How to Verify Gas-Lift Casing and Tubing Sensors
On a gas-lift well the whole diagnosis rests on knowing which pressure is which and that both are true, so verifying the casing and tubing pressure sensors is a job worth doing carefully. A swapped pair of tags or a blocked impulse line will make the well look like it is doing something it is not. This procedure verifies both sensors after installation or after any wiring change: it checks each against a test gauge, confirms the two are not swapped, and proves each responds to the well. It is a focused field verification, narrower than a full telemetry commissioning.
Verify Gas-Lift Pressure Sensors in one line: To verify gas-lift casing and tubing pressure sensors, compare each transmitter against a known-good test gauge on its own tap, confirm the casing sensor reads the higher, steadier injection pressure and the tubing sensor reads the lower, more variable flowing pressure so the tags are not swapped, and prove each responds to a real change - the casing to an injection adjustment, the tubing to a well change. Sensors that read impossibly relative to each other, or never respond, are swapped, blocked, or isolated and must be corrected.
Check Each Sensor Against a Test Gauge
Put a known-good test gauge on the same tap as each transmitter and compare. The casing pressure transmitter should agree with a gauge on the casing tap, and the tubing pressure transmitter with a gauge on the tubing tap, each within the transmitter's stated accuracy. A large, steady offset points to a zero error or a wrong range; a reading that will not move points to a blocked impulse line or a closed isolation valve. Fixing the offset or the blockage here is far cheaper than chasing a phantom well problem later.
Confirm each impulse line and isolation valve is clear and correctly lined up. A gas-lift casing tap can plug with debris or hydrate, and a tubing tap can plug with paraffin or scale, either of which freezes the reading at a stale value that looks like a real but unchanging pressure. Cracking the isolation and watching the reading respond, then restoring it, confirms the line is open. A sensor reading a plugged line is worse than no sensor because it looks trustworthy.
Verify the electrical signal is in its live band, not pegged. A transmitter reading at or past either end of its range is signalling an over-range or a loop fault rather than a real pressure, and that has to be resolved before the reading means anything. Each sensor deserves the same treatment as any field pressure loop, so confirm the loop as you would when you commission a pressure transmitter anywhere else on site.
Prove the Tags Are Not Swapped
Swapped casing and tubing tags are a classic gas-lift error, and the pressures themselves tell you when it has happened. On a normally injecting gas-lift well, the casing injection pressure is higher and steadier than the flowing tubing pressure, so a channel labeled casing that reads low and jumpy while the one labeled tubing reads high and steady is almost certainly swapped. Use the expected magnitude and behavior of each pressure to confirm the labels match reality.
Confirm the swap-check with a deliberate, coordinated change if the operation allows. A change to the injection supply should move the casing pressure, not the tubing pressure, first and most; a change in the well's production should move the tubing pressure. Watching which telemetered channel responds to which physical action is the definitive proof that each tag is wired to the right tap. Do this in coordination with whoever controls the injection, and only within safe operating limits.
Check the pressures make sense together as a final cross-check. The relationship between casing and tubing pressure is what drives the gas-lift valves, so the two should sit in the order and pattern the gas-lift design predicts. Pressures that cross impossibly or move in lockstep when they should be independent point to a swap or a scaling error, not a real event. Keeping the mechanics of a gas-lift valve in mind helps you judge whether the two pressures relate correctly.
Confirm Each Sensor Responds to the Well
A sensor that reads the right number but never changes may still be effectively dead. Watch each pressure over time and confirm it responds to the well's real behavior: casing pressure moving slowly with injection, tubing pressure fluctuating with production. A tubing pressure that is dead flat on a producing well, or a casing pressure that ignores an injection change, is likely reading a plugged line or a stuck sensor even if its instantaneous value once matched a gauge.
Establish that both pressures move within believable bounds. A pressure that pins to a fixed value, snaps between two values, or shows steps that do not correspond to any operation suggests a sensor or telemetry fault rather than the well. Comparing the live behavior against what a gas-lift well should do is a form of verification that a single gauge comparison cannot provide, because it tests the sensor over its working range, not just one point.
Once both sensors read true, are correctly labeled, and respond to the well, they are verified and ready to trend. Capture a short baseline of normal casing and tubing pressure so future changes are visible against it, and store both as monitored tags. Reliable casing and tubing telemetry is the foundation for reading valve operation, injection problems, and well loading remotely, which is the whole reason to instrument a gas-lift well.
Common Mistakes
The most common mistake is not checking for swapped tags, because a swap produces readings that look like a real but bizarre well condition. Confirm the casing channel reads the higher, steadier pressure and the tubing channel the lower, more variable one, and prove it with a coordinated change if you can.
The second is trusting a sensor that matched a gauge once but never moves afterward, which usually means a plugged impulse line reading a stale value. Confirm each sensor responds to a real change. The third is verifying only the instantaneous value and never checking that the two pressures relate to each other the way the gas-lift design requires.
Frequently Asked Questions
How do I know if my casing and tubing tags are swapped?
Use the expected behavior of each pressure. On a normally injecting gas-lift well, casing injection pressure is higher and steadier while tubing pressure is lower and more variable. A channel labeled casing that reads low and jumpy while the one labeled tubing reads high and steady is almost certainly swapped. Confirm it by making a coordinated injection change and watching which telemetered channel responds - the casing should move first, not the tubing.
Why does a pressure sensor read a value but never change?
Usually a blocked impulse line or a closed isolation valve, which freezes the sensor at a stale pressure that looks real but never moves. On gas-lift wells the casing tap can plug with debris or hydrate and the tubing tap with paraffin or scale. Crack the isolation and watch the reading respond, then restore it, to confirm the line is open. A sensor reading a plugged line is worse than no sensor because it looks trustworthy.
Do casing and tubing pressures need to relate in a certain way?
Yes. The relationship between casing injection pressure and tubing pressure is what drives the gas-lift valves, so on a healthy well the casing pressure exceeds the tubing pressure where gas enters, and the two move in patterns the gas-lift design predicts. Pressures that cross impossibly or move in lockstep when they should be independent indicate a swapped tag or a scaling error rather than a real well event.
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