Page 44 - Synthetic Fuels Handbook

32 CHAPTER TWO

Rich gas has a high heating value and a high hydrocarbon dew point. However, the terms
rich gas and lean gas, as used in the gas processing industry, are not precise indicators of gas
quality but only indicate the relative amount of natural gas liquids in the gas stream. When
referring to natural gas liquids in the natural gas stream, the term gallons per thousand
cubic feet of gas is used as a measure of hydrocarbon richness.
Thus, in the case of associated gas, crude oil may be assisted up the wellbore by gas
lift (Speight, 1993, 2007b). Thus, gas is compressed into the annulus of the well and then
injected by means of a gas lift valve near the bottom of the well into the crude oil column in
the tubing. At the top of the well the crude oil and gas mixture passes into a separation plant
that drops the pressure down to nearly atmospheric in two stages. The crude oil and water
exits the bottom of the lower pressure separator, from where it is pumped to tanks for sepa-
ration of the crude oil and water. The gas produced in the separators is recompressed and
the gas that comes out of solution with the produced crude oil (surplus gas) is then treated
to separate out the natural gas liquids that are treated in a gas plant to provide propane and
butane or a mixture of the two (liquefied petroleum gas, LPG). The higher boiling residue,
after the propane and butane are removed, is condensate, which is mixed with the crude oil
or exported as a separate product.
The gas itself is then dry and, after compression, is suitable to be injected into the natural gas
system where it substitutes for natural gas from the nonassociated gas reservoir. Pretreated
associated gas from other fields enters the system at this stage (Manning and Thompson,
1991). Another use for the gas is as fuel for the gas turbines on site. This gas is treated in
a fuel gas plant to ensure it is clean and at the correct pressure. The startup fuel gas supply
will be from the main gas system, but facilities exist to collect and treat low-pressure gas
from the various other plants as a more economical fuel source.

2.3.2 Nonassociated Gas
Nonassociated gas (sometimes called gas well gas) is produced from geologic formations
that typically do not contain much, if any, crude oil, or higher boiling hydrocarbons (gas
liquids) than methane. However, nonassociated gas can contain non–hydrocarbon gases
such as carbon dioxide and hydrogen sulfide.
The nonassociated gas recovery system is somewhat simpler than the associated gas
recovery system. The gas flows up the well under its own energy, through the wellhead
control valves and along the flow line to the treatment plant. Treatment requires the tem-
perature of the gas to be reduced to a point dependent upon the pressure in the pipeline
so that all liquids that would exist at pipeline temperature and pressure condense and are
removed.
The water in the gas must also be dealt with to stop the formation of gas hydrates
(Sec. 2.4.2) that may block the pipes. One method is to inject ethylene glycol (glycol) that
combines with the water and is later recovered by a glycol plant. The treated gas then flows
from the top of the treatment vessel and into the pipeline. The water is treated in a glycol
plant to recover the glycol. Any natural gas liquids are pumped as additional feedstock to
the liquefied petroleum gas plant.

2.3.3 Liquefied Natural Gas
When natural gas is cooled to a temperature of approximately −160°C (−260°F) at atmo-
spheric pressure, it condenses to a liquid (liquefied natural gas, LNG). One volume of
this liquid takes up about one six-hundredth the volume of natural gas. Liquefied natural
gas weighs less than one-half that of water, actually about 45 percent as much. Liquefied