The strength of
casing pipes refers to the ability of the material to resist external forces and prevent failure. In petroleum engineering, casing pipes play a crucial role in bearing underground pressure, preventing wellbore collapse, and isolating formations. Their strength is essential for maintaining the stability of the wellbore and ensuring safe operations.
The strength of casing pipes is closely related to the mechanical properties of the material, including yield strength, tensile strength, compressive strength, and shear strength. These mechanical performance indicators can be obtained through experiments or theoretical calculations. In practical applications, the strength of casing pipes must meet a series of design requirements to ensure the safe operation of the wellbore.
Yield Strength of Casing Pipes
The yield strength of casing pipes refers to the material's ability to undergo plastic deformation when subjected to stress. When a casing pipe is subjected to external forces and reaches its yield strength, it will undergo plastic deformation and will no longer be able to fully return to its original shape. This occurs because the internal crystal structure of the casing pipe material changes under stress.
What Is the Strength of API Casing?
API 5CT P110 casing pipes have higher strength levels than grades such as J55/K55, N80, L80, and T95, with a minimum tensile strength of 862 MPa and a yield strength ranging from 758 to 965 MPa. The ends of P110 casing pipes are marked with white bands to designate this material. They are widely used in specific environments for oil drilling and production, especially in deep oil wells.
Quality Requirements for Casing Pipes
During drilling,
drill pipes can cause severe impact and wear on the casing pipes. If there are problems with the casing string inside the well, it can lead to significant accidents, even resulting in the abandonment of the well. Therefore, the quality of casing pipes must meet the following requirements to ensure wellbore safety.
Material Requirements
Seamless steel pipes are generally selected based on parameters such as the depth and quality of the well. When selecting casing materials, the quality of the casing string should also be considered. For casing pipes weighing less than 10 tons, ordinary low-carbon steel grades can be used. For those weighing between 10 and 20 tons, DZ40-DZ50 steel grades are recommended, and for casing pipes weighing 20 tons or more, DZ50 and higher grades should be used. Additionally, the material for casing joints should be of a higher grade than the casing itself.
Casing Pipe Curvature Requirements The curvature of casing pipes refers to the degree of bending along the length of the pipe, represented by the arc height at the maximum bending point. This can be expressed as local curvature (mm/m) or total length curvature (%). In deep well drilling, the following curvature standards are generally required: for casing pipes with a diameter of ≤Φ89mm, the local curvature should be within 1mm/1.5m; for diameters greater than Φ89mm, the local curvature should be within 1mm/1m.
Ovality and Wall Thickness Uniformity Requirements The ovality (or out-of-roundness) of a casing pipe is the difference between the maximum and minimum outer diameters of the pipe's cross-section. Generally, the ovality should not exceed 80% of the outer diameter tolerance. For casing pipes with a diameter of ≤Φ89mm, the ovality should be controlled within 0.5mm, and for diameters greater than Φ89mm, the ovality should be controlled within 1.0mm. Wall thickness unevenness refers to differences in wall thickness across the pipe's cross-section and longitudinal body. This should not exceed 80% of the wall thickness tolerance. For diameters ≤Φ89mm, wall thickness unevenness should be controlled within 0.3mm, and for diameters greater than Φ89mm, it should be controlled within 0.5mm.
Casing Pipe Connection Methods and Processing Quality Requirements
Casing pipes can be connected through direct connection, seam welding, or coupling. Generally, for casing pipes weighing less than 10 tons, direct or seam welding is used, and for pipes weighing more than 10 tons, coupling connections are employed.
Heat treatment should be applied to deep well casing joints and couplings to enhance their strength.
The thread centerlines of both ends of the casing pipe joints should be aligned, with the coaxiality deviation not exceeding 0.25mm.
The threads of casing pipes, joints, and couplings must comply with geological core drilling and petroleum drilling standards. Deep well and large-diameter geological core drilling usually follow oil pipe thread standards.
The processing accuracy of threads on casing pipes, joints, and couplings must strictly follow the relevant specifications.
After forming, casing pipes, joints, and couplings should undergo a pass-through inspection with a standard inner diameter gauge.
API Casing Steel Grades
Most casing steel grades have similar chemical compositions, with the unique properties of each grade determined by the type of heat treatment used. The main differences between casing steel grades lie in yield strength, tensile strength, and hardness.
API 5CT requires additional testing for certain casing groups to ensure that each grade is suitable for its intended use.
Common Casing Applications (H40, J55, K55, M65, N80, R95)
Due to their lower cost compared to other steel grades, J55, K55, and N80 grades are popular choices for wells with less stringent requirements for corrosion resistance or strength.
M65 and R95 are newer grades, each with its specific applications. The primary differences between these casing grades lie in their yield strength and tensile strength.
H40
Yield Strength: 276-552 MPa (40-80 ksi)
Minimum Tensile Strength: 414 MPa (60 ksi)
API Color Code: None or 1 black band
J55
Yield Strength: 379-552 MPa (55-80 ksi)
Minimum Tensile Strength: 517 MPa (75 ksi)
API Color Code: 1 bright green band
K55
K55 has the same yield strength as J55 but approximately 26% higher minimum tensile strength.
Yield Strength: 379-552 MPa (55-80 ksi)
Minimum Tensile Strength: 665 MPa (95 ksi)
API Color Code: 2 bright green bands
M65
M65 is not commonly used and is the only steel grade among general-purpose casings with maximum hardness restrictions.
Yield Strength: 448-586 MPa (65-85 ksi)
Minimum Tensile Strength: 586 MPa (85 ksi)
API Color Code: 1 bright green band + 1 blue band
N80
(N80-1, N80-Q) N80-1 and N80-Q have the same yield and tensile strengths, but the quenched and tempered N80-Q offers better resistance to internal pressure and collapse. On the other hand, N80-1 is preferred due to its lower production cost and reduced need for collapse resistance.
Yield Strength: 552-758 MPa (55-80 ksi)
Minimum Tensile Strength: 689 MPa (100 ksi)
API Color Code: 1 red (N80-1), 1 red band + 1 bright green band (N80-Q)
R95
The yield strength and tensile strength of R95 are the same as those of T95, but R95 does not require SSC or hardness testing. As a result, R95 is a more cost-effective alternative to T95 for wells where there is no risk of sulfide stress cracking.
Yield Strength: 655-758 MPa (95-110 ksi)
Minimum Tensile Strength: 727 MPa (105 ksi)
Mechanical properties of casing pipe
Grade
|
Type
|
Total elongation
under load
(%)
|
Yield strength
(min)Mpa
|
Yield strength
(max)Mpa
|
Tensile strength
min Mpa
|
Hardness
Max (HRC)
|
Hardness
Max (HBW)
|
J55
|
-
|
0.5
|
379
|
552
|
517
|
-
|
-
|
K55
|
-
|
0.5
|
379
|
552
|
655
|
-
|
-
|
N80
|
1
|
0.5
|
552
|
758
|
689
|
-
|
-
|
N80
|
Q
|
0.5
|
552
|
758
|
689
|
-
|
-
|
L80
|
1
|
0.5
|
552
|
655
|
655
|
23
|
241
|
L80
|
9Cr
|
0.5
|
552
|
655
|
655
|
23
|
241
|
L80
|
13Cr
|
0.5
|
552
|
655
|
655
|
23
|
241
|
C90
|
-
|
0.5
|
621
|
724
|
689
|
25.4
|
255
|
C95
|
-
|
0.5
|
655
|
758
|
724
|
-
|
-
|
T95
|
-
|
0.5
|
655
|
758
|
724
|
25.4
|
255
|
P110
|
-
|
0.6
|
758
|
965
|
862
|
-
|
-
|
Q125
|
All
|
0.65
|
862
|
1034
|
931
|
-
|
-
|