Page 42 - Synthetic Fuels Handbook

30 CHAPTER TWO

below the surface. Other reservoirs are thousands, even tens of thousands of feet under-
ground. In the United States, several reservoirs have been discovered at depths greater than
30,000 ft. Many offshore wells are drilled in thousands of feet of water and penetrate tens
of thousands of feet into the sediments below the sea floor.
Natural gas reservoirs, like crude oil reservoirs, exist in many forms such as the dome
(syncline-anticline) structure (Fig. 2.2), with water below, or a dome of gas with a crude
oil rim and water below the oil. When the water is in direct contact with the gas, pressure
effects may dictate that a considerable portion of the gas (20 percent or more) is dissolved
in the crude oil as well as in the water. As gas is produced (or recovered) from the reservoir,
the reservoir pressure declines allowing the dissolved gas to enter the gas phase. In addi-
tion, and because of the variability of reservoir structure, gas does not always flow equally
to wells placed throughout the length, breadth, and depth of the reservoir and at equal pres-
sure. Recovery wells must be distributed throughout the reservoir to recover as much of
the gas as efficiently as possible.

Ground

Natural gas

Cap rock
Sedimentary
porous rock
FIGURE 2.2 An anticlinal reservoir containing unassociated natural gas.
As the gas pressure in the reservoir declines, the reservoir energy (i.e., reservoir pressure)
declines, and the gas requires stimulation for continued production. Furthermore, reduction
in the gas pressure may allow compaction of the reservoir rock by the weight of rock above
eventually resulting in subsidence of the surface above the reservoir. This can be gradual
process or a sudden catastrophic process depending on the structures of the geologic formation
above the reservoir.
A reservoir containing wet gas with a large amount of valuable natural gas liquids (any
hydrocarbons other than methane such as ethane, propane, and butane) and even light crude
oil and condensate has to be treated carefully. When the reservoir pressure drops below the
critical point for the mixture, the liquids may condense out and remain in the reservoir.
Thus it is necessary to implement a cycling process in which the wet gas is produced to
the surface and the natural gas liquids are condensed as a separate stream and the gas is
compressed and injected back into the reservoir to maintain the pressure.
Once brought from underground (recovered from the reservoir), natural gas is refined to
remove impurities like water, other gases, sand, and other compounds. Some hydrocarbons