Production Casing

What is production casing?

In oil drilling, production casing, also known as reservoir casing, is the final layer of casing lowered into the well and directly serves oil and gas production. It is the core of the entire wellbore structure and serves as a permanent infrastructure connecting the oil and gas reservoir to the surface. Its core functions include maintaining wellbore stability, isolating vertical seepage from oil/gas/water layers, and creating a high-speed channel for oil and gas production.

Functions of Production Casing:

The core functions of production casing can be summarized as follows:

1. Forming a Permanent Production Channel
It creates a complete flow channel from the oil and gas reservoir to the wellhead to ensure long-term production, providing a strong, stable, and sealed pathway for oil and gas to flow from reservoirs thousands of meters deep to the surface. This is its most fundamental function.

2. Effectively Isolating the Producing Zone

High-quality cementing completely isolates the oil and gas reservoir from other overlying formations (especially the water layer). This prevents water from entering the oil layer and oil and gas from entering other formations, ensuring only the desired oil and gas is produced and protecting the reservoir structure.

3. Accommodates Downhole Production Facilities
Inside the production casing, we run a complete set of production equipment, including tubing, packers, and safety valves. The production casing provides space for the installation and operation of this equipment.

4. Withstands Various Operating Pressures
It must withstand the downhole pressures of long-term production.
Especially importantly, it must withstand the extremely high pressures generated by stimulation measures (such as hydraulic fracturing and acidizing) without bursting or deforming.

5. Provides a Well Control Barrier
As part of the wellbore pressure control system, the production casing and its attached wellhead assembly form a critical barrier to prevent uncontrolled oil and gas leakage.

Key Features of Production Casing:

a. Last Layer: This is the last layer of casing run during drilling and reaches the deepest depth.

b. Highest Quality Requirements: Extremely high demands are placed on the pipe material for strength, sealing, and corrosion resistance, as it is designed to last for decades and is extremely expensive to repair or replace. 

c. Cementing quality is crucial: Cementing quality is key to ensuring effective isolation of the producing stratum and must be rigorously verified through methods such as sonic logging.

d. Small size: Because it is nested in the innermost layer, its outer diameter is typically smaller than the technical casing above. Common sizes include 7-inch and 5-1/2-inch (as a production liner).

Production Casing Pipe Size:

The size of the production casing is not selected arbitrarily; it is the final result of the overall wellbore design (casing procedure). It must match the size of the technical or surface casing above.

Most Common Sizes:
7-inch - This is the most widely used production casing size globally, in both conventional oil and gas wells and unconventional shale gas wells.
5-1/2-inch - Also extremely common, it is often used in relatively shallow wells or as a production liner.

Other Common Sizes:
9-5/8-inch - Often used in high-yield wells, high-pressure gas wells, or wells requiring a larger inner diameter to accommodate complex downhole tools. 4-1/2-inch - Commonly used in slim-hole wells, low-yield wells, or as a liner for deeper wells.

A typical casing sequence clearly illustrates this decision:

Surface casing: 13-3/8-inch
Technical casing: 9-5/8-inch
Production casing: Run 7-inch casing or 5-1/2-inch liner

Key point: The production casing must be able to pass smoothly through the ID of the upper casing (usually the technical casing). Sufficient annular space must be maintained between them to ensure smooth installation and maintain the thickness and quality of the cement sheath.

Main factors determining production casing size:

1. Reservoir characteristics and production requirements
High-yield wells: A larger ID is required to reduce flow resistance and increase oil and gas production, so 9-5/8-inch or 7-inch casing may be selected.
Gas wells: High gas flow rates generally require a larger ID to reduce frictional losses.

2. Production Stimulation Measures
For shale gas or tight oil wells requiring hydraulic fracturing, production casing must be able to withstand extremely high pumping pressures and flow rates. 7-inch casing, due to its excellent pressure-bearing capacity and large inner diameter, is the preferred choice for unconventional drilling.

3. Downhole Tools and Completion Methods
If the well completion design is complex and requires the use of large-sized packers, sliding sleeves, intelligent completions, and other tools, a sufficiently large production casing (such as 7-inch) must be selected to provide access for these tools.

4. Economics and Long-Term Production
The larger the casing, the higher the cost. Engineers need to strike a balance between initial cost and long-term production benefits. An undersized casing may limit future production increases or operational flexibility.

Production Casing vs. Oil Tubing:

Production casing: A permanent wellbore structure that isolates the formation and provides a path for the tubing.
Oil tubing: A replaceable pipe run inside the production casing to transport oil and gas.

The oil tubing is suspended from a tubing hook inside the production casing. Oil and gas are typically produced from within the tubing. Simply put, the production casing is the "wall," and the tubing is the "water pipe inside the wall."

Failure Analysis and Prevention:

Casing failures primarily occur due to thread galling (42%), pipe collapse (31%), and corrosion perforation (19%).
Key technologies for improving reliability include: optimizing string design using finite element analysis; developing steam injection thermal recovery casing to address thermal stress deformation; and applying nickel-based alloy coatings to resist H₂S corrosion.
In sour oil and gas fields, the casing material must have a hardness of ≤HRC22 and utilize composite protective technologies such as fiberglass lining or aluminizing.

If drilling a well is compared to building a building:

Conductor and surface casing: These are the foundation and underground waterproofing and load-bearing walls.

Technical casing: These are temporary supports added during construction to accommodate complex geology. 

Production casing: Once a building is completed, it serves as the final, permanent load-bearing wall and pipeline shaft within the building. All water, electricity, and gas lines (oil pipelines and equipment) pass through this section.

In short, production casing is the "backbone" of an oil and gas well. Its design and quality directly determine whether the well can safely, efficiently, and sustainably produce oil and gas.

Read more: Casing Pipe Price & Cost or API Seamless Casing Specifications