121
Exploration, Recovery, and Transportation
between an electrode at the surface and one that is lowered into the hole, the latter being uncased
and filled with drilling mud. Any change in the resistivity conditions around the moving electrode
affect the flow of current and voltage distribution around it. Voltage fluctuations can be measured
by a pair of separate electrodes used in conjunction with the moving electrode.
The natural radioactive properties of many constituents of rock have made it possible to develop
and use nuclear radiation detectors (radioactive logging) in the borehole or even in holes that have
already been cased. Two commonly used methods are γ-ray and neutron logging. In the first case,
the natural radiation from the rock is used. With the second, a neutron source is employed to excite
the release of radiation from the rock. The neutron source is usually a mixture of beryllium and
radium but it can be a miniature Van der Graaff particle accelerator. The neutron method is a means
for determining the relative porosity or rock formations; the γ-ray log helps define shale.
The acoustic logging method is quite similar to surface seismic work. Instead of explosives, an
electrically operated acoustic pulse generator is used. In one instrument, the generator is separated
from the receiver by an acoustic insulator. The design permits automatic selection and recording of
the travel times of the onsets of pulses that travel through the rock wall of the hole as the instrument
moves down or up. Signals are recorded continuously at the surface, being transmitted through a
cable on which the instrument is suspended. The velocity log provided by the instrument helps to
define beds and evaluate formation porosity.
Density can now be logged with a new technique that uses radioactivity (density logging). The
instrument consists of a radioactive cobalt source of γ-rays and a Geiger counter as a detector, which
is shielded from the source. The rock formation is bombarded with the γ-rays, some of which are
scattered back from the formation and enter the detector. The degree to which the original radiation
is adsorbed is a function of the density of the rock.
Test well sampling is another important method used in the search for oil (core sampling). Well
data obtained from the examination of formation samples taken from various depths in the borehole
are of considerable value in deciding further exploratory work. These samples can be cores, which
have been taken from the hole by a special coring device, or drill cuttings screened from the circulating drilling mud. The major purpose of sample examination is to identify the various strata in the
borehole and compare their positions with the standard stratigraphic sequence of all the sedimentary rocks occurring in the specific basin in which the hole has been drilled.
6.3 DRILLING
Generally the first stage in the extraction of crude oil is to drill a well into the underground reservoir. Often many wells (called multilateral wells) will be drilled into the same reservoir, to ensure
that the extraction rate will be economically viable. Also, some wells (secondary wells) may be
used to pump water, steam, acids, or various gas mixtures into the reservoir to raise or maintain the
reservoir pressure, and so maintain an economic extraction rate.
Drilling for oil is a complex operation and has evolved considerably over the past 100 years. The
older cable tool method, used almost extensively until 1900, involves raising and dropping a heavy
bit and drill stem attached by cable to a cantilever arm at the surface. It pulverizes the rock and
earth, gradually forming the hole. The cable tool system is generally preferred only for penetrating
hard rock at shallow depths and when oil reservoirs are expected at the shallow depths. The weight
of the column is usually enough to attain penetration but can be augmented by a hydraulic pressure
cylinder at the surface.
6.3.1 PrePArIng to drIll
Once the site has been selected, it must be surveyed to determine its boundaries, and environmental impact studies may need to be performed. Lease agreements, titles, and right-of way accesses
for the land must be obtained and evaluated legally. For offshore sites, legal jurisdiction must be
Exploration, Recovery, and Transportation
between an electrode at the surface and one that is lowered into the hole, the latter being uncased
and filled with drilling mud. Any change in the resistivity conditions around the moving electrode
affect the flow of current and voltage distribution around it. Voltage fluctuations can be measured
by a pair of separate electrodes used in conjunction with the moving electrode.
The natural radioactive properties of many constituents of rock have made it possible to develop
and use nuclear radiation detectors (radioactive logging) in the borehole or even in holes that have
already been cased. Two commonly used methods are γ-ray and neutron logging. In the first case,
the natural radiation from the rock is used. With the second, a neutron source is employed to excite
the release of radiation from the rock. The neutron source is usually a mixture of beryllium and
radium but it can be a miniature Van der Graaff particle accelerator. The neutron method is a means
for determining the relative porosity or rock formations; the γ-ray log helps define shale.
The acoustic logging method is quite similar to surface seismic work. Instead of explosives, an
electrically operated acoustic pulse generator is used. In one instrument, the generator is separated
from the receiver by an acoustic insulator. The design permits automatic selection and recording of
the travel times of the onsets of pulses that travel through the rock wall of the hole as the instrument
moves down or up. Signals are recorded continuously at the surface, being transmitted through a
cable on which the instrument is suspended. The velocity log provided by the instrument helps to
define beds and evaluate formation porosity.
Density can now be logged with a new technique that uses radioactivity (density logging). The
instrument consists of a radioactive cobalt source of γ-rays and a Geiger counter as a detector, which
is shielded from the source. The rock formation is bombarded with the γ-rays, some of which are
scattered back from the formation and enter the detector. The degree to which the original radiation
is adsorbed is a function of the density of the rock.
Test well sampling is another important method used in the search for oil (core sampling). Well
data obtained from the examination of formation samples taken from various depths in the borehole
are of considerable value in deciding further exploratory work. These samples can be cores, which
have been taken from the hole by a special coring device, or drill cuttings screened from the circulating drilling mud. The major purpose of sample examination is to identify the various strata in the
borehole and compare their positions with the standard stratigraphic sequence of all the sedimentary rocks occurring in the specific basin in which the hole has been drilled.
6.3 DRILLING
Generally the first stage in the extraction of crude oil is to drill a well into the underground reservoir. Often many wells (called multilateral wells) will be drilled into the same reservoir, to ensure
that the extraction rate will be economically viable. Also, some wells (secondary wells) may be
used to pump water, steam, acids, or various gas mixtures into the reservoir to raise or maintain the
reservoir pressure, and so maintain an economic extraction rate.
Drilling for oil is a complex operation and has evolved considerably over the past 100 years. The
older cable tool method, used almost extensively until 1900, involves raising and dropping a heavy
bit and drill stem attached by cable to a cantilever arm at the surface. It pulverizes the rock and
earth, gradually forming the hole. The cable tool system is generally preferred only for penetrating
hard rock at shallow depths and when oil reservoirs are expected at the shallow depths. The weight
of the column is usually enough to attain penetration but can be augmented by a hydraulic pressure
cylinder at the surface.
6.3.1 PrePArIng to drIll
Once the site has been selected, it must be surveyed to determine its boundaries, and environmental impact studies may need to be performed. Lease agreements, titles, and right-of way accesses
for the land must be obtained and evaluated legally. For offshore sites, legal jurisdiction must be