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doi:10.2204/iodp.proc.327.103.2011

Downhole measurements

A wireline logging string was deployed in Hole U1362A to determine the physical and hydrological properties of upper basement rocks and to identify suitable intervals for packer placement. The logging string consisted of tools with sensors for measuring natural spectral gamma ray, bulk density, borehole fluid temperature, borehole orientation, tool motion, ultrasonic images, and hole diameter, as well as a qualitative spontaneous potential (SP). Two passes were run over the entire open hole, and a third pass was run over two intervals of particular interest.

Calipers revealed a borehole that was highly enlarged over most of the open hole section, as expected, although two notable near-gauge sections were identified as suitable packer locations.

The ultrasonic borehole images are marred by rotational and heave-induced stick-slip tool motion, and the density measurement suffered from the enlarged borehole. Where the hole condition was good, logged density compares favorably with laboratory tests of core samples from the same depths.

Some elevated gamma ray values correspond loosely with an enlarged borehole. These zones could represent areas of greater alteration and may be indicative of past hydrothermal fluid flow. However, in other intervals gamma ray values and borehole size are inversely related.

Borehole fluid temperature data were acquired while running into the hole and during the uphole logging passes. The borehole temperature gradient increases steeply at the top of the primary in-gauge interval, and a temperature anomaly was observed near the casing shoe.

Logging tool string

The 26.4 m long wireline tool string consisted (from the top down) of the logging equipment head (LEH), which contained an electrode for generating a qualitative SP curve; the Hostile Environment Natural Gamma Ray Sonde (HNGS); the Hostile Environment Litho-Density Sonde (HLDS); the Modular Temperature Tool (MTT); the General Purpose Inclinometry Tool (GPIT); and the Ultrasonic Borehole Imager (UBI). For a description of these tools, see “Downhole measurements” in the “Methods” chapter.

Operations

After drilling the 9⅞ inch open hole section to a total depth of 528 mbsf, the hole was cleaned and left full of seawater. After the packer BHA was made up, the drill string was run to 264 mbsf, roughly 44.5 m above the 10¾ inch casing shoe.

The wireline tool string was rigged up and run out of the drill pipe and into the 10¾ inch casing, where it was held still while the wireline heave compensator (WHC) was started. Surface heave, as measured by accelerometers near the moonpool, was observed to be ~0.9 m peak-to-trough. Downhole tool motion, as measured by the accelerometers in the wireline string, was ~180% of surface motion both before and after the WHC was started.

The wireline string was lowered into the open hole and, to avoid damaging the sensitive UBI sensor head at the bottom of the tool string, was stopped at 507 mbsf (5 m above the last known bottom fill). Borehole fluid acoustic velocity (for UBI tool calibration) and temperature were logged during the descent.

A first pass was run at 600 ft/h using low-resolution UBI settings (250 kHz, 1.0 inch vertical resolution, and 140 samples per rotation) in an attempt to acquire usable, adequate data over as much of the open hole interval as possible. We expected a small (<13 inch diameter) hole over only a few intervals, particularly across the 453.8–472 mbsf section identified via core recovery, quality, and rate of penetration. The first pass confirmed a near-gauge hole section near 447 mbsf and another near 417 mbsf. The HLDS mechanical caliper—but not the UBI ultrasonic caliper—resolved an undergauge (7.8 inch diameter) interval between 364 and 374 mbsf.

After logging up into the casing shoe, we ran back to total depth in order to conduct a second medium-resolution pass (UBI settings of 250 kHz, 0.4 inch vertical resolution, and 180 samples per rotation). We logged at 400 ft/h over the best hole section and then sped up to 800 ft/h at 414 mbsf, above which the hole was too enlarged to obtain a meaningful UBI image. We observed no diagnostic fluctuation in cable or head tension as we passed up or down through the undergauge interval identified in the first pass. Once again, the UBI caliper did not resolve the undergauge section, but the HLDS caliper saw an even tighter (5.5 inch diameter) interval, only this time it appeared to be at 376 mbsf and only 3 m thick.

After completing the second pass into casing, the tool string was run back to total depth to conduct a final high-resolution pass (UBI settings of 500 kHz, 0.2 inch vertical resolution, and 180 samples per rotation) over the good hole interval. We ran up at 400 ft/h until we passed the good hole section at 447 mbsf and then pulled up rapidly (2000 ft/h) to log the apparent tight zone. At this point the Schlumberger logging engineer experimented with the logging configuration in the hope of acquiring an accurate ultrasonic caliper over the apparent tight spot. After that reconfiguration, the UBI images became noisy and speckled, so the original settings were restored at 388 m wireline log depth below seafloor (WSF). On this third pass over the suspected tight section, the HLDS caliper saw only hole with >12 inch diameter.

The logging and operations team decided to log a fourth caliper pass through the possible tight hole interval. Just before starting the pass with the caliper extended, the winch operator lowered the tool string slightly to fix a spooling problem on the winch. Head tension dropped as the HLDS caliper took some of the tool string weight. After pulling up on the string, the Schlumberger engineer was unable to close the mechanical caliper. The caliper continued to read hole diameter, and after a brief troubleshooting period we decided to continue to log up into casing. For the rest of the pass, the caliper did not extend beyond ~14 inches, nor did it register a hole restriction at the troublesome interval. The tool string entered the casing shoe and pipe without incident. Because of the damaged caliper, the planned WHC testing period was canceled, and the tool string was pulled to the surface to complete logging operations.

Data processing

Wireline logging data were acquired by the Schlumberger acquisition system and archived in digital log interchange standard (DLIS) format. Immediately after logging, field prints of the three logging passes were generated and delivered to the operations and science teams for packer-seat identification. Simultaneously, data were transferred via satellite to the Borehole Research Group at the Lamont-Doherty Earth Observatory. There, the data were processed and transferred back to the ship for distribution and archiving in the shipboard log database. Processing details can be found in “Downhole measurements” in the “Methods” chapter, as well as in the processing notes found with the data in the IODP US Implementing Organization (USIO) log database.

Depth shifting

Because the principal operational objective of logging was to identify packer intervals, we used the 10¾ inch casing shoe (identified by caliper and by a step in gamma ray values) as a depth reference, rather than the seafloor (identified on the downlog by an ambiguous gamma ray response). The downlog gamma ray identified the seafloor at either 2672 or 2675 m, and the shift to the casing shoe had the effect of placing the seafloor depth at 2675.5 m. The first and third logging passes and the downlog were shifted to match the reference from the second pass, resulting in the wireline log matched depth below seafloor (WMSF) scale.

Data quality

With the exception of the HLDS caliper, measurements for all tools were highly repeatable over the notable interval described above.

An enlarged borehole over most of the open hole section affected the density measurements, which require eccentralization and good contact with the borehole wall. Similarly, the ultrasonic images and caliper are only usable in the narrowest (less than ~13 inch) hole sections. Because wireline tools must be conveyed through drill pipe, an undersized UBI sonde head was used; the 3.56 inch OD sonde head was designed to run in holes ≤7⅝ inches. The gamma ray, temperature, and borehole orientation measurements are relatively unaffected by hole size, and it does not appear that borehole size has much influence on the SP curve.

The UBI images were severely affected by downhole tool motion despite the use of the WHC. Surface heave throughout the logging passes averaged 0.8 m peak-to-trough, with frequent excursions of >1.1 m. When logging upward, the apparent downhole heave-induced motion was reduced (compared to its motion when the tool string was held at a static depth), and downhole heave displacement averaged 0.8 m, with occasional excursions of >1.5 m. The apparent reduction in downhole heave-induced motion while logging can be a result of the HLDS caliper being extended and the tool string being pulled up at a steady speed. The third logging pass used the most ambitious UBI configuration and yielded the sharpest images over the near-gauge intervals of interest.

Preliminary results

The logging string consisted of tools with sensors for measuring SP, natural spectral gamma ray, bulk density and photoelectric effect (PEF), borehole fluid temperature, tool motion, oriented ultrasonic images, and hole diameter. Selected representative log curves from the second pass are shown in Figure F39.

Spontaneous potential

SP data from the three logging passes repeat remarkably well, exhibit little noise, and appear insensitive to hole condition. The measurement deflects noticeably at the casing shoe and then trends negatively throughout most of the open hole section. Over the 417 mbsf near-gauge interval the SP curve appears to flatten, and at the 447 mbsf near-gauge interval the SP curve reverses.

Natural gamma ray

Gamma ray measurements repeat well over the three passes. In the open hole, total gamma ray values are correlated with potassium content (Fig. F40). Values range between 2.1 and 7.4 API units, which is typical of basaltic oceanic crust (Bartetzko et al., 2001).

In a few intervals, like the one starting at 470 mbsf, an increase in gamma ray values corresponds with an increase in borehole size; intervals like these can represent zones of greater alteration and may be indicative of focused hydrothermal fluid flow through more fractured rock (Bartetzko et al., 2001; Fisher, Urabe, Klaus, and the Expedition 301 Scientists, 2005; Expedition 324 Scientists, 2010). Supporting this hypothesis, shipboard ICP-AES analyses of K₂O appear to correlate well with the HLDS caliper and with HNGS-determined potassium concentration (Fig. F41). On further inspection, however, HNGS potassium and HLDS caliper measurements correlate inversely (Fig. F42) over the bulk of the 9⅞ inch open hole section, suggesting that the apparent K₂O correlation is an artifact of sparse core sampling and the contribution from the near-gauge hole interval at 455 mbsf. Enlarged borehole size may be the primary cause of the apparent reduction in HNGS potassium concentration because of insufficient correction for borehole size during processing.

The HNGS total gamma ray curve does not correlate well with shipboard NGR analyses, the latter being both sparse and erratic (Fig. F43). The shipboard NGRL probably underestimates NGR when core samples are short and irregular in shape. For that reason, NGR measurements are highest for intact cores taken from the near-gauge hole interval starting at ~455 mbsf.

A pronounced increase in HNGS gamma ray values was observed at the casing shoe (Fig. F39), most likely indicating the presence of cement. According to the gamma ray signature, the top of cement outside the 10¾ inch casing sits at 2933 mbrf, ~47 m above the casing shoe.

Density and photoelectric effect

Wireline density and PEF measurements in Hole U1362A are impaired over most of the open hole section because of the large, washed-out borehole. Low density and PEF values correspond to intervals of enlarged borehole (Fig. F39). Where the hole is near-gauge, wireline density is consistent with MAD core sample measurements (Fig. F39). Between 417 and 428 mbsf, density values average 2.59 g/cm³ (standard deviation = 0.31), and between 447 and 472 mbsf, where hole diameter is <12 inches, they average 2.80 g/cm³ (standard deviation = 0.15). PEF values in the same intervals average 3.21 barns/e^– (standard deviation = 0.85) and 3.96 barns/e^– (standard deviation = 0.73), respectively.

Temperature

Temperature data were acquired while running into the hole and during the three uphole logging passes. The data reveal a highly repeatable borehole-fluid temperature profile (Fig. F39). The marked gradient increase at 447 mbsf may be indicative of more conductive conditions around the massive section from 447 to 472 mbsf. The return to nearly isothermal conditions at ~475 mbsf correlates with another enlarged borehole section and lower core recovery. Successive passes hint at a slight warming trend with time, although that progression might be an artifact of the different logging speeds used at different times (where faster logging speeds promote lower measured temperatures). A short temperature anomaly (a ~0.5°C rise followed by a 0.2°C drop) was observed ~8 m below the casing shoe. The gamma ray and SP curves have too much character to offer any correlation with the temperature anomaly, which occurs in the 14¾ inch rathole section of hole that is beyond the reach of the HLDS caliper.

Ultrasonic imaging

The ultrasonic borehole images are marred by both rotational and ship heave–induced stick-slip tool motion. Postlogging processing helped mitigate tool motion effects, but they are still pronounced and visible as smears and truncations (Fig. F44). Moreover, because the UBI sonde head is undersized (see “Data quality”), no images can be expected where the hole is even moderately out of gauge. Where the hole is near-gauge, some centimeter- to meter-scale dipping sinusoidal features are apparent. Images from the third pass, collected using the highest-resolution UBI configuration, reveal an interval of pillow basalt lying above a zone of sheet basalt at 458 mbsf.

Caliper

The wireline logging program for Hole U1362A was intended to deliver caliper data that would help identify suitable locations for the drill string and CORK packers. Before the run, certain intervals were predicted to be near-gauge on the basis of relatively high core recovery and core quality; the most likely zone was between 454 and 472 mbsf, whereas the intervals 415–430, 390–408, and 346–380 mbsf also held promise.

The three logging passes acquired data with the HLDS mechanical caliper arm, supplemented by the UBI ultrasonic caliper. The HLDS caliper identified a remarkably consistent near-gauge interval from 447 to 472 mbsf and a second, less impressive interval from 417 to 428 mbsf (Fig. F39). These two zones correlate remarkably well with those predicted by coring. The remaining two potentially near-gauge intervals were not observed. Because the HLDS caliper has a maximum reach of ~18 inches, hole size in the 14¾ inch rathole is possibly much larger.

Ultrasonic radial measurements confirmed the intervals identified with the mechanical caliper. However, the ultrasonic caliper was highly erratic and noisy, owing, perhaps, to the high degree of downhole tool motion and to the undersized sonde head. UBI caliper measurements are not valid where hole size exceeds 13 inches. Where the ultrasonic caliper maximum and minimum values appear meaningful, they indicate a nearly circular borehole through the near-gauge sections with minor-to-major axis ratios between 0.94 and 0.97. The major axis, which represents the direction of minimum horizontal stress, lies roughly north–south.

Although the HLDS caliper measurements were generally highly repeatable, an anomalous, apparently undergauge 7.7 inch diameter interval from 363.5 to 373.5 mbsf was observed during the first pass (Fig. F45). On the second pass, the apparent tight interval had only a 5.5 inch diameter and was observed between 376 and 379 mbsf, a few meters lower than before. Two new caliper spikes were observed at 359 and 413 m WMSF. A third pass of the HLDS caliper revealed no undergauge intervals.

The mechanical caliper was damaged before a planned fourth pass could take place (see “Operations” in this section). That failure at the end of the logging deployment is unrelated to the caliper anomalies observed during the first two passes. Even after the damage—with a known failure mode and diagnostic response at the Schlumberger surface system—the caliper still saw the entry into the casing shoe and drill pipe. It did not see the undergauge anomalies, which might represent sloughing or material working its way down the hole.

The ultrasonic caliper did not record the apparent undergauge sections shown by the HLDS caliper on the first and second passes. It is possible that the obstructions, if they existed, were dislodged before the UBI crossed the interval. It is also possible, though unlikely, that the UBI was unable to resolve diameters smaller than the expected bit size. To account for this possibility, the UBI was reconfigured midway through the third pass to measure an undergauge diameter. However, the UBI image became speckled, so the tool was reset to its original configuration.

Stratigraphy

Logging units were identified on the basis of petrophysical log response and borehole condition (Fig. F39). Logging Unit I is characterized by generally poor to moderate hole conditions and fairly stable gamma ray—except for a pronounced increase at the base of the unit—and corresponds roughly with lithologic Units 1 and 2. Logging Unit I extends from 346 mbsf (the top of the 9⅞ inch hole section) to 396 mbsf, where the gamma ray signature drops steeply.

Logging Unit II covers a zone of very poor hole condition, poor density data, and relatively low gamma ray values, all having a similar profile. As with Unit I, a gamma ray high followed by a sudden drop marks the base of the unit at 417 mbsf, as does a sharp reduction in hole size. In Unit II, which loosely matches lithologic Units 3 and 4, the SP curve exhibits step changes that are more blocky than those in Unit I.

Logging Unit III spans the interval of near-gauge hole between 417 and 428 mbsf. The unit has high density in its upper half, although surprisingly low values toward its base (considering the good hole quality). The transition from high to low density values occurs at a short zone of borehole enlargement that coincides with the base of lithologic Unit 4. The SP curve does not reflect a change in potential through logging Unit III, the very top of which also features a subtle increase in borehole fluid temperature.

Rugose, enlarged borehole is the primary feature of logging Unit IV, which is correspondingly characterized by low density readings. The SP curve begins to deflect rather sharply midway through the interval, and there is a pronounced temperature inflection toward the base of the unit. The unit ends at 447 mbsf and lies entirely within lithologic Unit 5.

Logging Unit V has generally good hole conditions (and similarly improved density measurements) and features highly variable gamma ray values. In the middle of the unit, the SP curve reverses after trending negatively since the casing shoe. The base of Unit V is marked by the apparent transition from pillow lava to sheet basalt, as observed at 458 mbsf in UBI images from the third pass, which also identifies the base of lithologic Unit 5.

The thick, consistent near-gauge hole section typifies logging Unit VI, which approximately matches lithologic Unit 6. Throughout, density values are high and correspond to those determined from core sample analyses. The SP curve increases steadily, and the gamma ray signature is remarkably low in this interval. A steep jump in gamma ray values marks the base of the unit at 470 mbsf.

Logging Unit VII extends to the base of the logged hole and is characterized by a worsening (followed by a slight improvement) of hole conditions, by a steady decrease in gamma ray values, and by a flattening and then steep inflection of both SP and temperature curves.

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