Two kinds of steel pipe define the architecture of nearly every oil and gas well: casing and tubing. They look similar but do very different jobs. Casing is the permanent structural lining of the wellbore; tubing is the working conduit inside it that actually carries the produced fluids to the surface.
Casing
Casing is run into the hole in a series of progressively smaller diameters and cemented in place. Each string is set to a planned depth and serves a specific purpose. From the surface down, the typical strings are:
- Conductor casing — the first, largest-diameter string near surface that supports the wellhead and prevents the top of the hole from caving.
- Surface casing — set below the conductor to protect shallow freshwater aquifers and provide a base for blowout-control equipment.
- Intermediate casing — run as needed to isolate troublesome zones and contain pressures while drilling deeper.
- Production casing — the innermost string set across the reservoir, through which the well will be completed and produced.
Because each string nests inside the previous one, the wellbore steps down in diameter with depth. Every casing string is cemented in its annulus, locking it to the rock and isolating the zones it passes through.
A length of steel pipe run to a set depth and cemented in place to line and stabilize part of the wellbore. Wells use several nested strings of decreasing diameter.
Tubing
Once the well is cased and ready to produce, tubing — a smaller-diameter pipe — is run inside the production casing. Oil and gas flow up through the tubing to the surface. Unlike casing, tubing is not cemented in place; it can be pulled and replaced during a workover to fix problems, change the completion or run downhole equipment.
Casing is cemented and permanent and lines the wellbore in nested decreasing sizes; tubing is the smaller, removable pipe inside the casing that carries oil and gas and can be pulled during workovers.
Why both are needed
Producing directly up the casing would expose that structural string — and the cement bond behind it — to the erosive, corrosive flow of well fluids and to pressure cycling, threatening long-term integrity. Tubing isolates the flow inside a dedicated, replaceable conduit, protecting the casing and allowing the well to be serviced without compromising its structure. The annular space between tubing and casing is typically sealed with a packer, separating produced fluids from the casing.
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Frequently asked
Casing is the cemented, permanent steel pipe that lines the wellbore in nested decreasing sizes. Tubing is the smaller, removable pipe inside the casing through which oil and gas flow; it can be pulled during workovers.
Casing is run in progressively smaller diameters so each string can nest inside the previous one as the well deepens. Conductor, surface, intermediate and production strings each set to a planned depth for a specific purpose.
Yes. Unlike cemented casing, tubing is not cemented in place, so it can be pulled and replaced during a workover to service the well or change the completion.