Geology _ Geophysics in Oil Exploration

It is based on the premise that the best resistivity

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Unformatted text preview: s in the determination of the porosity of a formation. The tool operates by bombarding the formation with high energy neutrons. These neutrons undergo scattering in the formation, losing energy and producing high energy gamma rays. The scattering reactions occur most efficiently with hydrogen atoms. The resulting low energy neutrons or gamma rays can be detected, and their count rate is related to the amount of hydrogen atoms in the formation. The source which emit neutrons is (Am-Be) source. Calibration: These tools are calibrated in blocks of limestone, sandstone and dolomite of high purity and accurately known porosity. The tools are calibrated, not to give readings in API neutron porosity units, but to give the porosity directly in percent. The calibration of the CNL tool is checked at the well site before and after each logging run by the use of a neutron source of accurately known activity placed a standard distance from each detector. Tool Operation: SNP: This tool is designed for use in open holes only. The tool has a source and a single detector with 16 inch spacing, which are mounted on a skid that is pressed against the borehole wall. Because the tool is pressed against the borehole wall, the drilling mud does not affect the measurement, and the attenuation due to the mud cake is reduced. The detector is sensitive to epithermal neutrons so the SNP tool readings are unaffected by the presence of chlorine in high salinity muds and formation fluids. CNL: This tool is designed to be sensitive to thermal neutrons, and is therefore affected by the chlorine effect. It has two detectors situated 15 inch and 25 inch from the source. The CNL tool has a very strong source of neutrons to ensure that the measured count rates are sufficiently high to obviate any significant errors associated with statistical fluctuations. The Hydrocarbon Effect: The presence of hydrocarbon liquid (oil) does not affect the tool response as it has approximately the same hydrogen index as fresh water. Hydrocarbon gas, however, has a much lower hydrocarbon index resulting from its low density, and its presence will give rise to underes...
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