120
The Chemistry and Technology of Petroleum
a measure of porosity and permeability since the data are affected markedly by the ability of the
drilling fluid to penetrate the formation. The resistivity measurements define the position of formation boundaries and the lithological character of the sediments.
Three resistivity logs are usually taken: (1) one having a shallow penetration to define the location of the formation boundaries and two others having (2) intermediate and (3) deep penetration.
These last two logs are used to determine the extent to which the drilling fluid has penetrated into
the formations and the true resistivity of the formation present. The various measurements taken
in conjunction provide a valuable tool not only for studying conditions in a given well but also for
carrying out correlation studies between wells and thus defining geological structure and horizontal
changes in lithology.
6.2.5 eleCtromAgnetIC metHods
Electromagnetic methods are based upon the concept that an alternating magnetic field causes an electrical current to flow in conducting material. Measurements are carried out by connecting a source of
alternating current to a coil of wire, which acts as a source for a magnetic field similar to that which
will be produced by a short magnet located on the axis of the coil. A receiving system consisting of a
second coil connected to a voltmeter is mounted so that there is free rotation about a horizontal axis.
The receiving coil should be mounted so that rotation is on an axis perpendicular to that of the
induced magnetic field. In this case, the induced voltage (in the absence of a conductor) will vary
from zero (when the coil plane is parallel to the plane of the applied field) to a maximum (when the
coil plane is perpendicular to the plane of the applied field). However, if a conductor is present, the
induced current in the conductor sets up a secondary magnetic field that distorts the primary field
and gives a value that is not horizontal except directly over the conductor. By using an inclinometer
to record the angle of the moving search coil when in the null position, the location of a conductor
can be determined as the crossover (inflection) point on a profile across the body.
Another variation of this method is to have both the receiver and transmitting coils in the horizontal plane. In this arrangement the voltage developed over nonconducting ground is a function of
the construction of the coils, which are usually moved across the ground with a constant separation.
The presence of a conductor is indicated by changes in the voltage values from the normal values
for this configuration.
6.2.6 rAdIoACtIve metHods
In the disintegration of radioactive minerals three spontaneous emissions take place, the election
of an electron (β-ray), a helium nucleus (α-ray), and short-wavelength electromagnetic radiation
(γ-rays). The instruments used in radioactive exploration are the Geiger counter and the scintillometer. In addition to prospecting for radioactive minerals the radioactive method is extensively
applied in borehole studies of subsurface stratigraphy as might be deemed necessary when prospecting for oil. Different sedimentary rocks are naturally characterized by different concentrations
of radioactive materials. Shale and volcanic ash give the highest γ-ray count and the limestone the
lowest γ-ray count.
6.2.7 BoreHole loggIng
Another valuable exploration method is geophysical borehole logging, which involves drilling a
well and use of instruments to log or make measurements at various levels in the hole by such means
as electrical resistivity, radioactivity, acoustics, or density. In addition, formation samples (cores) are
taken for physical and chemical tests.
The use of electrical logging is based on the fact that the resistivity of a rock layer is a function of
its fluid content. Oil-filled sand has a very high resistivity. The method consists of passing a current
The Chemistry and Technology of Petroleum
a measure of porosity and permeability since the data are affected markedly by the ability of the
drilling fluid to penetrate the formation. The resistivity measurements define the position of formation boundaries and the lithological character of the sediments.
Three resistivity logs are usually taken: (1) one having a shallow penetration to define the location of the formation boundaries and two others having (2) intermediate and (3) deep penetration.
These last two logs are used to determine the extent to which the drilling fluid has penetrated into
the formations and the true resistivity of the formation present. The various measurements taken
in conjunction provide a valuable tool not only for studying conditions in a given well but also for
carrying out correlation studies between wells and thus defining geological structure and horizontal
changes in lithology.
6.2.5 eleCtromAgnetIC metHods
Electromagnetic methods are based upon the concept that an alternating magnetic field causes an electrical current to flow in conducting material. Measurements are carried out by connecting a source of
alternating current to a coil of wire, which acts as a source for a magnetic field similar to that which
will be produced by a short magnet located on the axis of the coil. A receiving system consisting of a
second coil connected to a voltmeter is mounted so that there is free rotation about a horizontal axis.
The receiving coil should be mounted so that rotation is on an axis perpendicular to that of the
induced magnetic field. In this case, the induced voltage (in the absence of a conductor) will vary
from zero (when the coil plane is parallel to the plane of the applied field) to a maximum (when the
coil plane is perpendicular to the plane of the applied field). However, if a conductor is present, the
induced current in the conductor sets up a secondary magnetic field that distorts the primary field
and gives a value that is not horizontal except directly over the conductor. By using an inclinometer
to record the angle of the moving search coil when in the null position, the location of a conductor
can be determined as the crossover (inflection) point on a profile across the body.
Another variation of this method is to have both the receiver and transmitting coils in the horizontal plane. In this arrangement the voltage developed over nonconducting ground is a function of
the construction of the coils, which are usually moved across the ground with a constant separation.
The presence of a conductor is indicated by changes in the voltage values from the normal values
for this configuration.
6.2.6 rAdIoACtIve metHods
In the disintegration of radioactive minerals three spontaneous emissions take place, the election
of an electron (β-ray), a helium nucleus (α-ray), and short-wavelength electromagnetic radiation
(γ-rays). The instruments used in radioactive exploration are the Geiger counter and the scintillometer. In addition to prospecting for radioactive minerals the radioactive method is extensively
applied in borehole studies of subsurface stratigraphy as might be deemed necessary when prospecting for oil. Different sedimentary rocks are naturally characterized by different concentrations
of radioactive materials. Shale and volcanic ash give the highest γ-ray count and the limestone the
lowest γ-ray count.
6.2.7 BoreHole loggIng
Another valuable exploration method is geophysical borehole logging, which involves drilling a
well and use of instruments to log or make measurements at various levels in the hole by such means
as electrical resistivity, radioactivity, acoustics, or density. In addition, formation samples (cores) are
taken for physical and chemical tests.
The use of electrical logging is based on the fact that the resistivity of a rock layer is a function of
its fluid content. Oil-filled sand has a very high resistivity. The method consists of passing a current