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doi:10.2204/iodp.proc.344.107.2013

Downhole logging

Logging operations

Coring in Hole U1413C ended with Core 344-U1413C-43R reaching 582.2 mbsf at 1350 h on 29 November 2012 (all times are local Costa Rica time, UTC – 6). A wiper trip was then started to prepare the hole for logging. The hole was drilled without major problems, but during the wiper trip, high torque was encountered below ~250 mbsf. While the hole was being displaced with heavy mud (10.5 ppg), high standpipe pressures were noted. Moreover, 20,000–30,000 lb of overpull was observed when raising the drill pipe to logging depth. Because of poor hole conditions, it was decided to first deploy a slick triple combo tool string without a radioactive source.

The triple combo tool string included (from top to bottom) the Enhanced Digital Telemetry Cartridge (EDTC), Hostile Environment Natural Gamma Ray Sonde (HNGS), Hostile Environment Litho-Density Sonde (HLDS; no radioactive source, measuring just the caliper), and Dual Induction Tool (Fig. F40). For explanation of the acronyms and details on the wireline tools, see the “Methods” chapter (Harris et al., 2013). Rig-up of the triple combo started at 1045 h on 30 November, and the tool string started down the hole at 1200 h. After exiting the drill pipe, an obstruction was encountered with the base of the tool string at 187 mbsf (the mbsf unit used here corresponds to the wireline depth below seafloor [WSF] depth scale; see the “Methods” chapter [Harris et al., 2013]). Repeated attempts to pass the obstruction were unsuccessful, and at 1320 h we started to log the 84 m section of open hole from the obstruction to the base of the drill pipe (observed on the gamma ray log at 103 mbsf). The seafloor was detected from a step on the NGR log at 550 mbrf (the driller’s seafloor depth for this hole was 551.4 mbrf). The triple combo reached the rig floor and was rigged down at 1435 h. In the short hole interval that could be logged, the triple combo caliper showed sections that were nearly in gauge. After consultation with the Co-Chief Scientists, we decided to run two additional short tool strings focused on imaging borehole breakouts, the UBI and the FMS (Fig. F40).

The second tool string included (from top to bottom) the EDTC, HNGS, General Purpose Inclinometry Tool (GPIT), and UBI. Rig-up started at 1435 h on 30 November, and the UBI tool string started down the hole at 1600 h. After reaching the obstruction encountered in the previous run at 187 mbsf, we ran two complete passes. The UBI tool string returned to the rig floor and was rigged down at 1930 h.

The third tool string included (from top to bottom) the EDTC, HNGS, GPIT, and FMS. Rig-up started at 1930 h, and the FMS tool string started down the hole at 2010 h. Again, the tool string encountered an obstruction at 186 mbsf, and we ran two complete passes in the open hole. The FMS images in the first pass looked good up to the base of the drill pipe, and we asked the drillers to raise the drill pipe to increase the length of open hole available for logging. The second pass encountered the base of the drill pipe at 93.5 mbsf (from the NGR log). The FMS tool string reached the rig floor at 2220 h, and logging operations at Hole U1413C ended when the rig down of the wireline system was completed at 2300 h on 30 November.

Downhole log data quality

The downhole log data collected in Hole U1413C were processed to convert to depth below seafloor and to match depths between different logging runs. The resulting depth scale is wireline matched depth below seafloor (WMSF; see the “Methods” chapter [Harris et al., 2013]), and from here on mbsf denotes the WMSF depth scale.

A key factor that influences downhole log data quality is the size and irregularity of the borehole. Measurements of the diameter of Hole U1413C are summarized in Figure F41. The LCAL track in this figure is measured by a single caliper arm on the HLDS and shows a hole that is larger than the RCB bit diameter (9.875 inches) in the two intervals below 169 mbsf and above 148 mbsf, with values that are locally greater than the maximum measurable range (~18 inches). In contrast, the hole size is close to the bit diameter in the 148–169 mbsf interval. The FMS tool measures hole diameter in two orthogonal directions from the aperture of the two pairs of arms that support the microresistivity measurement pads. These orthogonal hole diameter measurements, shown in the C1 and C2 tracks in the figure, show that the borehole is enlarged in both directions below 169 mbsf. In contrast, above 148 mbsf the hole diameter is clearly greater than the bit size in only one direction. The larger borehole diameter in this interval is measured by Caliper C1 in the first FMS pass and by Caliper C2 in the second (see “Borehole images and breakouts”).

The overall quality of the logging data can be assessed from the repeatability of measurements acquired in different runs or passes. In general, the downhole log data acquired in Hole U1413C show excellent repeatability. Figure F41 compares spectral gamma ray logs acquired by the HNGS tool in the UBI and FMS tool strings. The gamma ray measurement is highly attenuated when the tool is inside the BHA (above 93–103 mbsf in Hole U1413C), and data in this interval should only be used qualitatively.

Log characterization and logging units

The logging measurements acquired in Hole U1413C are summarized in Figure F42. The total gamma ray data measured by the HNGS tool are generally comparable to the NGR measurements made on cores. The total gamma ray values measured by the HNGS tool are expressed in an American Petroleum Institute (gAPI) scale based on a standard artificial formation built to simulate about twice the radioactivity of a typical shale and conventionally set to 200 gAPI (Ellis and Singer, 2007). The units of the NGR measurements made on whole core sections are in counts per second (cps; for a detailed description of the NGR apparatus, see Vasiliev et al., 2011). The comparison of log and core NGR measurements in Figure F42 shows that their curves overlap if 1 cps equals ~2 gAPI. The general agreement in the trend of the total gamma ray log and core values above 169 mbsf suggests that hole conditions did not impact the gamma ray measurements in this interval.

Figure F42 also shows the abundances of the three naturally radioactive elements (U, Th, and K) measured by the spectral HNGS tool. The measured resistivities are generally between 1 and 2 Ωm; low values <1 Ωm measured at 170–172 mbsf and below 182 mbsf are likely due to a locally enlarged borehole (Fig. F41). Finally, the figure shows the borehole radius image obtained by the UBI. The measured traveltimes of the ultrasonic pulse from the UBI transducer to the borehole wall give a detailed image of the borehole radius as a function of azimuth.

We distinguished three logging units in the interval logged in Hole U1413C. Logging Unit 1 (93–148 mbsf) is characterized by total gamma ray values between 38 and 46 gAPI, by a relatively low U content between 1.4 and 2.7 ppm, and by relatively low resistivities just above 1 Ωm. This is the interval where the UBI images show vertical bands with large reflection radiuses and the borehole diameter is large in one of the directions measured by the FMS caliper arms (Fig. F41). In contrast, the borehole is almost circular and nearly in gauge throughout Logging Unit 2 (148–169 mbsf). Compared to Unit 1, Unit 2 displays a higher total gamma ray (~60 gAPI), higher U content (~3 ppm), and higher resistivity. As the resistivity of sedimentary formations is mostly controlled by porosity, the increase in resistivity implies a decrease in porosity in Logging Unit 2. This decrease in porosity and the circular, in gauge borehole of Unit 2 suggest a more consolidated formation than in Unit 1. At the boundary between Logging Units 1 and 2, magnetic susceptibility and NGR data measured on cores also show a significant change around 148 mbsf in Hole U1413A (see “Physical properties”). The borehole seems to be washed out in all directions in Logging Unit 3 (169–184 mbsf), and the low values of natural radioactivity and resistivity measured at 170–172 mbsf are likely artifacts caused by a pronounced borehole enlargement.

Borehole images and breakouts

Borehole images collected by the UBI ultrasonic tool and the FMS microresistivity tool are shown in Figure F43. The three images display the UBI amplitude (which is related to the small-scale roughness of the borehole wall), the UBI borehole radius image (which is proportional to the measured traveltime), and the FMS images collected by the four microresistivity pads.

The UBI images show an irregular, large-radius borehole in Logging Unit 3 (below 169 mbsf) and a borehole that is smooth and has a nearly constant radius in Logging Unit 2 (148–169 mbsf). Within Logging Unit 1 (above 148 mbsf), the images show two nearly vertical bands of high rugosity (low amplitude) and large borehole radius (large traveltime). These two bands are on opposite sides of the borehole and span sectors of ~30°–90°. For a nominal 10 inch borehole diameter, the width of these bands is 6.5–20 cm.

The FMS images from Logging Unit 1 show low resistivity values (dark) in the two opposite pads that are in the same direction as the high-rugosity/large-radius bands in the UBI images. The FMS caliper measurements in this interval (Fig. F41) show that the pads measuring low resistivities also measure the larger borehole diameter. As the FMS tool is pulled up, the 6 cm wide microresistivity pads get stuck in these large-diameter borehole sectors, and the measured low resistivity is likely caused by the rough borehole surface that prevents close contact with the pad. The azimuth of the pair of pads measuring the larger borehole diameter in the second pass of the FMS run is also apparent in Figure F43, showing that the UBI and FMS measurements of these large-diameter borehole sectors are entirely consistent.

The nearly vertical bands of large-diameter, rugose borehole in Logging Unit 1 are borehole breakouts. When a vertical borehole is drilled in a region with significant deviatoric (maximum–minimum) horizontal stress, the resulting hoop stress along the borehole wall reaches a maximum compressive value at the azimuth of the minimum horizontal stress. If the hoop stress is greater than the rock strength, the rock fails, giving rise to characteristic borehole breakouts (e.g., Zoback et al., 2003). Borehole breakouts have been imaged with ultrasonic and four-arm caliper logs in many other studies (e.g., Bell and Gough, 1979; Plumb and Hickman, 1985; Zoback et al., 2003; Lin et al., 2010). The minimum horizontal stress in the 148–169 mbsf interval in Hole U1413C is oriented approximately north–south.

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