Intermediate Casing

What is an Intermediate Casing?

Intermediate casing, also known as technical casing, is one or more layers of casing pipe that must be installed when drilling encounters complex formations below the surface casing, making further drilling unsafe. Located between the surface casing and the reservoir casing, its primary function is to isolate complex formations prone to collapse, leaks, high pressure, and salinity. Through cementation, it forms a protective barrier, ensuring safe drilling of the lower wellbore section and the structural integrity of the wellbore.

Main Functions of Intermediate Casing:

As a key component of the wellbore structure, the intermediate casing serves as a connecting link between upstream and downstream wellbore protection. Its core functions are isolation and transition. Specifically, it is used to:

1. Isolate complex formations and resolve drilling challenges

Isolate high-pressure formations: When drilling into abnormally high-pressure oil, gas, or water zones, installing and cementing intermediate casing can prevent blowouts. 

Isolating low-pressure loss zones: When drilling into formations with well-developed fractures and low pressure, drilling fluid can lose significant amounts, leading to wellbore instability. Intermediate casing can isolate these loss zones.

Isolating unstable formations: Such as easily collapsing mudstone and creeping salt-gypsum layers prevents them from jamming the drill string and causing the wellbore to fail.

2. Protecting upper formations

By isolating the lower, high-pressure formations, the relatively weaker formations above are protected from fracturing. After cementing the technical casing, cement slurry is injected into the annular space between the casing and the wellbore, forming a cement sheath-casing composite barrier. This barrier effectively supports the wellbore and prevents formation collapse.

3. Creating conditions for subsequent drilling

After running and cementing the intermediate casing, a new, safe bottom hole is established. Drilling fluids of varying densities can be used to continue drilling into deeper, more complex formations under controlled conditions.

4. Carrying the weight of subsequent casing
Deeper casing (such as production casing) is typically suspended from the intermediate casing by a hanger, so it must bear this weight.

Key Features of Intermediate Casing:

a. Required: It is not required in every well. If the geology is simple and drilling can proceed safely from the surface casing shoe to the target depth, then there is no need to run an intermediate casing. The more complex the geology, the more layers of intermediate casing may be required, and the higher the cost of the well.
b. Intermediate Position: Its position in the wellbore structure is within the surface casing and outside the production casing.
c. Highly Flexible: Its size, depth, and number of layers depend entirely on the actual geology encountered, allowing for significant design flexibility.

Intermediate Casing Pipe Size:

The size of the intermediate casing is a combination of the dimensions of the upper casing and the planned production casing. It must fit smoothly through the upper casing and leave ample space for the production casing below. This type of casing typically has an outside diameter ranging from 5 to 13 3/8 inches. It is manufactured from steel grades such as J55 and N80 and must meet API standard threading specifications. The final size is determined by factors such as the projected well depth and geological complexity (the depth and number of intervals to be isolated).

Most Common Sizes:
9-5/8-inch – This is the most widely used intermediate casing size in onshore and offshore drilling worldwide.
10-3/4-inch – Also very common.
13-3/8-inch – Often used as the first intermediate casing in deeper wells or the only intermediate casing in shallower wells.

Other Possible Sizes:
7-inch / 7-5/8-inch – Often used as a deeper, smaller intermediate casing (or "casing liner").

11-3/4-inch / 16-inch – Used when a larger wellbore or more complex wellbore configuration is required.

Running Depth Range:

Because intermediate casing is designed on demand, its running depth range is very wide:
Onshore conventional wells: May be between 1,000 and 3,000 meters.
Deep/ultra-deep wells: May be between 3,000 and 5,000 meters or deeper.
Offshore wells: Running depths vary even more due to water depth and complex geological conditions.

Structural Features:

This type of casing utilizes a multi-layer string design:
The minimum yield strength of the tubing is 55,000 psi (J55 steel grade) and the maximum is 110,000 psi (P110 steel grade);
The casing connections utilize API SPEC 5B standard threads, including long circle threads (LC/LTC), short circle threads (STC), and traverse threads (BC/BTC);
The cement seal must extend to at least 200 meters above the top of the isolated formation, with the cement seal at the casing shoe at least 20 meters long.

Installation Process:

The installation process includes the following key quality control points:
Precise torque (±5% tolerance) is applied using power tongs to ensure airtightness of the threaded connections;
Automatic grouting equipment is deployed to control pressure fluctuations, with grouting pressure deviations not exceeding ±0.3 MPa;
Drift testing (100% pass rate) and hydrostatic pressure testing (pressure hold for ≥5 seconds) are performed;

Casing centralizers are installed at intervals of 15-20 feet, and the mud scraper frequency is maintained at 20 revolutions per minute.

Read more: Types of Drilling Casing Pipe or Oil Casing Application: Key Technology from Borewell to Oil and Gas Well