Casing is the steel pipe that lines a wellbore and holds the whole well together - it keeps the hole from collapsing, isolates freshwater aquifers from hydrocarbons, and gives every other piece of well equipment something to seat against. A well is built from several nested casing strings, each cemented in place. This guide explains what casing is, the sequence of casing strings, and why cement matters as much as the steel.
Casing in one line: Well casing is steel pipe run into a drilled wellbore and cemented in place to line and support the hole. It prevents the wellbore from collapsing, seals off freshwater and unwanted zones, contains pressure, and provides the anchored structure that tubing, packers, and the wellhead rely on.
A well is lined not with a single pipe but with a series of casing strings run at increasing depth and decreasing diameter, each passing inside the last. Conductor casing is the shallow, widest string that stabilizes the top of the hole. Surface casing goes deeper to protect freshwater aquifers and anchor the blowout preventer. Intermediate casing seals off troublesome zones - unstable shales or high-pressure formations - on the way down. Production casing is the innermost string run across the reservoir, through which the well ultimately produces.
Because each string nests inside the previous one, casing is described by outside diameter - a surface string might be 9 to 13 inches while the production string is 4.5 to 7 inches. Not every well uses all four: a shallow well may need only surface and production casing, while a deep or complex well adds several intermediate strings and a liner.
Steel pipe alone does not seal a well - cement does. After a casing string is run, cement slurry is pumped down the inside and up the annulus between the casing and the borehole wall, then allowed to set. This cement sheath bonds the casing to the rock, isolates one formation from another, and blocks fluids from migrating up the outside of the pipe. A good cement job is fundamental to well integrity and to protecting groundwater, which is why bond quality is verified with cement bond logs and pressure tests and is a focus of regulators. Casing and cement together, not either alone, make a well a sealed, controllable conduit.
Once a well produces, the space between the production casing and the tubing - the casing annulus - is monitored for pressure. Casing pressure that rises unexpectedly can indicate a tubing or packer leak, gas migration, or an integrity problem, so operators track it alongside tubing pressure. Some artificial-lift methods, such as gas lift, deliberately use casing pressure in normal operation. At surface, casing pressure is read by a transmitter and digitized into a tag by a wellsite RTU or flow computer. A cloud SCADA such as Merobix polls those casing pressure tags over Modbus or DNP3, so an operator can watch annulus pressure across many wells and be alerted when a trend signals a developing integrity issue.
Casing lines the wellbore to keep it from collapsing, isolates freshwater aquifers from hydrocarbons, seals off unwanted or high-pressure zones, contains well pressure, and provides the anchored structure that tubing, packers, and the wellhead attach to. Cemented in place, it is the backbone of well integrity.
Casing is the larger-diameter pipe cemented permanently into the wellbore to line and seal the hole. Tubing is the smaller pipe run inside the production casing to carry produced fluids to surface and is retrievable for workovers. Casing is structural and permanent; tubing is the removable production conduit.
It is pressure that builds up in a casing annulus and returns after being bled off, usually signaling a leak in the cement, casing, tubing, or a packer. It is a well-integrity concern that operators monitor with casing pressure gauges and report, because it can indicate fluid or gas migrating where it should not.
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.
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