Induction log
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The induction log was invented in 1947 to measure formation resistivity in non-conductive boreholes containing oil-based muds or drilled without fluid. It works using a transmitter coil that generates a magnetic field around the borehole, which is measured by a receiver coil. Factors like borehole size, mud properties, and bed thickness can influence readings. Induction logs are used for saturation determination, lithology identification, and locating hydrocarbon zones, and provide accurate resistivity readings, especially in low resistivity formations. The tool is minimally affected by drilling fluid resistivity.
Induction log
- 4. INDUCTION LOG CONCEPTS The induction log was invented by Henry Doll of Schlumberger and described in 1947. Induction logging was originally developed to measure formation resistivity's in boreholes containing oil-based muds and in air-drilled boreholes because electrode devices could not work in these non-conductive boreholes. Because the tools were easy to run and required much less in the way of chart corrections than laterals or normal, induction tools were used in a wide range of borehole salinity soon after their introduction.
- 5. Principal Two-coil sonde consists of a transmitter and receiver mounted coaxially on a mandrel. Typical coil separations range from 1 to 10 ft apart. In practice, each coil can consist of from several to 100 or more turns, with the exact number of turns determined by engineering considerations. The operating frequency of commercial induction tools is in the tens to hundreds of kilohertz range, with 20 kHz being the most commonly used frequency before 1990. The induction transmitter coil is driven by an alternating current that creates a primary magnetic field around the transmitter coil.
- 7. Factors Bore hole size Mud cake type and thickness Invasion diameter Bed thickness and tool resolution Depth of investigation Unit The unit of induction logging is ohm.
- 8. Applications Saturation determination Lithology identification Source Rock identification Locating of hydrocarbon bearing zones Determination of shale volume Localization of over-pressured zones Correlation purposes
- 9. The Induction Gives High Accuracy of Reading in Low Resistivity Beds Due to the use of a conductivity scale, it becomes extremely easy to differentiate between 0.25 and 0.3 ohms--whereas such readings on the conventional Electric Log are the width of the trace. For example, the difference of resistivity between 8133-40 and 8146-50 is 4 large divisions on the conductivity scale of the Induction Log. This high accuracy is necessary in high porosity sands where the difference between 0.6 and 0.7 ohms can mean the difference between water and oil production.
- 10. The Drilling Fluid has Negligible Influence on the Induction Log The Induction Log, originally designed for oil-base muds, and for cable tool holes (without fluid in the bore hole) is negligibly affected by the resistivity of the drilling fluid. This has a great advantage because nearly identical logs will be obtained in muds of 3.00 or 0.3 ohms resistivity; in such cases the electrical logs may exhibit considerable changes in character.
- 11. Induction Log has a "Great Affinity" for Water Sands The two water sands above and below the oil zone are very clearly shown on the Induction log. This reduces the uncertainty, and, therefore, the number of side-wall cores required. Clean water sands are, therefore, always positively identified. Shally sands may also show quite low resistivity's even when oil bearing, and a very accurate determination of their true resistivity is essential for log analysis. These sands should always be side-wall sampled.
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