This method of logging formation characteristics works by characterizing the rock or
sediment in a borehole by measuring its electrical resistivity. Resistivity is a fundamental material property
which represents how strongly a material opposes the flow of electric current. Resistivity is measured using
the standard Kelvin four electrical probes which eliminates the resistance of the contact leads.
Under this configuration, a very small portion of the current leaks from the casing to the adjacent rock, and the casing current can be studied. The difference in the vertical current at Electrode i and at Electrode 2 is the leakage current that left to the formation. From the leakage current and the potential of the casing, the apparent resistivity of the formation can be calculated.
In this technique, oxygen is irradiated with high-energy neutrons from a neutron generator
and, consequently, the oxygen forms an unstable isotope of nitrogen with a 7.13-second half-life. When the nitrogen
decays back to oxygen, a gamma ray is generated. These generated gamma rays are detected, recorded, and analyzed. The
near and far detectors record the generated gamma radiation from the decay of the activated nitrogen in water.
The requirement for a calibration measurement in a zero-flow zone of the well is one of the challenges associated with this technique, as is the steady-state nature of the measurement.
X-ray or high-energy X(γ)-ray generation is required for the ray to penetrate the casing, to investigate the formation, and be backscattered back through the casing to a detector. Detection of backscattered photons depends on the source energy and flux, the distance to the formation, downhole temperature, and the density of intervening material. X-ray interaction with the intervening material could be in the form of coherent scattering, incoherent scattering, fluorescence, or the Auger effect.
Two wireline spectral gamma-ray tools are used to measure and classify natural radioactivity in the formation: the natural gamma-ray tool (NGT) and the hostile environment natural gamma-ray sonde (HNGS). The NGT uses a sodium-iodide scintillation detector and five-window spectroscopy to determine concentrations of K (percent), Th (parts per million), and U (parts per million), the three elements whose isotopes dominate the natural radiation spectrum. The HNGS is similar to the NGT, but it uses two bismuth germanate scintillation detectors for a significantly improved tool precision. The HNGS filters out gamma-ray energies below 500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy. Although the NGT response is sensitive to borehole diameter and the weight and concentration of bentonite or KCl present in the drilling mud, corrections for these effects are routinely made during processing.
Formation density is determined with the hostile environment lithodensity sonde (HLDS). The
sonde contains a radioactive cesium (137Cs) gamma-ray source (622 keV) and far and near gamma-ray detectors mounted on a
shielded skid, which is pressed against the borehole wall by a hydraulically activated eccentralizing arm. Gamma rays
emitted by the source undergo Compton scattering, which involves the transfer of energy from gamma rays to the electrons
in the formation via elastic collision. The number of scattered gamma rays that reach the detectors is directly related
to the number of electrons in the formation, which is in turn related to bulk density. Porosity may also be derived from
this bulk density if the matrix density is known.
The HLDS also measures photoelectric absorption as the photoelectric effect (PEF). Photoelectric absorption of the gamma rays occurs when they reach <150 keV after being repeatedly scattered by electrons in the formation. As PEF depends on the atomic number of the elements in the formation, it also varies according to the chemical composition of the minerals present and is essentially independent of porosity. For example, the PEF of calcite = 5.08 barn/e-; illite = 3.03 barn/e-; quartz = 1.81 barn/e-; and kaolinite = 1.49 barn/e-. Good contact between the tool and borehole wall is essential for good HLDS logs; poor contact results in underestimation of density values.
Formation porosity was measured with the accelerator porosity sonde. The sonde incorporates a
minitron neutron generator that produces fast (14.4 MeV) neutrons and five neutron detectors (four epithermal and one
thermal) positioned at different spacings from the minitron. The measurement principle involves counting neutrons that
arrive at the detectors after being slowed by neutron absorbers surrounding the tool. The highest energy loss occurs when
neutrons collide with hydrogen nuclei, which have practically the same mass as the neutron (the neutrons simply bounce off
heavier elements without losing much energy). If the hydrogen (i.e., water) concentration is small, as in low-porosity
formations, neutrons can travel farther before being captured and the count rates increase at the detector. The opposite
effect occurs when the water content is high. However, because hydrogen bound in minerals such as clays or in hydrocarbons
also contributes to the measurement, the raw porosity value is often an overestimate.
Upon reaching thermal energies (0.025 eV), the neutrons are captured by the nuclei of Cl, Si, B, and other elements, resulting in a gamma-ray emission. This neutron capture cross section (Σf) is also measured by the tool.
Gamma Ray Tools
The Hostile Environment Gamma Ray Sonde (HNGS) measures natural gamma radiation from isotopes of potassium, thorium, and uranium and uses a five-window spectroscopic analysis to determine concentrations of radioactive potassium (in weight percent), thorium (in parts per million), and uranium (in parts per million). The HNGS uses two bismuth germanate scintillation detectors for gamma ray detection with full spectral processing. The HNGS also provides a measure of the total gamma ray emission (SGR) and uranium-free or computed gamma ray emission (CGR) that are measured in American Petroleum Institute units (gAPI). The HNGS response is influenced by the borehole diameter.
Gamma/Gamma Density Measurements
Gamma/gamma density measurement is based on absorption of gamma rays as a function of density. This technique is often referred to as the gamma ray density. In this technique, gamma rays are emitted by a source and the backscattered gamma rays are sensed by a detector. The intensity of the backscattered gamma rays is related to the density of the intervening material.
In this technique, neutrons are generated by use of a particle accelerator. A high voltage
current passes through a material called the "filament," which is a source of deuterium. As the current passes through
it, the filament will heat up and, consequently, deuterium atoms are released. The released deuterium atoms are bombarded
by electrons generated by a high voltage current passing through a cathode. The current heats up the cathode, and
electrons are then released. By creating an electric field, the released electrons are directed toward the deuterium
atoms. Collisions cause the deuterium atoms to lose electrons, resulting in positivecharge atoms. At this time, a
high voltage current passes through a material that is called a "target," which is a source of tritium. When current
passes through it, the target will heat up and release tritium atoms. By creating a strong electric field, collisions
between tritium atoms and the positive-charge deuterium atoms result, producing helium atoms and neutrons.
Neutrons are able to travel hundreds or even thousands of meters through air. They can easily penetrate iron and steel; however, they are stopped by hydrogen-rich materials such as water. When an individual neutron collides with the nuclei of atoms, it loses energy. When the neutron reaches an intermediate energy level, it is called an epithermal neutron. As epithermal neutrons collide with more nuclei, they lose additional energy and come to a general equilibrium with the surrounding nuclei. At this level, they are called thermal neutrons. Thermal neutrons are easily captured by nearby nuclei, causing gamma rays to be released. The number of neutrons captured and gamma rays released is proportional to the amount of hydrogen in the medium.
Pulses of light generated by a laser are sent through an optical fiber and are reflected repeatedly from the fiber walls. The fiber and its coating form a waveguide with total internal reflection such that light is not lost through the fiber walls. A sensor or combination of sensors can be placed along the fiber and record measurements of pressure, temperature, seismic, mechanical stresses, chemicals, flow, and other properties.