Downhole Measurements

Measuring Downholes Using a Variety of Methods

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 that represents how strongly a material opposes the flow of electric current. Resistivity is measured using the standard Kelvin four electrical probes which eliminate 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 1 and Electrode 2 is the leakage current that is 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 the 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, 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 the 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 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 the 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 gamma ray emission. This neutron capture cross-section (Σf) is also measured by the tool.

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 measurement is based on the 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 the 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 positive-charge 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 the 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 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 with the fiber and record measurements of pressure, temperature, seismic, mechanical stresses, chemicals, flow, and other properties.