Multi-well gas lift operations

A common trend in the last several years has been to use gas lift as an intermediary artificial lift system between a free flowing well to rod lift. We now know that it is possible to draw down the well more effectively with gas lift while producing at higher rates.

To lower compressor rental costs, operators will often install one or two large compressors per pad that will inject into numerous wells, oftentimes up to eight wells per pad.

A lot of times, the folks sitting in the office just see rates and pressures on a computer screen, but when it comes to actually optimizing the system on site, it takes some work. When you show up on a pad that has multiple wells with different required injection rates, here are a few things to keep in mind.

Verify that each well is getting the right volume of gas injected

Orifice meter

One of the main challenges is balancing the total compressor capacity between a number of wells. Gas-engine driven, reciprocating compressors are typically used and the flow rate is a function of the suction pressure, discharge pressure, and RPM speed of the engine.

The injection rate for each well is typically controlled by an electronically-actuated valve (or manual choke) and the rate is adjusted at the RTU (remote terminal unit). The control valves will typically try to match the injection rate setpoint with the gas volume flowing through the injection meter run.

Adjusting the control valves

Manual choke

1. If a manual choke is installed to control the injection rate, you will need to make a small adjustment to the choke and see how the measured rate changes. Changing injection rates on wells that have manual chokes to control injection is often easier done with a partner.

 2. Make sure the actuators aren’t frozen. Adjust the rates at the RTU and see if the valves adjust. If not they may need to be reset.

 3. If the compressor output is lower than the volume demanded by all the wells, you may need to adjust the rates. Try lowering the rates to where the sum of the injection set points is equal to the compressor output. You’ll have to think about which wells need more gas than others and do a little back of the envelope math to make sure you give each well the right proportion of total compressor output.

However, if the compressors are outputting more / less gas than the total gas demanded by all the meters, the injection rates can start acting a bit strange. Each control valve will try to match the setpoint, which can cause swings in the valves opening and closing.

Adjusting the rate at the compressor

Compressor skid

1. If running multiple compressors, you can shut each compressor down one at a time to see how much each compressor is moving. This can help identify which compressor might need adjustments made to it.

2. Increase / decrease the suction pressure. The throughput of the compressor is a function of suction pressure. A higher suction pressure will increase gas throughput.

3. Speed up or slow down the compressor. The compressor supplier will be able to give you the compressor’s operating range.

4. Check compressor valve temperatures with a temp gun. If one of the valves is running hotter than the rest, there may be a leaking valve that is lowering capacity.

Once you get the compressors and injection valves lined out and humming, you can really start optimizing for production.

Injecting only what the well needs

Every well is designed to have a certain amount of injection during unloading, but once the well starts also producing gas, the injection rate can often be lowered as the produced gas will help keep the well unloaded. As casing pressure decreases and lower valves are utilized, the injection rate can often be reduced. 

A systematic way of prioritizing which wells are over-injecting or under-injecting is to check the amount of gas injected against the amount of fluid produced of all the wells on the pad. It is a good goal to aim to use roughly the same MCF of gas to lift one barrel of fluid out of each well on the pad. If you think a change is necessary, bump the rate on a well in 25 MCFD increments (adjusting rate at the compressor as well) and see if there’s a change in production over the next few days.

Monitoring the recycle system

Lastly, the recycle system should be monitored as casing (and compressor discharge) pressures change over time. As the wells are drawn down casing pressures will typically decrease, which causes the compressors’ discharge pressures to also decrease. As the discharge pressure decreases, make sure the pilot on the recycle line gets adjusted to prevent over-injecting on any one well. This will also help prevent freezing across the valve in wintertime as pressure drop will be lower. 

The goal is to minimize gas usage and compressor rental costs while keeping the well flowing at its maximum productivity as long as possible. With a little bit of patience and tweaking of the system, utilizing a few compressors to inject on a number of wells can be a great solution.
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