A METHOD TO INTERPRET MULTIPLE PROPAGATION RESISTIVITY DATA IN ENLARGED HOLES TO REVEAL TRUE RESISTIVITY

摘要

Logging while drilling resistivity is expected to have none to very little influence from invasion and hole enlargement while drilling as it is logged shortly after having been drilled. This paper shows studies over several areas where multiple propagation resistivity are logged over the same depth interval with several days between the logging passes. These curves are affected differently based on the frequency, 400 kHz or 2 MHz, and the attenuation and phase difference. Often, the resistivity measurements are misinterpreted as the borehole size is unknown in unstable formations. This data set has been modeled to establish the washed out borehole size based on the eight compensated LWD resistivity curves. Understanding the order of the different resistivity measurements based on frequency and mode from a controlled data set will enable further understanding of the resistivity measurement responses in areas where one logging pass is the only available data set.Extensive resistivity modeling is used to understand and invert back to the original hole size and the resistivity in the rock without the extensive influence from the borehole seen in the later logging passes. In this well, logged with an oil based mud system, several of the resistivity responses were opposite of what one would expect as reasonable based on first principles resistivity responses. The reason for this difference is explained in this paper. By turning the dataset around and take the most washed out data and invert back to the initial logged data with an in-gauge hole gives a method for calculating true resistivity, Rt, in washed out or collapsed boreholes. This method can then be extended into a broader use and enhance the resistivity interpretation to further reveal true resistivity while drilling. The data investigated also cover different magnitude of overburden giving rise to understanding of rock mechanics in the near borehole environment.