Gas Compression – Why Natural Gas Needs a Push
Gas comes out of a new well with plenty of pressure. Give it a few years, and that same well is struggling to push gas into a line that's running at higher pressure than the well can deliver. The gas is still there, but it can't get where it needs to go. Out on the lease, a loud skid with a big engine and a stack of finned coolers fixes that problem every day. Why does natural gas need compression, and how does gas compression work?
Gas compression raises the pressure of natural gas so it can flow from a lower-pressure point, like a tired well or a stock tank, into a higher-pressure pipeline. Gas only moves from high pressure to low pressure, so when a well's pressure falls below the line pressure, or when gas has to travel long distances, compressors make up the difference. Compressors work by squeezing gas into a smaller volume, either with pistons (reciprocating compressors), meshing rotors (screw compressors) or high-speed impellers (centrifugal compressors).
They show up at the wellhead, at central gathering stations, at processing plants and along transmission pipelines. Without them, a lot of gas would stay in the ground or get flared.
Why Can't Gas Just Flow on Its Own?
It can, as long as it has enough pressure. The trouble is that pressure gets used up along the way. Friction in pipelines eats pressure over distance. Reservoir pressure falls as a field is produced. And pipelines need gas delivered at or above their operating pressure.
So compression is needed in a few classic spots:
- Wellhead and field compression, to help weak wells flow into gathering lines.
- Gathering and central stations, to collect gas from many wells and boost it to the plant.
- Processing plants, to recompress gas after treating.
- Transmission pipelines, with compressor stations spaced along the route to keep gas moving.
- Gas lift systems, to boost produced gas back up to injection pressure.
- Storage fields, to push gas underground for later use.
The EIA overview of natural gas delivery and storage shows where compressor stations fit in the bigger pipeline system.
What Does a Wellhead Compressor Do?
A wellhead compressor pulls gas from one well or a small group of wells and pushes it into the gathering line. By lowering the pressure at the wellhead, it lets the reservoir push more gas into the wellbore.
That lower pressure also helps with liquid loading. When gas wells slow down, liquids start building up in the tubing and choke off flow. Dropping the wellhead pressure raises the gas velocity in the tubing, so it can carry liquid out again. A small wellhead compressor can bring a dying well back to life and extend its producing years.
Reciprocating vs Screw Compressor: What's the Difference?
The reciprocating vs screw compressor choice comes down to pressure ratio, volume and maintenance. Here's how the main types stack up.
| Reciprocating | Rotary screw | Centrifugal | |
|---|---|---|---|
| How it works | Pistons in cylinders with suction and discharge valves | Two meshing helical rotors squeeze gas as they turn | Spinning impellers speed gas up, diffusers turn speed into pressure |
| Strong points | High pressure ratios, flexible to changing conditions | Smooth, simple, handles low suction pressure well | Very high volumes, few wearing parts |
| Weak points | More wearing parts (valves, rings, packing) | Lower pressure ratios per stage | Less flexible at changing rates |
| Common uses | Field gathering, gas lift, many wellsite jobs | Wellhead boosting, vapor recovery | Big transmission lines and plants |
Most field compressors are driven by natural gas engines using fuel gas from the line. Electric motors are common where power is available, and gas turbines drive many large pipeline units.
Why Do Compressors Work in Stages?
Squeezing gas makes it hot, and the bigger the pressure jump, the hotter it gets. Push too much pressure rise into one step and the discharge temperature climbs past what the valves, seals and lubricants can handle.
So compressors split the job into stages. Each stage raises the pressure partway, then the gas runs through a cooler before the next stage. Cooling between stages saves horsepower, protects the machine and drops out liquids that a scrubber can catch. A wellhead unit might need only one or two stages, while a unit lifting gas from a low-pressure tank to a high-pressure line needs more.
What's on a Typical Compressor Skid?
A field compressor package is more than just the compressor. You'll usually find:
- Inlet scrubber, to catch liquids before they reach the compressor.
- Compressor cylinders or rotors, often in more than one stage.
- Coolers between stages and after the last stage, since compressing gas makes it hot.
- Interstage scrubbers, to remove liquids that drop out as gas cools.
- Driver: engine or motor.
- Control panel with shutdowns for pressure, temperature, vibration and liquid level.
- Relief valves and blowdown systems.
What Goes Wrong With Gas Compressors?
Compressors are some of the hardest-working equipment in the field, and they let you know when they're unhappy. Here's what you'll see and where to look first. Your mechanic, the lease operator and site procedures set the fix.
- Liquid slugs. Knocking or a shutdown on high scrubber level. Liquid in a reciprocating cylinder can wreck it. Check scrubbers and dump valves.
- High discharge temperature. Dirty coolers, failed fans or bad valves. Clean and inspect before running harder.
- Valve failures. Lost capacity, odd noises, temperature changes on one cylinder. Valves are a top maintenance item on reciprocating units.
- Vibration. Loose mounts, misalignment or worn bearings. Don't ignore it.
- Frequent shutdowns. Usually a sensor or setpoint problem, or a real issue the panel is catching. Find the root cause instead of just resetting.
Don't make this mistake: bypassing a shutdown switch to keep a compressor running. Those switches protect people and equipment from fires, overpressure and mechanical failure. If it keeps tripping, fix the cause.
How Does Compression Cut Flaring and Venting?
Without compression, low-pressure gas from tanks, separators and weak wells often has nowhere to go. It ends up vented or flared. Vapor recovery units, often small screw compressors, capture that low-pressure gas and push it into the sales line instead.
That's good for the operator, who gets to sell the gas, and good for the air, since methane is a powerful greenhouse gas. It's also one reason you'll see fewer flares in fields with good compression and pipeline access. If you're curious about the flame side of that story, read about gas flaring on oil rigs and why it happens.
Why Does Gas Compression Matter So Much?
Gas compression gives natural gas the push it needs to move from low pressure to high pressure, from wells into gathering lines, through plants and down long pipelines.
Wellhead compressors keep tired wells flowing and fight liquid loading. Reciprocating units handle high pressure ratios, screw compressors handle low suction pressure and vapor recovery, and centrifugal units move big volumes on transmission lines.
If you work around compressors, learn the shutdowns and the scrubbers first. Most bad days on a compressor start with liquid where it shouldn't be or a safety device somebody decided to ignore.