DEEP? HOW DEEP AND WHAT? THE VAGARIES AND MYTHS OF “LOOK AROUND” DEEP-RESISTIVITY MEASUREMENTS WHILE DRILLING

摘要

Azimuthal resistivity measurements that can readdeeply (5 m ～17 ft) into a formation while drilling havebecome common. These measurements help to reduceuncertainty during well placement within the sweet-spotof a reservoir (often termed reservoir navigation orgeosteering). These measurements also help addressother important formation evaluation problems such assaturation, formation dip, anisotropy, etc. Azimuthaldeep-resistivity technologies all use multi-spacing,multi-component and multi-frequency inductionmeasurements. Some questions arise: what are thesemeasurements really measuring, and what do we reallyunderstand about their sensitivity? Even very basiccharacteristics of the deep-reading azimuthal resistivitytechnologies, such as depth of investigation (DOI) anddirectional sensitivity, are not very well understood.Providers also get accused of “black box” interpretationsolutions, which fail to communicate the understandingof the underlying tool responses. As we push to seeeven deeper while drilling - with these frequency-basedmeasurements - questions only intensify, as the geologybecomes more unpredictable and discontinuous, finallyto the extent that we have many unknowns anduncertainties.In this paper, we investigate some of the myths of deepreadingazimuthal resistivity technologies. We discusstheir characteristics based on a sensitivity analysisalgorithm that allows the calculation of uncertainties ofthe model parameters due to the noise present in thedata for any combination of measurements. Thisapproach allows us to analyze DOI and directionalsensitivity by comparing uncertainties in distance-tobedand formation resistivities for different toolconfigurations in various formation models. In thispaper, we consider some field examples of geosteeringapplications using deep-resistivity technologies. Weconclude with a discussion of an important element ofthis study: how much is “look around” versus howmuch is “look ahead”.