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Exploration, Recovery, and Transportation
containing proppants (sand, walnut shells, aluminum pellets) is pumped down the well and out of
the perforations. The pressure from this fluid makes small fractures in the sandstone that allow
oil to flow into the well, while the proppants hold these fractures open. Once the oil is flowing, the
oil rig is removed from the site and production equipment is set up to extract the oil from the well.
A well is always carefully controlled in its flush stage of production to prevent the potentially
dangerous and wasteful gusher. This is actually dangerous condition, and is (hopefully) prevented
by the blowout preventer and the pressure of the drilling mud. In most wells, acidizing or fracturing
the well starts the oil flow.
As already noted (Chapter 3), crude oil accumulates over geological time in porous underground rock formations called reservoirs that are at varying depths in the Earth’s crust, and in
many cases elaborate, expensive equipment is required to get it from there. The oil is usually found
trapped in a layer of porous sandstone, which lies just beneath a dome-shaped or folded layer of
some nonporous rock such as limestone. In other formations the oil is trapped at a fault, or break
in the layers of the crust.
Generally, crude oil reservoirs sometimes exist with an overlying gas cap, in communication
with aquifers, or both. The oil resides together with water and free gas in very small holes (pore
spaces) and fractures. The size, shape, and degree of interconnection of the pores vary considerably
from place to place in an individual reservoir. Below the oil layer the sandstone is usually saturated
with salt water. The oil is released from this formation by drilling a well and puncturing the limestone layer on either side of the limestone dome or fold. If the peak of the formation is tapped, only
the gas is obtained. If the penetration is made too far from the center, only salt water is obtained.
Oil wells may be either on land or under water. In North America many wells are offshore in the
shallow parts of the oceans. The crude oil or unrefined oil is typically collected from individual
wells by small pipelines.
The oil in such formation is usually under such great pressure that it flows naturally, and sometimes with great force, from the well. However, in some cases this pressure later diminishes so that
the oil must be pumped from the well. Natural gas or water is sometimes pumped into the well to
replace the oil that is withdrawn. This is called repressurizing the oil well.
Thus the anatomy of a reservoir is complex and is site specific, microscopically and macroscopically. Because of the various types of accumulations and the existence of wide ranges of both rock
and fluid properties, reservoirs respond differently and must be treated individually.
Conventional crude oil is a brownish green to black liquids of specific gravity in a range from
about 0.810 to 0.985 and having a boiling range from about 20°C (68°F) to above 350°C (660°F),
above which active decomposition ensues when distillation is attempted. The oils contain from
0% to 35% or more of gasoline, as well as varying proportions of kerosene, hydrocarbons, and
higher boiling constituents up to the viscous and nonvolatile compounds present in lubricant oil and
asphalt. The composition of the crude oil obtained from the well is variable and depends not only on
the original composition of the oil in situ but also on the manner of production and the stage reached
in the life of the well or reservoir.
For a newly opened formation and under ideal conditions the proportions of gas may be so high
that the oil is, in fact, a solution of liquid in gas that leaves the reservoir rock so efficiently that a core
sample will not show any obvious oil content. A general rough indication of this situation is a high
ratio of gas to oil produced. This ratio may be zero for fields in which the rock pressure has been
dissipated. The oil must be pumped out to as much as 50,000 ft 3 or more of gas per barrel of oil in
the so-called condensate reservoirs, in which a very light crude oil (0.80 specific gravity or lighter)
exists as vapor at high pressure and elevated temperature.
New methods to drill for oil are continually being sought, including directional or horizontal
drilling techniques, to reach oil under ecologically sensitive areas, and using lasers to drill oil wells.
Directional drilling is also used to reach formations and targets not directly below the penetration
point or drilling from shore to locations under water (Figure 6.5). A controlled deviation may also
be used from a selected depth in an existing hole to attain economy in drilling costs. Various types
Exploration, Recovery, and Transportation
containing proppants (sand, walnut shells, aluminum pellets) is pumped down the well and out of
the perforations. The pressure from this fluid makes small fractures in the sandstone that allow
oil to flow into the well, while the proppants hold these fractures open. Once the oil is flowing, the
oil rig is removed from the site and production equipment is set up to extract the oil from the well.
A well is always carefully controlled in its flush stage of production to prevent the potentially
dangerous and wasteful gusher. This is actually dangerous condition, and is (hopefully) prevented
by the blowout preventer and the pressure of the drilling mud. In most wells, acidizing or fracturing
the well starts the oil flow.
As already noted (Chapter 3), crude oil accumulates over geological time in porous underground rock formations called reservoirs that are at varying depths in the Earth’s crust, and in
many cases elaborate, expensive equipment is required to get it from there. The oil is usually found
trapped in a layer of porous sandstone, which lies just beneath a dome-shaped or folded layer of
some nonporous rock such as limestone. In other formations the oil is trapped at a fault, or break
in the layers of the crust.
Generally, crude oil reservoirs sometimes exist with an overlying gas cap, in communication
with aquifers, or both. The oil resides together with water and free gas in very small holes (pore
spaces) and fractures. The size, shape, and degree of interconnection of the pores vary considerably
from place to place in an individual reservoir. Below the oil layer the sandstone is usually saturated
with salt water. The oil is released from this formation by drilling a well and puncturing the limestone layer on either side of the limestone dome or fold. If the peak of the formation is tapped, only
the gas is obtained. If the penetration is made too far from the center, only salt water is obtained.
Oil wells may be either on land or under water. In North America many wells are offshore in the
shallow parts of the oceans. The crude oil or unrefined oil is typically collected from individual
wells by small pipelines.
The oil in such formation is usually under such great pressure that it flows naturally, and sometimes with great force, from the well. However, in some cases this pressure later diminishes so that
the oil must be pumped from the well. Natural gas or water is sometimes pumped into the well to
replace the oil that is withdrawn. This is called repressurizing the oil well.
Thus the anatomy of a reservoir is complex and is site specific, microscopically and macroscopically. Because of the various types of accumulations and the existence of wide ranges of both rock
and fluid properties, reservoirs respond differently and must be treated individually.
Conventional crude oil is a brownish green to black liquids of specific gravity in a range from
about 0.810 to 0.985 and having a boiling range from about 20°C (68°F) to above 350°C (660°F),
above which active decomposition ensues when distillation is attempted. The oils contain from
0% to 35% or more of gasoline, as well as varying proportions of kerosene, hydrocarbons, and
higher boiling constituents up to the viscous and nonvolatile compounds present in lubricant oil and
asphalt. The composition of the crude oil obtained from the well is variable and depends not only on
the original composition of the oil in situ but also on the manner of production and the stage reached
in the life of the well or reservoir.
For a newly opened formation and under ideal conditions the proportions of gas may be so high
that the oil is, in fact, a solution of liquid in gas that leaves the reservoir rock so efficiently that a core
sample will not show any obvious oil content. A general rough indication of this situation is a high
ratio of gas to oil produced. This ratio may be zero for fields in which the rock pressure has been
dissipated. The oil must be pumped out to as much as 50,000 ft 3 or more of gas per barrel of oil in
the so-called condensate reservoirs, in which a very light crude oil (0.80 specific gravity or lighter)
exists as vapor at high pressure and elevated temperature.
New methods to drill for oil are continually being sought, including directional or horizontal
drilling techniques, to reach oil under ecologically sensitive areas, and using lasers to drill oil wells.
Directional drilling is also used to reach formations and targets not directly below the penetration
point or drilling from shore to locations under water (Figure 6.5). A controlled deviation may also
be used from a selected depth in an existing hole to attain economy in drilling costs. Various types