- Industry: Oil & gas
- Number of terms: 8814
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A measurement of the spectrum of gamma rays emitted by a formation bombarded by high-energy neutrons. Neutrons are emitted by a high-energy neutron generator (14. 1 MeV). The neutrons interact with different nuclei, which may emit characteristic gamma rays through inelastic neutron scattering, fast-neutron reactions and neutron capture. When pulses from a neutron generator are used, it is possible to separate the different interactions in time after each neutron pulse. Inelastic and fast-neutron interactions occur very soon after the neutron burst, while most of the capture events occur later. The two types can therefore be separated to give a so-called inelastic spectrum and a capture spectrum. The spectra are analyzed either by counting gamma rays in windows placed at the main peaks for the elements concerned, or by comparison with spectral standards, or by combining the two (alpha processing). <br><br>The resultant logs are known as pulsed neutron spectroscopy logs, the most common of which are the carbon-oxygen log and the elemental capture spectroscopy log.
Industry:Oil & gas
A measurement of the slowing down and capture of neutrons between a source and one or more thermal neutron detectors. The neutron source emits high-energy neutrons that are slowed mainly by elastic scattering to near thermal levels. Thermal neutrons have about the same energy as the surrounding matter, typically less than 0. 4 eV. The slowing-down process is dominated by hydrogen. At thermal levels, the neutrons diffuse through the material until they undergo thermal capture. Capture is dominated by hydrogen and other thermal neutron absorbers. <br><br>Typical thermal neutron measurements use a chemical neutron source and two thermal neutron detectors. An accelerator source (neutron generator) is sometimes used. Some, mainly earlier, devices measure the gamma rays emitted by thermal capture, rather than thermal neutrons.
Industry:Oil & gas
A measurement of the formation resistivity made on drillpipe at a frequency in the range of 100 kHz to 10 GHz, most commonly 2 MHz. The basic measurement is accomplished using a transmitter and two receivers. At these frequencies, the response is best explained as the propagation of a wave. Thus, the phase shift and attenuation of the wave between the receivers are measured and transformed to give the phase shift and the attenuation resistivity. In practice, multiple transmitters may be used to obtain different depths of investigation and achieve borehole compensation. The wavelength is such that the borehole has a minor effect, but one for which correction may be needed.
Industry:Oil & gas
A measurement of the high frequency (about 1 GHz) dielectric properties of the formation. In a typical tool, a microwave transmitter is placed a few inches below two receivers separated by 4 cm (1. 6 in. ). At this frequency, the response is best explained as the propagation of a wave. Thus the phase shift and attenuation of the wave between the receivers are measured and transformed to give the log measurements of propagation time and attenuation. Because of the short spacings, the measurement has excellent vertical resolution and reads within inches of the borehole wall except at high resistivity. Different transmitter and receiver spacings and orientations are used, leading to different arrays, such as the endfire array and the broadside array. <br><br>An ideal measurement would give the plane wave properties of the formation. However, the geometry of the measurement precludes this, so that a correction, known as the spreading-loss correction, is needed for the attenuation and to a much smaller extent for the propagation time. The measurement is also affected by the dielectric properties and thickness of the mudcake. Borehole compensation is used to correct for sonde tilt or a rough borehole wall.
Industry:Oil & gas
A measurement of the bulk density of the formation, based on the reduction in gamma ray flux between a source and a detector due to Compton scattering. The gamma ray source, usually <sup>137</sup>Cs (cesium), is chosen so that gamma ray energies are high enough to interact by Compton scattering but not by pair production. The detectors discriminate against low gamma ray energies that may have been influenced by photoelectric absorption. Although Compton scattering depends on electron density and not bulk density, density logs are calibrated to give the correct bulk density in the majority of sedimentary rocks. Due to the Z/A effect there are small differences in some formations. <br><br>The measurement responds to the average density of the material between source and detector. In the wireline measurement, care is taken to minimize the mud between the sensors and the formation by pressing a pad against the borehole wall, with source and detector focused into the formation. In the logging-while-drilling measurement, a sleeve may be mounted on the collar around the sensors to exclude the mud. The detectors measure the gamma rays scattered from the formation. Even then, mudcake or borehole rugosity can affect the measurement. It is common practice to compensate for the mudcake by using two or more detectors at different spacings.
Industry:Oil & gas
A measurement made by a type of electrode device in which the current flow and hence the measurement is focused in a disk that is concentric with and perpendicular to the sonde. The term usually refers to a Laterolog-3 device. Guard logs may be recorded by either wireline or measurements-while-drilling tools.
Industry:Oil & gas
A measure of the volume of treated oil stored in stock tanks. A stock tank barrel is commonly abbreviated as STB.
Industry:Oil & gas
A measure, or indicator, of the length that a useable perforation tunnel extends beyond the casing or liner into the reservoir formation. In most cases, a high penetration is desirable to enable access to that part of the formation that has not been damaged by the drilling or completion processes.
Industry:Oil & gas
A measurement based on the slowing down of neutrons between a source and one or more detectors that measure neutrons at the epithermal level, where their energy is above that of the surrounding matter, between approximately 0. 4 and 10 eV. The slowing-down process is dominated by hydrogen, and is characterized by a slowing-down length. By measuring the neutrons at the epithermal level, rather than the thermal level, the response is a purer estimate of hydrogen index, unaffected by thermal absorbers. On the other hand, the count rate is smaller for the same source and source-detector spacing. <br><br>Epithermal measurements have been made with both the compensated neutron technique and by using a pad pressed against the borehole wall with detectors focused into the formation.
Industry:Oil & gas
A means of assessing reservoir performance by measuring flow rates and pressures under a range of flowing conditions and then applying the data to a mathematical model. In most well tests, a limited amount of fluid is allowed to flow from the formation being tested. The formation is isolated behind cemented casing and perforated at the formation depth or, in openhole, the formation is straddled by a pair of packers that isolate the formation. During the flow period, the pressure at the formation is monitored over time. Then, the formation is closed (or shut in) and the pressure monitored at the formation while the fluid within the formation equilibrates. The analysis of these pressure changes can provide information on the size and shape of the formation as well as its ability to produce fluids.
Industry:Oil & gas
