Well Integrity

Sustained Casing Pressure: Monitoring the Annuli That Will Not Bleed Off

Sustained casing pressure, or SCP, is pressure that builds in the casing annuli after a well is shut in, and that returns after being bled down instead of dropping to zero. It usually means a barrier downhole has failed, making it one of the most common well integrity problems an operator faces. This article covers what it is, why it matters, how to diagnose it, and how to monitor it.

What is sustained casing pressure?

Every well is built with concentric strings of casing and tubing, and the spaces between them are the annuli. The innermost space, the A annulus, sits between the production tubing and the production casing. The B annulus sits between the production casing and the intermediate casing, and the C annulus follows. Under normal conditions these annuli are sealed and hold a stable pressure from the completion fluid or a designed gas cap.

SCP is different from normal annular pressure, which stays flat. SCP builds after the well is shut in, and when the annulus is bled down, the pressure comes back. A bleed-down that goes to zero and stays at zero means the pressure was trapped, not sustained. A bleed-down that rebuilds means gas or liquid is migrating into the annulus, and a barrier that should hold it back is no longer doing its job.

Why SCP matters

SCP is a symptom of a failed barrier. The barriers that keep formation fluids out of the annuli include the cement sheath, the casing, and the packer that seals the tubing-casing annulus. When pressure keeps rebuilding, one of those barriers is leaking. The failure may be small today, but it tends to get worse.

If annular pressure exceeds the rating of the wellhead or the casing, the wellhead can be damaged or the casing can burst, putting people at risk and releasing gas or oil at surface. Annular pressure can also drive gas migration behind casing into shallow formations, where it can threaten groundwater. An untreated SCP condition is a liability that grows with time.

Regulators treat annular pressure seriously. For onshore wells, API Recommended Practice 90, "Annular Casing Pressure Management for Onshore Wells", is the industry practice document for monitoring and diagnosing annular pressure. It is guidance, not a law, but it reflects standard field practice. For offshore wells on the U.S. Outer Continental Shelf, the Bureau of Safety and Environmental Enforcement (BSEE) and 30 CFR Part 250, the well operations regulations, treat casing pressure monitoring as part of well integrity expectations.

How to diagnose SCP

The first step is a bleed-down test. Record the annulus pressure, bleed it to zero, close the valve, and watch. Note two things: what comes out, and how fast. If the bleed stream is gas, the leak is coming from a gas-bearing zone above the packer or through a leaking seal. If it is liquid, the source is usually completion fluid or formation water moving up a channel. Pressure that rebuilds in minutes points to a direct gas migration path. Pressure that takes days or weeks suggests a much smaller leak path.

After the test, classify the behavior. API RP 90 describes categories of annular pressure, such as pressure that bleeds to zero and stays there, pressure that rebuilds, and pressure that fluctuates with operations. Put the well in the right category, then start trending. Plot the shut-in pressure and the rebuild rate over weeks and months. A stable reading at a modest level is not the same as a reading that climbs every month.

The trending data drives the decision. If pressure is stable and below thresholds, the well can continue in service with a monitoring schedule. If pressure is climbing, or the bleed reveals gas that should not be there, the well likely needs a workover to restore the failed barrier, whether that means repairing the packer, squeezing cement, or planning plugging and abandonment. The point is to know which case you are in before the pressure decides for you.

A practical monitoring workflow

Monitoring comes down to routine readings on a schedule. Decide how often each annulus is checked, put it on the calendar, and do not skip it. Wells with known SCP get checked more often. Wells with a clean history get a longer schedule, but one must still exist.

Record every reading per annulus. The A, B, and C annuli each have their own gauge and need their own entry: gauge number, pressure, and bleed history. Log readings against the well's established baseline and thresholds. The baseline is what the annulus normally holds. The threshold is the level at which the well requires attention, including a shutdown and a workover decision. Compare every new reading to both.

• Take readings on a fixed schedule.
• Record A, B, and C annuli separately with gauge readings and bleed results.
• Compare every reading against the baseline and alert threshold.
• Flag rising trends immediately.
• Document every bleed-down test so rebuild rates can be compared.
• Escalate any reading that approaches a wellhead or casing pressure rating.

The weakest part of most SCP programs is record keeping, not measurement. A pressure that climbs slowly is easy to miss when the readings sit in a logbook nobody reviews. Keep the well integrity records current and track them in one place where the trend is visible. Software like OpsFlo, built for field tickets, dispatch, approvals, timesheets, and documents, is one way to keep monitoring data and integrity records attached to the well, where the field crew can update them and the office can review them.

When to act

Not every annulus with pressure needs a workover, but each needs a reason on file for why it is acceptable. The acceptable cases are stable, understood, and monitored. The unacceptable cases climb, bleed gas when they should not, or are not being read at all.

Set the rules before the problem shows up. Know the pressure rating of the wellhead and each casing string, know the threshold that triggers action, and know who makes the call when a reading crosses it. When a workover is required, address the barrier failure directly rather than bleeding the annulus and hoping the problem stays away.

Treat casing pressure monitoring the same way you treat wellhead maintenance: as routine, scheduled, documented work. Most failures are small pressures never written down, never compared, never acted on until the pressure makes the decision alone.

Sources and further reading

• API Recommended Practice 90
• Bureau of Safety and Environmental Enforcement
• eCFR 30 CFR Part 250