In well design, classifying casing by its function is the foundation of petroleum engineering. Every casing string has a specific mission, ranging from preventing surface soil collapse to protecting deep, high-pressure oil and gas reservoirs.
Drawning of casing structural
Conductor Casing
This is the first string run into the well after drilling begins.
Core Functions:
Wellhead Support: Prevents the loose surface soil (dirt, sand, gravel) from washing out or collapsing during drilling.
Establishing Circulation: Provides a path for the drilling fluid to return to the surface.
Weight Bearing: In some designs, it helps support a portion of the weight of the subsequent casing heads.
Typical Data:
Outside Diameter (OD): Usually 20" to 30" (508mm - 762mm).
Setting Depth: Very shallow, typically between 10m and 100m.
Connection: Often welded or uses large-diameter specialized connectors.
Surface Casing
The surface casing is the first major safety barrier of the well.
Core Functions:
Protecting Freshwater: Isolates shallow freshwater aquifers (groundwater) to prevent contamination by drilling mud or salt water from deeper formations.
BOP Foundation: Provides a stable base for the installation of the Blowout Preventer (BOP) to handle high-pressure zones in subsequent drilling.
Supporting Loads: Suspends the weight of all subsequent intermediate and production casing strings.
Typical Data:
Outside Diameter (OD): Commonly 9-5/8", 13-3/8", or 20".
Cementing Requirement: Cement must return to the surface to ensure total isolation of freshwater zones.
Common Grades: J55, K55, N80.
Intermediate Casing (Technical Casing)
When a well passes through complex formations, a layer of "armor" is needed.
Core Functions:
Pressure Transition: If a well passes through both "low-pressure loss zones" and "high-pressure kick zones" simultaneously, intermediate casing is used to isolate them so that mud weight can be adjusted.
Shielding Troubled Zones: Isolates salt domes, unstable shales, or severe lost-circulation zones.
Tapering the Hole: Provides a smaller, safer hole size for deeper drilling toward the target reservoir.
Typical Data:
Outside Diameter (OD): Commonly 7", 9-5/8", or 11-3/4".
Number of Strings: Depending on well depth, there may be one or more intermediate strings.
Common Grades: N80, L80, P110.
Production Casing
The final full-length casing string that reaches the target reservoir.
Core Functions:
Production Conduit: Provides the main channel for oil and gas (tubing is usually run inside it).
Zonal Isolation: Completely separates the target production zone from other non-productive or water-bearing formations.
Pressure Barrier: Must withstand the high internal pressure (Burst Pressure) that may occur over the life of the well.
Typical Data:
Outside Diameter (OD): Commonly 4-1/2", 5-1/2", or 7".
Common Grades: P110 (high-pressure deep wells), L80 (sour service), C110.
Connections: Often requires Gas-Tight Premium Connections or LTC.
Liner
A liner is a special type of casing that does not extend to the wellhead.
Core Functions:
Cost Savings: It is hung from the bottom of the previous casing string, saving the cost of the steel pipe from the wellhead to the hanging point.
Weight Reduction: Reduces the total load on the wellhead and hanging equipment.
Flexibility: Commonly used in the final section of deep wells or as a remedial measure during workovers.
Typical Data:
Installation: Secured using a Liner Hanger.
Overlap: Usually overlaps with the previous string by 50m - 100m.
Functional Classification
| Function Name | Abbreviation | Primary Purpose | Common Sizes | Grade Requirements |
| Conductor | CON | Stabilize wellhead, prevent washout | 20" - 30" | H40 |
| Surface | SUR | Protect freshwater, base for BOP | 9-5/8" - 20" | J55 / K55 |
| Intermediate | INT | Isolate complex zones/pressures | 7" - 11-3/4" | N80 / L80 / P110 |
| Production | PRO | Protect reservoir, lifelong use | 4-1/2" - 7" | P110 / L80 / C110 |
| Liner | LNR | Save cost and weight | Various | P110 / Q125 |
GNEE API 5CT Casing Factory equipments
GNEE API 5CT Casing test equipments
GNEE API 5CT OCTG Casing certificate
FAQ
What is range 3 casing?
Range 3 is the longest length category for casing as defined by the API 5CT standard.
Length: 36 to 48 feet (10.97 to 14.63 meters).
Key Advantage: Because the pipes are longer, you need fewer connections (couplings) to reach the bottom of the well. This reduces installation time and minimizes potential leak points.
Common Use: Typically used in deep wells or offshore rigs with high masts that can handle longer pipe sections.
What are the common sizes of casing?
Casing sizes are measured by their Outside Diameter (OD). The most common sizes used in the oil and gas industry are:
Production Casing (Small): 4-1/2", 5", 5-1/2", and 7".
Intermediate Casing (Medium): 7", 9-5/8", and 11-3/4".
Surface Casing (Large): 13-3/8", 16", and 20".
Conductor Casing (Extra Large): 20", 24", and 30".
For surface casing, which is more cost-effective: ERW or Seamless?
For large-diameter surface casing, ERW (Electric Resistance Welded) offers more uniform wall thickness and a lower price, which is sufficient for low-pressure environments. However, for high-pressure intermediate and production casing, buyers typically trust only Seamless to ensure burst pressure safety.
