Field Testing of a Propagation At-Bit Resistivity Tool

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Despite its great potential in geosteering, geostopping, well placement, and other applications, at-bit propagation resistivity technology has seen little progress in the past 40 years. No commercial tools are available today on the markets. Compared to conventional logging-while-drilling (LWD) resistivity tools, at-bit resistivity tools bring measurements right at or close to the bit, substantially reducing the blind time for wellbore adjustment decision making. Today, conventional LWD resistivity technologies, primarily propagation and azimuthal resistivity technologies, are routinely available for commercial uses, but at-bit resistivity technologies are rare except for a few electrode-type tools. The latter not only are limited in depth of investigation but also often experience difficulties in oil-based muds or other nonconducting drilling fluids. In this paper, we report some of the latest progress in the at-bit resistivity technology. We shall discuss the design and field testing of a propagation-type at-bit resistivity tool. The new tool, by design, measures both attenuation and phase difference at low MHz frequencies. Much of the development effort was centered on the challenge posed by short-spaced antennas. Because of the restriction on the tool length for the sake of BHA steerability, much shorter coil spacings were to be used as compared to conventional LWD resistivity tools. As a result, the attenuation and phase difference quantities to be measured would be much smaller, in some cases even orders of magnitude smaller, than those of conventional LWD tools. Higher frequencies help increase the measurability of the quantities but may greatly reduce the depths of investigation of the tool. Moreover, high frequencies may also introduce large dispersion effects into the measurements, making the at-bit resistivity data more difficult to interpret. To test the tool design, especially the selection of the frequencies, prototype tools were built for lab experiments and field trials. A water tank was used to simulate a conducting medium. Both attenuation and phase difference data were acquired and compared against the numerical models of the water tank. The water tank data was also used to help define the limits of the resistivity measurements. More lab experiments were designed to verify the azimuthal resolution capability of the tool as predicted by the numerical modeling. Both the water tank and metal reflectors were used to demonstrate the azimuthal resolution of the tool. As an integral part of the tool development effort, thorough numerical modeling was performed to study the tool response to various important scenarios, including (1) resistivity anisotropy, (2) borehole effects, (3) tool eccentricity effects, and (4) bed boundary effects. In this paper, we shall report some of the important results from the numerical modeling studies. Our emphasis will be on the field testing of the new tool. We shall discuss a few case studies from the US and Canada. We shall present field data from dual-sensor (resistivity and gamma) at-bit tools. We shall discuss how the at-bit resistivity data compare with the conventional LWD resistivity data. We shall also discuss how the dual-sensor data can be used as a means to validate both at-bit resistivity and gamma data when an independent resistivity log is not available for comparison.