INNOVATIVE TECHNIQUES APPLIED TO LOGGING-WHILEDRILLING AZIMUTHAL DENSITY DATA FOR REDUCING STRUCTURAL UNCERTAINTIES

摘要

Uncertainties in geological interpretation present significant challenges when drilling reservoirs related to salt tectonics. Despite recent advances in seismic technology, seismic imaging remains a major issue, especially for subsalt reservoirs. Seismic-based structural interpretation relies heavily on interpreted salt geometry, and it frequently underestimates the dip angle of steeply dipping beds below salt. Well logging data plays an instrumental role in constraining geological interpretation. In subsalt reservoirs in particular, integration of reliable log-derived structural dip is crucial when calibrating seismic-derived structural models. Various types of wireline logging tools, such as high-resolution micro-resistivity borehole imagers and multi-component induction resistivity tools, are available to address this challenge. Complex wellbore designs and economic constraints, however, usually limit the use of such technologies to the main objective section of the well. Logging-while-drilling (LWD) can be advantageous compared to wireline, because it does not require additional rig time to collect data, and is less prone to sticking. Improvements to data quality since the introduction of LWD to the industry, coupled with advances in real-time data acquisition, have made LWD the method of choice for formation evaluation in deep-water settings. A common LWD system for providing dip information in all mud types, including oil-based mud, is the azimuthal density tool. Although often considered inferior to the wireline micro-resistivity imager, LWD azimuthal density continues to fill the data gap where wireline data acquisition is impractical. Additionally, LWD real-time borehole images have proven valuable in terms of assessing the accuracy of the geological model and target location while the well is being drilled, so that the wellbore trajectory can be adjusted, if necessary. This paper provides best practices for obtaining reliable structural-dip information using LWD azimuthal density, from the pre-job planning stage to post-drilling data-processing and interpretation. Conventional LWD density dip interpretation involves selecting all visible sinusoid features in static and dynamically enhanced images. From these dip picks, structural dip components must be distinguished from stratigraphic features, noise, and other artifacts. Innovative techniques are presented that have consistently improved image data quality. These techniques include the use of single-detector density data, delta density and delta density derivative processing, and delta density display. The improved image quality helps the interpreter distinguish real geologic features from artifacts, thus helping increase the accuracy of dip interpretation. Although the invention of these techniques was driven by bed dip uncertainties in subsalt reservoirs, they are applicable in all situations where improved image quality and accurate dip interpretation are necessary.