For oil fields with gas compression facilities, gas lift is an established method of enhancing production, especially late in field life when reservoir pressure may decline or water production increases. Gas lift works by reducing the density and therefore the hydrostatic pressure created by produced fluids in production tubing. Consequently, the drawdown between the reservoir and the bottom of the wellbore is increased, and higher production is achieved.
Gas lift involves circulating gas down the A annulus between the production tubing and the production casing to as deep a location in the tubing as possible. The gas enters the production stream via a gas lift valve (GLV). The GLV is furnished with an orifice valve, which is sized for critical flow to ensure any fluctuations in tubing pressure due to situations like slugging are not exacerbated by resultant fluctuations in lift gas rate.
The deeper the lift gas can be injected, the greater the benefit. This is usually constrained by the discharge pressure of the gas compressor and the pressure of the column of liquid in the tubing when initiating gas lift or when restarting following a shutdown.
Injection pressure-operated (IPO) unloading GLVs often are used to overcome this constraint. They are designed to transfer the lift gas injection depth to successively deeper locations within the well. Once the transfer process is completed, it is important that the unloading valves close reliably to ensure that all of the available lift gas is injected from the deepest possible location.
Historically, the only reliable solution to the gas lift system design and performance optimization challenge described above has been to:
- Conservatively select a lift gas injection depth that will work under the worst-case combination of reservoir pressures; and
- Install a retrievable dummy GLV at a deeper location in the well that could be accessed if the worst-case combination does not materialize or if conditions change, such as a fall in reservoir pressure.
In the event deeper gas lift became possible, the dummy valve would be retrieved along with the existing orifice GLV using wireline intervention techniques. The dummy valve would then be replaced with a new orifice valve, and the orifice valve would be replaced with a new IPO unloading valve.
This activity is among the most challenging of all wire-line operations and usually takes a few days to complete. Furthermore, in deviated wells or wells where scale precipitation has occurred, it has been known to take a number of weeks. At the worst, loss of wireline can result in the requirement for workover if fishing operations prove unsuccessful. Consequently, it can become very expensive, especially in subsea wells.
A new solution
PTC has developed a new IPO unloading GLV design that has been field proven in hundreds of wells. The GLV is designed to be equally effective as both an orifice/operating valve and as an IPO unloading valve. It flips between these modes as needed.
Consequently, a gas-lifted well now can be completed without using a dummy valve at the deepest envisaged lift gas injection depth. Instead, an orifice valve can be installed at that depth, and an IPO unloading valve can be installed at the depth the conservative design assumes the lift gas injection will be restrained to.
Depending on the actual reservoir and well characteristics, the IPO unloading valve then can be reliably used as an orifice valve or as an unloading valve without the requirement for wireline intervention.
Traditional valve design
Original IPO GLVs incorporated a metal bellows charged with high-pressure gas to close the valve when the annulus pressure fell below the bellows charge pressure. The design has not changed much since the first patent in 1944.
PTC technicians identified numerous limitations in this original design, most critically that the bellows move both axially and radially as the annulus pressure changes, and the amount they move is not physically constrained. Consequently, these bellows have a relatively low pressure rating, which limits the depth to which they can be installed in the well.
They also have to be relatively stiff, resulting in a slow response time and only a very short valve stem travel between the open and closed positions. For this reason, long-term gas lift operation through a traditional IPO unloading valve would most likely result in damage to the bellows and/or the unloading valve seal. It usually is not recommended.
The new valve design
The new IPO unloading valve design is modular, incorporating in series both a standard orifice check valve module and an unloading valve module. The latter incorporates a patented double-acting bellows design rated to 10,000 psi.
Despite this high-pressure rating, the double-acting bellows facilitate unusually long valve stem travel. This is because the charge pressure does not act on the internals of the bellows, which are instead fully sealed and filled with silicone oil. In this case, the charge (or dome) pressure acts only externally on the upper cross section of the bellows.
When the dome pressure is higher than the annulus pressure, the dome side of the bellows is fully compressed, and the well side of the bellows is fully extended (and vice versa when the annulus pressure is higher than the dome pressure). In both cases, the extent to which the bellows moves is limited by the amount that the double-acting bellows can be compressed.
Consequently, the bellows can be designed to travel a relatively long distance with a very small pressure differential. This facilitates an extremely responsive mechanism and long unloading valve stem travel, which addresses the potential for erosion and facilitates rig time savings during unloading.
The double-acting bellows also is very reliable, having been tested by an operator to more than 100,000 cycles without fail. The major benefit of this is that when the bellows is in the open position, the unloading valve becomes almost hydrodynamically invisible, meaning the valve can be used reliably as either an IPO unloading valve or as an orifice/operating valve.
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