An advanced logging-while-drilling (LWD) tool has been introduced for resistivity, imaging and at-bit measurements. A new engineering approach enabled the concurrent design and testing of 4-3/4-in. and 6-3/4- in. collar versions. Both sizes are equipped with nine current-sensing electrodes that are arranged in three rows of three electrodes each for use with rotating and non-rotating drillstrings. Two multi-frequency transmitters provide for compensated operation in resistivity and imaging modes. In the "at-bit" measurement mode, a toroidal receiver senses the collar current emitted by the tool.

A recent field test campaign has verified the concept of quantitative imaging, which combines resistivity imaging with laterolog features. The image acquisition is compensated, based on two, opposed transmitters (symmetrically arranged for the medium spacing) and is repeated at multiple frequencies. The traditional "laterolog" ring electrode is split into discrete imaging pads, enabling directional resistivity measurements that are of particular relevance in high-angle and/or horizontal wells. Three different transmitter-to-receiver spacings enable optimization of depth-of-investigation (DOI), invasion profiling and image contrast.

The paper discusses log examples from a variety of nvironments, calculation and visualization of complex resistivity profiles, achievable resolution and image quality, as well as log quality control and comparisons with high-resolution wireline imaging logs.

Figure 1

(Available In Full Paper)

Resistivity borehole imaging is an essential element of modern formation evaluation, particular in high-angle or horizontal wells and/or in complex lithologies. LWD in water-based muds is an attractive platform for electrical imaging methods because the drillstring rotation and the rather slow rate of penetration (ROP) provide for uniform azimuthal coverage and dense sampling, as long as an acceptable standoff-vs.- resolution compromise can be found (Bonner et al., 1994; Ritter et al., 2004).

The existing tools, however, differentiate between the laterolog measurement that provides a medium-depth, compensated resistivity value averaged over the borehole azimuth (Bonner), and the imaging measurement, which is very shallow and is not compensated (Bonner, Ritter). Fig. 1 illustrates the roblem at hand (previous page). Shown is a 30-ft. nearly-horizontal section of a well drilled in the Middle East in late 2006. The borehole penetrates a highly resistive bed of approximately 2 ft. thickness that is spread out over almost 20 ft. on the log. A laterolog, because it averages over the borehole circumference, barely shows the existence of this layer. A resistivity imaging device readily picks out the bed?s presence, but a shallow, uncompensated reading in the presence of invasion and standoff cannot provide an accurate value of the bed?s resistivity.

A new hardware solution was found (Bittar, 2002) and the term "Azimuthally Focused Resistivity" (AFR) was coined for this new measurement. The novel combination of resistivity imaging with an LWD laterolog-type measurement made possible quantitative imaging, the result of which is shown in Fig. 1. The interpretational problem is addressed by producing a resistivity image that is resolved in 32 azimuthal bins and generated by two symmetrically spaced transmitters located at distances of ±30 in. from the receivers. The effective depth of investigation (DOI) is approximately seven (7) inches.