Gas Compression

Click photo above to expand

A gas compressor has the job of taking low-pressure gas that flows from shallow, low-pressure reservoirs such as coal beds, then raising the pressure of such gas so that it can be injected into gas pipelines for transport and subsequent sale at a defined interconnect with the pipeline of a gas buyer.

While this two-pipeline junction point of sale enables the gas producer to obtain revenues for his produced gas, it is often the case that the buyer's gas pipe is moving gas at a higher pressure than the seller's gas. In such cases, it is the gas producer's responsibility to increase the pressure of his gas in order to inject it into the buyer's pipeline with its higher pressure regime.

In order to do this, the gas producer uses a skid-mounted, unitized natural gas compressor which essentially comes in three connected parts: 1) the gas-fueled reciprocating engine that drives the compressor; 2) the high-speed, reciprocating compressor that compresses the low pressure wellstream gas in various stages in its cylinders to higher desired pressures; 3) the intercooler that cools down the compressed gas in between each stage of compression, since the laws of physics are such that the gas undergoes the heat of compression as the gas molecules are crammed together within the confines of the reciprocating compressor cylinders.

Click photo above to expand

For example, a "3-Stage" compressor can take low pressure gas at an intake pressure of, for example, 10 psig ("pounds per square inch gauge"). The first stage of that compressor can compress the gas safely to about 60 psig. This higher pressure gas must then go through an intercooler before it enters the second stage of compression. The second stage can take that 60 psig gas and compress it to about 195 psig, adjusting for pressure losses in the gas cooler. After intercooling, that gas will then go to the third stage for compression up to 610 psig, where it may be of high enough pressure such that it can successfully be measured and injected into the gas buyer's sale line.

The energy for this compression work in the above illustration has come from the burning of natural gas as fuel in the compressor engine (the first connected part described in the third paragraph above). This compressor engine is identical in design to a common automobile engine, with the exception that its combustion cylinders are much larger and the engine turns at only about 1,000 RPM, compared with an automobile engine that turns in a range between 2,000 RPM and 3,000 RPM under normal operating conditions. Otherwise, the compressor engine requires lube oil, anti-freeze and spark plugs just like the ordinary auto engine.

The working, or compression side of the compressor (the "frame") takes low pressure gas into large cylinders and compresses such gas with large, snug-fitting pistons inside the large cylinders. The compressor engine turns the gas compressor frame via a connected driveshaft, and both the frame and the engine have reciprocating pistons driven by crankshafts.

The fuel used by the compressor engine is taken from the wellstream gas being compressed and usually represents no more than 5% of the total gas being compressed. If natural gas prices are in a range of $8/MMBtu, then you can see that the "opportunity cost" of this fuel can be as high as $300/day while being burned in order to generate in excess of $5700/day in gas sale revenues.