Annulus Definition / Meaning
The annulus is the ring-shaped space between two concentric things in a well. During drilling it is the gap between the drill string and the wall of the hole, or the inside of the last casing string. In a completed well it is the gap between the production tubing and the production casing, and between each casing string and the one outside it. Almost everything that happens in a well happens either inside the pipe or in an annulus, and the two are kept deliberately separate.
The plural is annuli, and a well with several casing strings has several of them, named from the inside out: the A annulus between tubing and production casing, the B annulus between production and intermediate casing, and so on.
The Annulus While Drilling
Drilling mud is pumped down the inside of the drill string, out through the bit, and back to surface up the annulus. That return trip does three jobs. It carries the cuttings out of the hole, which is why the annular velocity, the speed of the mud in the gap, has to be high enough to lift them. It supports the hole wall with the weight of the mud column. And it is the path a kick takes: formation fluid that enters the well rises up the annulus, so a rising pit level and a change in annular pressure are the first signs of one.
Annular Pressure and the Blowout Preventer
Because the annulus is open to the formation at the bottom, the pressure in it at surface is a direct reading of what the well is doing. During a well-control event the driller shuts the well in and reads the shut-in casing pressure, the pressure trapped in the annulus, alongside the drill-pipe pressure to work out how heavy the mud must become. The blowout preventer stack includes an annular preventer, a doughnut of rubber that closes on whatever pipe is in the hole and seals the annulus around it.
The Annulus After Casing
| Annulus | Between | What is in it |
|---|---|---|
| Casing-to-formation | Each casing string and the rock | Cement, ideally from the shoe to the planned top of cement, sealing zone from zone |
| Casing-to-casing (B, C…) | One casing string and the next outside it | Cement at the bottom, then the fluid the casing was run in; monitored at the wellhead for pressure |
| A annulus | Production tubing and production casing | Packer fluid above the packer; the route for gas-lift gas or for chemical injection |
Sustained Casing Pressure
An annulus that should be sealed and static but shows pressure at the wellhead, and rebuilds it after being bled off, has sustained casing pressure: somewhere a barrier has failed and formation fluid is finding its way in, usually through a poor cement job or a leaking connection. It is one of the most common well-integrity problems in mature fields and is diagnosed by which annulus carries the pressure and how fast it returns.
Usage Example
“Returns dropped and the pit gained twelve barrels, so we shut in on the annular and read 350 on the casing side against 200 on the drill pipe: a gas kick, and it was on its way up the annulus.”
Why It Is Watched for the Life of the Well
The annulus is where the well’s barriers either hold or fail. Cement in it isolates aquifers from hydrocarbons; packer fluid in it protects the casing from corrosion; pressure in it is the earliest warning of a leak. Regulators in most jurisdictions require annulus pressures to be recorded and limits to be set, and at the end of life the plug and abandonment program has to prove that every annulus, not just the bore, is sealed.