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Through Casing Formation Evaluation
INTERFACING & TOOL COMBINATIONS
Compensated Dual-Spaced Neutron (DSN/CNL) Compensated Dual-Spaced Density (DSD) Cased Hole Sonic / Cement Bond Log (CBL)
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The Compensated Neutron tool, DSN (also known as CNL – Compensated Neutron Log), consists of 2 He3 thermal neutron detectors which provide compensation for borehole, casing and cement effects. Neutron particles are emitted from an isotopic source with an average energy of 4 - 6 MeV. These “fast” neutrons collide with atomic nuclei and are slowed down until they reach a “thermal” energy level and are captured by a hydrogen nucleus. In situations where an abundance of hydrogen is present, such as high porosity water/oil-filled formations, the neutrons are thermalized quickly near the source. If a small amount of hydrogen is present, such as in low porosity water/oil-filled reservoirs, the neutrons must travel further before being thermalized and captured. The counts of thermal neutrons captured between the two He3 detectors are inversely proportional to the equivalent water-filled porosity of the reservoir. Porosity measurments are calibrated for the specific matrix being logged.
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Compensated Dual-Spaced Density (DSD)
The Compensated Dual Spaced Density (DSD) uses a 150mCi (5.5GBq) Cs-137 gamma source, housed within an attached sub at the tool base. The DSD performs best when run centralized and requires adequate centralization in deviated and horiozontal applications, although in vertical wells it can be run eccentralized. Deployment is available through multiple options such as slickline, coil tubing and tractor. The DSD is ideally placed at the base of the tool string to avoid formation ‘heating’ from additional sources in stack (DSN/CNL), however it is capable of being stacked at any point to accommodate multiple tool configurations.
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Gamma rays are detected by a high temperature sodium iodide crystal and amplified using a 10 stage photo-multiplier to a measurable level. The detector is temperature compensated to minimize photo-multplier drift. Calibration is maintained by a wrap-around calibrator with a distributed thorium source of very low radioactive content. The difference between background and calibrated levels provides a ratio between tool count rate and API units.
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Cased Hole Sonic / Cement Bond Log (CBL)
A traditional Cement Bond Log (CBL) measures the amplitude of a sonic signal passing along the casing; the signal is reduced where the casing is bonded to the cement. While cement bonds that are either complete, or totally absent, can be clearly identified, signals showing partial bonds provide insufficent data to determine hydraulic isolation. In wellbores with good cement bond a compressional delta T can be computed using the CBL data.
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