00 &#; NL &#; NL 0.99 &#; 0.020 0,020 &#; Q125 3 &#; 0.50 &#; 1.90 &#; NL &#; NL 0.99 &#; 0.030 0.010 Q125 4 &#; 0.50 &#; 1.90 &#; NL &#; NL 0.99 &#; 0.030 0.020 &#; a The carton content for L80 may be increased up to 0.50% maximum if me product is oil-quenched.
b The molybdenum content for Grade C90 Type 1 has no minimum tolerance if the wall thickness it less than 17.78 mm
c The carton content for C95 may be increased up to 0.55% maximum if the product is oil-quenched.
d The molybdenum content for T95 Type 1 may be decreased to 0.15% minimum if the wall thickness is less than 17.78 mm
e For EW Grade P110, the phosphorus content shall be 0.020% maximum and the sulfur content 0.010% maximum
NL = No limit elements shown shall be reported in product analysis.