2 Natural gas
The natural gas used by consumers is composed almost entirely of
methane. However, natural gas found at the wellhead, though still composed
primarily of methane, is not pure. Raw natural gas comes from three types of
wells: oil wells, gas wells, and condensate wells.
Natural gas that comes from oil wells is typically termed “associated gas.”
This gas can exist separately from oil in the formation (free gas), or
dissolved in the crude oil (dissolved gas). Natural gas from gas and
condensate wells in which there is little or no crude oil, is termed “nonassociated gas.”
Gas wells typically produce only raw natural gas. However condensate wells
produce free natural gas along with a semi-liquid hydrocarbon condensate.
Whatever the source of the natural gas, once separated from crude oil (if
present), it commonly exists in mixtures with other hydrocarbons, principally
ethane, propane, butane, and pentanes. In addition, raw natural gas
contains water vapor, hydrogen sulfide (H2S), carbon dioxide, helium,
nitrogen, and other compounds.
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3.1.3 Condensates
While the ethane, propane, butane, and pentanes must be removed from
natural gas, this does not mean that they are all waste products. In fact,
associated hydrocarbons, known as natural gas liquids (NGL), can be very
valuable byproducts of natural gas processing. NGLs include ethane,
propane, butane, iso-butane, and natural gasoline. These are sold
separately and have a variety of different uses such as raw materials for oil
refineries or petrochemical plants, as sources of energy, and for enhancing
oil recovery in oil wells. Condensates are also useful as diluents for heavy
crude.
3.2 The reservoir
The oil and gas bearing structure is typically porous rock, such as sandstone
or washed out limestone. The sand may have been laid down as desert sand
dunes or seafloor. Oil and gas deposits form as organic material (tiny plants
and animals) deposited in earlier geological periods, typically 100 to 200
million years ago, under, over or with the sand or silt, are transformed by
high temperature and pressure into hydrocarbons.
Anticline Fault Salt dome
Porous rock
Impermeable rock
Gas
Oil
Fossil water in porous reservoir rock
Figure 5. Reservoir formations
For an oil reservoir to form, porous rock needs to be covered by a nonporous layer such as salt, shale, chalk or mud rock that prevent the
hydrocarbons from leaking out of the structure. As rock structures become
folded and raised as a result of tectonic movements, the hydrocarbons
migrate out of the deposits and upward in porous rock and collect in crests
under the non-permeable rock, with gas at the top and oil and fossil water at
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the bottom. Salt is a thick fluid, and if deposited under the reservoir, it will
flow up in heavier rock over millions of years. This process creates salt
domes with a similar reservoir-forming effect. These are common e.g. in the
Middle East.
This extraordinary process is ongoing. However, an oil reservoir matures in
the sense that an immature formation may not yet have allowed the
hydrocarbons to form and collect. A young reservoir generally has heavy
crude, less than 20 API, and is often Cretaceous in origin (65-145 million
years ago). Most light crude reservoirs tend to be Jurassic or Triassic (145-
205/205-250 million years ago), and gas reservoirs where the organic
molecules are further broken down are often Permian or Carboniferous in
origin (250-290/290-350 million years ago).
In some areas, strong uplift, erosion and cracking of the rock above have
allowed hydrocarbons to leak out, leaving heavy oil reservoirs or tar pools.
Some of the world's largest oil deposits are tar sands, where the volatile
compounds have evaporated from shallow sandy formations, leaving huge
volumes of bitumen-soaked sands. These are often exposed at the surface
and can be strip-mined, but must be separated
from the sand with hot water, steam and
diluents, and further processed with cracking
and reforming in a refinery to improve fuel
yield.
The oil and gas is pressurized in the pores of
the absorbent formation rock. When a well is
drilled into the reservoir structure, the
hydrostatic formation pressure drives the
hydrocarbons out of the rock and up into the
well. When the well flows, gas, oil and water
are extracted, and the levels shift as the
reservoir is depleted. The challenge is to plan
drilling so that reservoir utilization can be
maximized.
Seismic data and advanced 3D visualization
models are used to plan extraction. Even so,
the average recovery rate is only 40%, leaving
60% of the hydrocarbons trapped in the
reservoir. The best reservoirs with advanced enhanced oil recovery (EOR)
allow up to 70% recovery. Reservoirs can be quite complex, with many folds
and several layers of hydrocarbon-bearing rock above each other (in some
areas more than ten). Modern wells are drilled with large horizontal offsets to
101 kPa
10 °C
20 MPa
100 °C
40 MPa
200 °C
Reservoir hydrostatic pressure
pushes oil and gas upwards.
Gas expands
and pushes oil
downwards
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reach different parts of the structure and with multiple completions so that
one well can produce from several locations.
