A well is a set of concentric pipes, and the spaces between them are the annuli. Each annulus is a pressure barrier, a flow path, and an integrity signal all at once, which is why operators name and monitor them carefully. This guide explains what a well annulus is, how the A, B, and C annuli are defined, and why annular pressure matters.
Well Annulus in one line: A well annulus is the ring-shaped space between two concentric strings of pipe in a well - for example, between the production tubing and the production casing, or between two casing strings. Annuli are lettered from the inside out: the A annulus is between tubing and the innermost casing, the B annulus is between that casing and the next, and so on. Each annulus is a monitored pressure boundary central to well integrity.
Starting from the produced-fluid path outward, the tubing carries the production. The first annular space around it - between the tubing and the production casing - is the A annulus. Outside the production casing, the next space (between it and the intermediate casing) is the B annulus, and the next is the C annulus. Each annulus is bounded by pipe walls, sealed at surface by the appropriate wellhead spool, and often isolated downhole by cement or a packer.
The A annulus is the most active: it is normally sealed at the bottom by the production packer and at the top by the tubing hanger, and it may be used deliberately for gas lift, chemical injection, or as a monitored pressure space. The outer annuli (B, C) are usually meant to be static and cemented, so any pressure appearing in them is a warning sign rather than a design feature.
Because each annulus is a sealed barrier, its pressure tells you whether that barrier is intact. Pressure that builds in an annulus that should be dead - called sustained casing pressure or sustained annular pressure - indicates a leak somewhere: a tubing or casing leak, a failed packer, poor cement, or gas migration up a channel behind pipe. Operators diagnose the source by bleed-down and buildup behavior at the annulus outlet.
Annular pressure limits are set from the casing and wellhead pressure ratings, and exceeding them risks burst or collapse of a string. This is why annulus monitoring is a regulated well-integrity practice, especially offshore and on high-pressure wells. Each annulus is accessed at surface through valved outlets on its wellhead spool - the A annulus at the tubing head, outer annuli at the casing head and casing spools.
Annulus management is fundamentally about watching pressures over time and reacting to changes. A pressure transmitter on each annulus outlet turns annular pressure into a signal that the field controller reads and reports, so an operator can see every annulus at once and get an alarm when one that should be dead begins to build.
A cloud SCADA such as Merobix reads those annulus pressure tags from the RTU or flow computer over Modbus, DNP3, or OPC UA, so annular pressures across a whole field can be trended and alarmed from a browser. The wellhead outlets provide access to each annulus; the transmitters and controller turn them into monitored values, and SCADA makes the long-term trend and alarm visible for integrity management.
The A annulus is the innermost annular space, between the production tubing and the innermost (production) casing. It is normally sealed by the production packer at the bottom and the tubing hanger at the top, and it may be used for gas lift or chemical injection. Its pressure is closely monitored for well integrity.
Each annulus is a sealed pressure barrier, so its pressure reveals whether that barrier is intact. Pressure building in an annulus that should be dead - sustained casing pressure - signals a tubing or casing leak, failed packer, poor cement, or gas migration. Monitoring protects well integrity and is often required by regulation.
Annuli are lettered from the inside out. The A annulus is between the tubing and the innermost casing, the B annulus is between that casing and the next one out, and the C annulus is the next space beyond that. Each is accessed at surface through valved outlets on its wellhead spool.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.