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Many oil and gas companies are currently evaluating Logging-While-Drilling (LWD) or Measurement-While-Drilling (MWD) results as a possible alternative to conventional wireline logging measurements. Based on preliminary comparisons, in the Offshore Northwest Java Sea (ONWJ) area of Indonesia, some differences occur between the conventional resistivities and those obtained from LWD/MWD in the same wells. Most of these differences can be interpreted and many reveal valuable information about the formation.
Some of the differences observed between the LWD/MWD and conventional resistivities can be used to locate permeable hydrocarbon zones which have become flushed with mud filtrate and therefore exhibit a difference in resistivity over time. Other non similarities can often be explained by differences in instrument response and design. Both laterolog and induction type conventional logs are used in the comparisons to LWD/MWD.
In several cases, the LWD/MWD measurements indicate higher resistivities, which leads to more hydrocarbon pay, when compared to the conventional resistivities, apparently due to less mud filtrate invasion at the time of the LWD/MWD measurement. In water bearing sands, however, the deep laterolog resistivities often read higher than either the LWD/MWD or deep induction measurements.
There are certain advantages as well as limitations to each type of resistivity device. Comparison of these different resistivity measurements, in the same wells, can help determine which type of resistivity data is appropriate for future wells in an area. However, borehole conditions, deviation angles, and economic considerations are also major factors in deciding which type of resistivity instrument to use.
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