There are three main types of conventional wells. The most common well is an oil
well with associated gas. Natural gas wells are wells drilled specifically for natural
gas, and contain little or no oil. Condensate wells are wells that contain natural gas,
as well as a liquid condensate. This condensate is a liquid hydrocarbon mixture that
is often separated from the natural gas either at the wellhead, or during the
processing of the natural gas. Depending on the type of well that is being drilled,
completion may differ slightly. It is important to remember that natural gas, being
lighter than air, will naturally rise to the surface of a well. Because of this, in many
natural gas and condensate wells, lifting equipment and well treatment are not
necessary, while for oil wells many types of artificial lift might be installed,
particularly as the reservoir pressure declines during years of production.

Reservoir and Wellheads

There are three main types of conventional wells. The most common well is an oil
well with associated gas. Natural gas wells are wells drilled specifically for natural
gas, and contain little or no oil. Condensate wells are wells that contain natural gas,
as well as a liquid condensate. This condensate is a liquid hydrocarbon mixture that
is often separated from the natural gas either at the wellhead, or during the
processing of the natural gas. Depending on the type of well that is being drilled,
completion may differ slightly. It is important to remember that natural gas, being
lighter than air, will naturally rise to the surface of a well. Because of this, in many
natural gas and condensate wells, lifting equipment and well treatment are not
necessary, while for oil wells many types of artificial lift might be installed,
particularly as the reservoir pressure declines during years of production.

Crude oil and Natural gas

Crude Oil

Crude Oil is a complex mixture consisting of up to 200 or more different organic
compounds, mostly hydrocarbons. Different crude contain different combinations
and concentrations of these various compounds. The API (American petroleum
institute) gravity of a particular crude is merely a measure of its specific gravity, or
density. The higher the API number, expressed as degrees API, the less dense
(lighter, thinner) the crude. Conversely, the lower the degrees API, the more dense
(heavier, thicker) the crude. Crude from different fields and from different
formations within a field can be similar in composition or be significantly different.
In addition to API grade and hydrocarbons, crude is characterized for other nonwanted
elements like sulfur which is regulated and needs to be removed.
Crude oil API gravities typically range from 7 to 52 corresponding to about 970
kg/m3 to 750 kg/m3, but most fall in the 20 to 45 API gravity range. Although light
crude (i.e., 40-45 degree API) is good, lighter crude (i.e., 46 degree API and above)
is not necessarily better for a typical refinery. Looking at the chemical composition
of crude, as the crude gets lighter than 40-45 degrees API, it contains shorter
molecules, or less of the desired compounds useful as high octane gasoline and
diesel fuel, the production of which most refiners try to maximize. Likewise, as
crude gets heavier than 35 degrees API, it contains longer and bigger molecules that
are not useful as high octane gasoline and diesel fuel without further processing.

For crude that have undergone detailed physical and chemical property analysis, the
API gravity can be used as a rough index of the quality of the crude of similar
composition as they naturally occur (that is, without adulteration, mixing, blending,
etc.). When crude of different type and quality are mixed, or when different
petroleum components are mixed, API gravity cannot be used meaningfully for
anything other than a measure of the density of the fluid.
For example, consider a barrel of tar that is dissolved in 3 barrels of naphtha (lighter
fluid) to produce 4 barrels of a 40 degree API mixture. When this 4-barrel mixture is
fed to a distillation column at the inlet to a refinery, one barrel of tar plus 3 barrels of
lighter fluid is all that will come out of the still. On the other hand, 4 barrels of a
naturally occurring 40 degree API South Louisiana Sweet crude when fed to the
distillation column at the refinery could come out of the still as 1.4 barrels of
gasoline and naphtha, 0.6 barrels of kerosene (jet fuel), 0.7 barrels of diesel fuel, 0.5
barrels of heavy distillate, 0.3 barrels of lubricating stock, and 0.5 barrels of
residuum (tar).

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