API 5CT C95 is a high-strength casing grade defined by the American Petroleum Institute (API) Specification 5CT. It is primarily designed for use in sour service environments where resistance to Sulfide Stress Cracking (SSC) is required.
Key Characteristic: C95 is a restricted yield strength grade, meaning its yield strength range is narrower and its hardness is controlled to enhance its performance in wells containing Hydrogen Sulfide (H₂S).
Comparison: It offers higher strength than the common L80 grade while providing better SSC resistance than the high-strength P110 grade, which is not typically recommended for sour service.
Types: Within the C95 specification, there are different types (e.g., Type 1) that may have variations in chemical composition and heat treatment to achieve the required properties.
1. Mechanical Properties Table
The following table outlines the key mechanical properties as per API 5CT.
| Property | Value & Requirements | Notes |
|---|---|---|
| Grade Designation | C95 | - |
| Specified Minimum Yield Strength (SMYS) | 95,000 psi (655 MPa) | The stress at which the pipe begins to deform plastically. |
| Minimum Tensile Strength | 105,000 psi (724 MPa) | - |
| Maximum Yield Strength | 110,000 psi (758 MPa) for OD ≤ 4.5" | This restricted yield strength range (95-110 ksi) is critical for SSC resistance. |
| 115,000 psi (793 MPa) for OD > 4.5" | ||
| Maximum Hardness | HRC 25.4 | This is a key control for sour service performance. |
| Elongation | As per API 5CT formula | Minimum elongation depends on the specimen size and grade. |
2. Chemical Composition Requirements
The chemical composition is strictly controlled to ensure the desired mechanical properties and SSC resistance. The following are typical maximum percentages, with exact requirements depending on the manufacturing process and type.
| Element | Typical Maximum % (Weight) | Purpose & Impact |
|---|---|---|
| Carbon (C) | 0.35 - 0.45 % | Increases strength and hardness. |
| Manganese (Mn) | 1.50 % | Increases strength and hardenability. |
| Molybdenum (Mo) | 0.40 - 0.70 % | Significantly improves strength and SSC resistance. |
| Chromium (Cr) | 0.90 - 1.30 % | Improves corrosion resistance and strength. |
| Sulfur (S) | 0.010 % | Low levels are mandatory to minimize susceptibility to SSC. |
| Phosphorus (P) | 0.020 % | Low levels improve toughness and SSC resistance. |
Note: The composition for C95 Type 1 is typically a Cr-Mo steel, similar to AISI 4130/4140 modified for sour service.
3. Dimensional and Mass Data (Example for Common Sizes)
The table below provides examples for common sizes. The full range of sizes is specified in API 5CT.
| Outside Diameter (OD) | Weight (Nominal) | Wall Thickness (Nominal) | Drift Diameter (Min.) | ||||
|---|---|---|---|---|---|---|---|
| in | mm | lb/ft | kg/m | in | mm | in | mm |
| 7 | 177.8 | 26.00 | 38.69 | 0.362 | 9.19 | 6.151 | 156.23 |
| 7 | 177.8 | 29.00 | 43.15 | 0.408 | 10.36 | 6.094 | 154.79 |
| 7 | 177.8 | 32.00 | 47.62 | 0.453 | 11.51 | 6.035 | 153.29 |
| 9 5/8 | 244.5 | 40.00 | 59.53 | 0.395 | 10.03 | 8.681 | 220.50 |
| 9 5/8 | 244.5 | 47.00 | 69.94 | 0.472 | 11.99 | 8.535 | 216.79 |
| 9 5/8 | 244.5 | 53.50 | 79.63 | 0.545 | 13.84 | 8.397 | 213.28 |
Weight: Nominal linear weight. Actual weight has a tolerance (typically -3.5% to +6.5%).
Drift Diameter: The minimum diameter of a cylindrical mandrel that must pass freely through the pipe. Ensures compatibility with downhole tools.
4. Performance Data (Example Calculations)
Performance data like collapse and burst pressure are calculated based on the specific dimensions and mechanical properties. The following are approximate values for a 7", 29 lb/ft C95 casing.
| Performance Property | Approximate Value | Condition / Formula |
|---|---|---|
| Internal Yield Pressure (Burst) | ~10,900 psi | Calculated using Barlow's formula: P = 2 * SMYS * t / OD |
| Collapse Resistance | ~8,500 psi | Varies greatly with D/t ratio; this is for Yield Strength Collapse. |
| Pipe Body Yield Strength | ~1.12 million lbf | F = SMYS * Cross-Sectional Area |
Important: These are illustrative values. For engineering design, always use the precise values provided in the manufacturer's pipe mill certificate or calculated using industry-standard formulas (API TR 5C3).
Key Applications
Sour Gas Wells: Production casing and intermediate casing in reservoirs with H₂S.
Deep Wells: Where higher strength than L80 is needed to handle pressure and tensile loads.
High-Pressure/High-Temperature (HPHT) Wells: As an intermediate strength option with good environmental cracking resistance.
Summary
API 5CT C95 is a critical casing grade that bridges the gap between medium-strength and high-strength applications in corrosive environments. Its carefully controlled chemical composition, heat treatment (quenching and tempering), and restricted mechanical properties make it a reliable choice for challenging drilling and production conditions where both strength and resistance to sulfide stress cracking are paramount.
