How to Read a Gas-Lift Casing Pressure Trend

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Recognize Steady, Stable Injection

The healthy baseline is a casing injection pressure that sits relatively steady near its design setpoint with only slow, small movement. Steady casing pressure means gas is entering the tubing through a single operating point at a consistent rate, which is what continuous gas lift is supposed to do. When the casing trend is flat and the tubing pressure shows the normal fluctuation of a flowing well, the well is injecting stably and the valves are behaving. This is the reference every other pattern is judged against.

Learn this well's own steady baseline, because absolute pressures depend on the gas-lift design, depth, and injection setpoint. A casing pressure that is steady but sitting at a different level than the design intended may indicate injection through a different valve than planned, so compare the current steady level against the commissioning baseline. The point is not a universal number but the shape and stability of the trend against this well's own history.

Confirm the tubing pressure agrees. Stable injection should produce a tubing pressure that flows within its normal band without wild swings. If the casing looks steady but the tubing is erratic, the interpretation is not simply stable, and you should look at the pair together. Reading the casing and tubing trends as a matched pair is the core skill, and it connects to the broader idea of casing head pressure in gas lift.

Spot Heading and Instability

The most important pattern to recognize is heading: a regular cycling of casing and tubing pressure that indicates unstable, surging lift. In heading, gas builds in the casing, dumps into the tubing, unloads a slug of liquid, and the cycle repeats, producing a sawtooth in the pressures and a surging production rate. On the trend it looks like a repeating rise-and-fall in casing pressure, often mirrored or offset in tubing pressure. Heading wastes gas and stresses equipment, so recognizing it is high value.

Distinguish heading from a normal slow movement by its regularity and amplitude. Heading is a repeating cycle with a consistent period and a meaningful pressure swing, not random small noise. Once you see the cycle, the fix is an injection or valve question - adjusting injection rate, choke, or valve design to stabilize the well - which belongs to the gas-lift engineer. Your job in reading the trend is to name the instability and quantify its period and amplitude, which the definition of heading instability describes in more detail.

Watch for instability that comes and goes with operating changes. A well that heads only at low injection rates, or only after an injection change, is telling you where the stable operating window is. Correlating the onset of cycling with injection-rate or choke changes on the trend helps the engineer find a stable setpoint. The casing trend is where this instability is most visible, so it is the primary tool for diagnosing and then confirming the cure for heading.

Read Valve Shifts and Lost Injection

A casing pressure that steps to a new steady level, without a matching injection change, often means the injection has moved to a different gas-lift valve. During normal operation gas should inject through the deepest valve that will pass it; if casing pressure shifts, a valve may have opened or closed unexpectedly, or the well may be multipointing through more than one valve. Reading the casing trend for these level shifts is how you catch a valve problem remotely before it shows up as lost production.

A gradual, sustained fall in casing pressure usually means the injection gas supply is dropping - a choke closing off, a compressor problem, or a supply-line issue upstream - rather than a downhole valve event. Distinguish this from a valve shift by its shape: lost supply is a slow decline toward a lower pressure, while a valve shift is a step to a new level. A casing pressure falling toward the tubing pressure risks the well dying, so this pattern deserves prompt attention.

Always confirm the casing interpretation against tubing pressure and, where available, injection rate. A casing level shift with a corresponding tubing pressure change tells a consistent story about where gas is entering; a casing change with no tubing response may be a supply or measurement issue. The table below pairs the common casing trend patterns with their likely meaning so you can match what you see to a diagnosis quickly.

Casing trend pattern	Tubing behavior	Likely meaning
Steady near setpoint	Normal flowing fluctuation	Stable injection through the operating valve
Regular sawtooth cycle	Cycles with casing, surging	Heading instability
Step to a new steady level	Corresponding change	Injection moved to a different valve or multipointing
Slow sustained decline	Falls toward casing	Lost injection supply, well may die

Common Mistakes

The most common mistake is reading the casing trend alone. Casing and tubing pressure together tell the story of valve operation and stability, and a casing pattern can be misread without its tubing partner. Always overlay the two before concluding.

The second is confusing normal slow movement with heading; heading is a regular, meaningful cycle, not random small noise, so quantify the period and amplitude before calling it instability. The third is mistaking a slow lost-supply decline for a valve shift, when the shape differs - a decline is gradual toward a lower level, a valve shift is a step to a new level - and the causes and fixes are entirely different.

Frequently Asked Questions