One of the nice things about gaslift, is that events that cause lost production, can sometimes be resolved by performing troubleshooting at surface. I like to quote JPT Emerging Technology Senior Editor, Stephen Rassenfoss:
“When a gas lift system starts performing poorly, there is a good chance no one will notice. It is not an event that demands attention like a broken pump. A gas lift system will continue injecting gas into wells and oil will continue to come out. Just not as much oil as there could be.....”
If you happen to notice that your well is producing less fluid, there are things you can do to troubleshoot the issue. This article will focus on Troubleshooting Gas Lift Wells.
With 50 years experience, Tom Nations is an expert in Gas-Lift and NODAL Analysis. In our latest guest article, Tom breaks down the process of trouble-shooting surface problems on a Gas-lifted well. He explains how carrying out the correct checks and diagnosing these issues early can avoid the added expense of having to perform a costly workover.
General Gas-Lift System problems are often associated with three areas:
- Inlet (surface)
- Outlet (surface)
In this edition of the Blog we are going to focus on problems that occur at the surface of a gas-lifted well.
The surface is the easiest place to start looking for a problem and is often also the most likely place where a problem may be found. Surface gas-lift problems are typically associated with either the injection gas inlet to the well or the fluid outlet from the well.
You should thoroughly explore all potential surface problems before incurring the expense of a wireline unit to investigate potential downhole causes.
Keep in mind that poor optimization is most often caused by inaccurate instrumentation readings. Inaccuracies are introduced by malfunction, blockage, or the lack of calibration.
Always troubleshoot your well at the surface before you call a rig!
Changes in casing pressure and gas volume typically indicate a problem with the inlet.
Low Casing Pressure
If there is a choke on the wellhead, check if it is plugged, frozen or too small. Verify the gauge readings to be sure the problem is real. A check of gas volume being injected will help you rule out low casing pressure due to a hole in the tubing or cut out valve. If the lift gas injection choke is frozen, the problem can often be remedied by continuous injection of methanol in the injection gas.
High Casing Pressure
Check the gas injection volume. the injection choke being too large will allow high gas injection volumes. Gas injection volumes greater than the capacity of the downhole gas-lift valves, will cause a high casing pressure. The high gas injection pressure will cause the upper valves to re-open.
If high gas injection pressure is accompanied by low injection gas volumes, the operating valve may be partially plugged. Flowing tubing temperatures higher than originally designed for will raise the operating pressure of IPO gas-lift valves. Flowing tubing pressure higher than designed for will reduce the differential across the operating gas-lift valve downhole preventing the valve from delivering the correct injection gas volume which in turn will cause the surface gas injection pressure to increase. Any surface flowline and choke restrictions should be removed.
Low Gas Usage
Ensure the control valve on the gas-lift line from the compressor to the wellhead is fully open and that any choke in the line is not too small, frozen, or plugged.
Verify that the available operating gas injection pressure is in the range required for downhole gas-lift valve operation. Determine if the gas volume is being delivered to the desired well or if nearby wells on the same manifold system may be robbing gas from the subject well. For a system that supplies injection gas to multiple wells, it is imperative that each well has both a control valve and a gas volume meter downstream of the manifold.
Excessive Gas Usage
Check the casing pressure. The casing choke may be too large or the casing pressure too excessive. Both may cause upper pressure valves to re-open. If excessive gas volume is accompanied by low casing pressure, a tubing leak or cut out valve may be to blame.
Check the wellhead casing and tubing pressures with a calibrated gauge. Inaccurate gauges can cause false indications of high or low casing pressures.
High back pressure is a common indicator of a problem with the outlet.
High Back Pressure
If there is a choke in present in the flowline check that the choke and even the choke body are equal to or larger in size than the flowline I.D. If this is not true, remove the choke body if possible.
Check for paraffin or scale buildup in the flowline. Hot oiling the line will generally remove paraffin. Scale can be reduced and managed with methods such as chemical washes, high-pressure steam clean outs or continual chemical injections.
If high back pressure is the result of long flowlines, increasing the size of the flowline or adding an additional parallel line should reduce the back pressure. Flowline I.D. smaller than the tubing I.D. will have a similar effect as a long flowline. A partially open check valve in the flowline may also cause excessive back pressure.
Common or shared flowlines and excessive 90° turns should be avoided or removed if feasible.
Separator Operating Pressure
The separator pressure should be maintained as low as possible for gas-lift wells. Often a well may be flowing to a high or intermediate pressure system when it dies and is placed on gas-lift. Ensure the well is switched to the lowest pressure system available. Sometimes an undersized orifice plate in the meter at the separator will cause high back pressure.
Check all valves at the tree and headers are fully open. And verify the valves are sized properly (for example, a 2-inch I.D. valve should be used in a 2-inch flowline). A smashed or crimped flowline is another possibility. Inspect the flowline in places where it crosses a road.