THE VAGARIES AND MYTHS OF LOOK-AROUND DEEP-RESISTIVITY MEASUREMENTS WHILE DRILLING

Azimuthal resistivity measurements that can read deeply (5 m similar to 17 ft) into a formation while drilling have become common. These measurements help to reduce uncertainty during well placement within the sweet-spot of a reservoir (often termed reservoir navigation or geosteering). These measurements also help address other important formation evaluation problems such as saturation, formation dip, anisotropy, etc. Azimuthal deep-resistivity technologies all use multi-spacing, multi-component and multi-frequency induction measurements. Some questions arise: what are these measurements really measuring, and what do we really understand about their sensitivity? Even very basic characteristics of the deep-reading azimuthal resistivity technologies, such as depth of investigation (DOI) and directional sensitivity are not very well understood. Providers also get accused of "black box" interpretation solutions, which fail to communicate the understanding of the underlying tool responses. As we push to see even deeper while drilling - with these frequency-based measurements - questions only intensify, as the geology becomes more unpredictable and discontinuous, finally to the extent that we have many unknowns and uncertainties. In this paper, we investigate some of the myths of deep-reading azimuthal resistivity technologies. We discuss their characteristics, based on a sensitivity analysis algorithm that allows the calculation of uncertainties of the model parameters due to the noise present in the data for any combination of measurements. This approach allows us to analyze DOI and directional sensitivity by comparing uncertainties in distance-to-bed and formation resistivities for different tool configurations in various formation models. In this paper, we consider some field examples of geosteering applications using deep-resistivity technologies. We conclude with a discussion of an important element of this study: how much is "look around" versus how much is "look ahead".