When a gas-lift well is first put on injection, gas cannot simply reach the deepest valve and start lifting - the tubing is full of kill fluid that has to be pushed out of the way first. The unloading sequence is the staged process by which injection gas works its way from the top valve progressively downward, transferring from one unloading valve to the next until it finally reaches the operating valve at the deepest point. Getting a well to unload cleanly is the first test of a gas-lift design, and reading the casing and tubing pressures during the process is how an operator confirms each transfer is happening as intended.
Gas Lift Unloading Sequence in one line: A gas lift unloading sequence is the staged process of starting a gas-lift well, in which injection gas enters the top valve, U-tubes fluid out of the tubing, then transfers deeper valve by valve until injection reaches the operating valve at the deepest point. Casing and tubing pressure trends confirm each successive transfer.
A newly completed or freshly killed gas-lift well starts with the tubing and casing annulus both full of fluid up to surface. To unload it, gas is injected into the casing annulus, and its pressure begins pushing the fluid in the annulus down. Because the annulus and tubing connect at the bottom, forcing fluid down the annulus displaces it up and out through the tubing - the well behaves like a U-tube, with gas on one side pushing liquid up and out the other. This U-tubing action is how the first fluid is removed.
As the annulus fluid level falls, it eventually uncovers the top gas-lift valve, which by design is open. Gas now enters the tubing through that valve, mixes into the fluid column above it, and lightens it. That lighter column is easier to lift, so the well begins flowing and the annulus level keeps falling. The whole point of starting at the top valve is that it is the only depth the available injection pressure can reach while the tubing is still full and heavy - you cannot get gas any deeper until the column above has been lightened.
Injecting through the top valve alone is not the goal; it is only the first step. Gas entering high in the tubing lifts only a short column and produces little, and the injection point sits far above where the reservoir would most benefit. The purpose of the top valve is simply to get the process started - to begin lightening the column so that injection can be pushed progressively deeper. Everything after this is about transferring the injection point downward toward the operating valve.
Once gas is lifting through the top valve, the fluid column below it grows lighter and the annulus level continues to drop until it uncovers the second valve. Now gas can enter at two depths, but the design intends for it to move fully to the deeper one. As the deeper valve begins passing gas, the injection point transfers downward, and the shallower valve is meant to close so all the gas goes through the new, deeper point. That handoff - injection stopping at the upper valve and taking over at the next one down - is the transfer, and it is the heart of unloading.
The unloading valves are engineered to make those transfers happen automatically as conditions change. Each valve is set to close as the injection or tubing pressure drops during unloading, so that once the deeper valve is uncovered and injecting, the shallower one is nudged shut. Step by step the injection point walks down the string: gas transfers from valve one to valve two, then to valve three, and so on, each transfer lightening the column a little more and letting the annulus level fall to uncover the next valve below.
The sequence ends when injection reaches the operating valve, the deepest point the design intends gas to enter continuously. At that point all the unloading valves above should be closed, and gas injects through the single operating valve alone. Reaching that deepest injection point is the objective of the whole exercise, because injecting as deep as possible lightens the longest fluid column and maximizes the drawdown and production the reservoir can deliver. A well that fails to transfer all the way down - stalling on an upper valve - never achieves its designed lift.
Unloading is invisible from surface except through pressure, so casing (injection) and tubing (production) pressure are the instruments an operator watches to know what is happening downhole. As gas is injected and the annulus fills, casing pressure rises; each time the injection point transfers to a deeper valve, the casing pressure characteristically peaks and then falls back to a lower stabilized level, because the deeper valve requires less injection pressure to keep gas flowing than the shallower one did. That saw-tooth of rising-then-dropping casing pressure is the classic fingerprint of successive valve transfers.
Tubing pressure and production tell the complementary half of the story. As the well unloads and the injection point moves deeper, the produced fluid rate and the character of the tubing pressure change, and a well that has reached its operating valve settles into a steadier, higher production with a stable injection pressure. Watching casing pressure step down through several plateaus while production builds is how an operator confirms the well has walked its injection point all the way to the operating valve rather than stalling partway.
This is where continuous SCADA monitoring turns unloading from a guessing game into an observable process. A cloud platform such as Merobix trending casing pressure, tubing pressure, and injection-gas rate together lets an engineer watch each transfer happen in real time and, just as important, replay it afterward. A slow or failed unload leaves a distinct trace - casing pressure that never steps down, or an injection rate that will not deepen - and having that history remotely available means a stuck unload can be diagnosed and the design or injection rate adjusted without a trip to the wellsite. The pressure trends are the record that the well is lifting from the depth it was designed to.
U-tubing is the initial action when a gas-lift well starts up, where injection gas pushes fluid down the casing annulus and forces it up and out through the tubing, because the annulus and tubing connect at the bottom like a U-tube. This displaces the first fluid and lowers the annulus level until it uncovers the top valve so gas can enter the tubing. U-tubing is how injection begins before any valve is passing gas into the production string.
Multiple unloading valves exist because injection gas cannot reach the deepest point while the tubing is still full of heavy fluid. The valves are spaced up the string so gas can start at the top, lighten the column, and transfer progressively deeper one valve at a time until it reaches the operating valve. Each unloading valve is a stepping stone that lets the injection point walk down to the depth where the well is designed to lift.
Casing pressure is the main indicator. As the injection point transfers to a deeper valve, casing pressure typically peaks and then drops to a lower stabilized level, because the deeper valve needs less injection pressure to keep passing gas. Watching casing pressure step down through successive plateaus while production builds confirms the injection point is walking deeper toward the operating valve.
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