98 PETROLEUM TECHNOLOGY, ECONOMICS, AND POLITICS
constituents up to the viscous and nonvolatile compounds present in lubricant oil and in 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 (specific gravity <0.80) exists
as vapor at high pressure and elevated temperature.
Thus, fluids produced from a well are seldom pure crude oil: in
fact, a variety of materials may be produced by oil wells in addition to liquid and gaseous hydrocarbons. The natural gas itself may
contain as impurities one or more non-hydrocarbon substances.
The most abundant of these impurities is hydrogen sulfide, which
imparts a noticeable odor to the gas. A small amount of this compound is considered advantageous as it gives an indication of leaks
and where they occur. However, a larger amount makes the gas
obnoxious and difficult to market. Such gas is referred to as sour
gas and much of it is used in the manufacture of carbon black.
A few natural gases contain helium, and this element does in fact
occur in commercial quantities in certain gas fields; nitrogen and
carbon dioxide are also found in some natural gases. Gas is usually
separated at as high a pressure as possible, reducing compression
costs when the gas is to be used for gaslift or delivered to a pipeline.
Lighter hydrocarbons and hydrogen sulfide are removed as necessary to obtain a crude oil of suitable vapor pressure for transport
yet retaining most of the natural gasoline constituents.
By far the most abundant extraneous material is water. Many
wells, especially during their declining years, produce vast quantities of salt water, and disposing of it is both a serious and an expensive problem. Furthermore, the brine may be corrosive, which
necessitates frequent replacement of casing, pipe, and valves, or it
may be saturated so that the salts tend to precipitate upon reaching the surface. In either case, the water produced with the oil is
a source of continuing trouble. Finally, if the reservoir rock is an
constituents up to the viscous and nonvolatile compounds present in lubricant oil and in 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 (specific gravity <0.80) exists
as vapor at high pressure and elevated temperature.
Thus, fluids produced from a well are seldom pure crude oil: in
fact, a variety of materials may be produced by oil wells in addition to liquid and gaseous hydrocarbons. The natural gas itself may
contain as impurities one or more non-hydrocarbon substances.
The most abundant of these impurities is hydrogen sulfide, which
imparts a noticeable odor to the gas. A small amount of this compound is considered advantageous as it gives an indication of leaks
and where they occur. However, a larger amount makes the gas
obnoxious and difficult to market. Such gas is referred to as sour
gas and much of it is used in the manufacture of carbon black.
A few natural gases contain helium, and this element does in fact
occur in commercial quantities in certain gas fields; nitrogen and
carbon dioxide are also found in some natural gases. Gas is usually
separated at as high a pressure as possible, reducing compression
costs when the gas is to be used for gaslift or delivered to a pipeline.
Lighter hydrocarbons and hydrogen sulfide are removed as necessary to obtain a crude oil of suitable vapor pressure for transport
yet retaining most of the natural gasoline constituents.
By far the most abundant extraneous material is water. Many
wells, especially during their declining years, produce vast quantities of salt water, and disposing of it is both a serious and an expensive problem. Furthermore, the brine may be corrosive, which
necessitates frequent replacement of casing, pipe, and valves, or it
may be saturated so that the salts tend to precipitate upon reaching the surface. In either case, the water produced with the oil is
a source of continuing trouble. Finally, if the reservoir rock is an