Oil and Gas at Your Door? (2005 Edition) - Earthworks

Oil and Gas Development and Impacts
Natural gas coming directly from a well contains many natural gas liquids
(NGLs), e.g., ethane, propane, butane, iso-butane, and natural
gasoline. NGLs often have a higher value when sold as separate products,
making it economical to remove them from the gas stream. The
removal of natural gas liquids usually takes place in a centralized processing
plant, and uses techniques similar to those used to dehydrate
natural gas. 111
In addition to the processes mentioned above, scrubbers and heaters
are installed either at or near the wellhead. The scrubbers remove
sand and other large-particle impurities. The heaters ensure that the
temperature of the gas does not drop too low and form natural gas
hydrates, which are solid or semi-solid compounds that resemble ice
crystals. Should these hydrates accumulate, they can impede the passage
of natural gas through valves and gathering systems. In addition
to wellhead heaters, small natural gas-fired heating units are typically
installed along the gathering pipe wherever it is likely that hydrates
may form. 112
FIGURE I-14. WELLHEAD COMPRESSORS
With and without sound abatement.
While some of the processing can be accomplished at or near the wellhead,
the complete processing of natural gas takes place at a processing
plant. The extracted natural gas is transported to these processing
plants through a network of gathering pipelines, which are
small-diameter, low pressure pipes. Some gathering systems are quite
complex, consisting of thousands of miles of pipes that connect the
processing plant to as many as 100 wells in the area. Should natural
gas from a particular well have high sulfur and carbon dioxide contents,
a specialized sour gas gathering pipe must be installed.
If the natural gas is being piped into larger pipelines, such as interstate
pipelines, it must be compressed. To ensure that the natural gas
flowing through any one pipeline remains pressurized, compressor
stations are usually placed at 40-100-mile intervals along the pipeline.
The natural gas enters the compressor station, where it is compressed
by a gas-powered turbine, electric motor, or gas powered engine.
FIGURE I-15. COMPRESSOR STATION Source of NO X .
It is not uncommon for a certain amount of water and hydrocarbons to
condense out of the gas stream while in transit. Thus, in addition to
compressing natural gas, compressor stations often contain a liquid
separator that has scrubbers and filters to remove liquids or other
undesirable particles from the natural gas in the pipeline.
In some regions, such as the Appalachian states, natural gas might
not require sweetening or extensive dehydration. Therefore, the gas
may be piped directly from the wellhead to a main transmission line
and, in some cases, directly to the customer. Compressor stations are
located as needed along the pipelines that run between the wellhead
and the main transmission line or the customer to maintain pressure
in the lines. 113
FIGURE I-16. SEPARATOR AND STORAGE TANK FOR
THE SEPARATED LIQUIDS
Compressors vary in size. Some compressors serve an individual well
(wellhead compressors); others may serve a number of wells.
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