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doi:10.2204/iodp.proc.335.103.2012

Downhole logging

Complete logging programs had been run in Hole 1256D during previous expeditions above ~1420 mbsf, and logging operations during Expedition 335 were designed to focus mostly on the deepest section of the hole. Another objective was to record a full caliper log over the entire hole, both to assess the results of the cementing operations earlier in the expedition and to help plan the final cementing to stabilize Hole 1256D for future expeditions.

A predrilling temperature log and water-sampling temperature probe (WSTP) geochemical sampling were originally scheduled but did not occur because of the hole obstruction at ~920 mbsf. Temperature sondes were included in every tool string scheduled to estimate the thermal rebound of the hole.

Operations

At the end of the run of the magnetic fishing tool (bit Run 22), the hole was displaced with 200 bbl of freshwater to decrease the resistivity contrast between the borehole fluid and the formation below ~1250 mbsf, where formation resistivity was known to exceed 10,000 Ωm. After recovering the magnetic fishing tool, a logging bit was run to 3864.3 mbrf (218.9 mbsf), ~50 m above the casing shoe, to ensure that the longest tool string would be able to exit the pipe while still fully inside the casing. Preparations for logging on the rig floor started at 2000 h on 26 May 2011 (times in local time, Universal Time Coordinated [UTC] – 6). Table T13 provides complete details on the sequence and timing of events during the logging operations.

First deployment: triple combo

Shortly after exiting the casing, the tool string was stopped at 4000 mbrf to continue the ongoing evaluation of the wireline heave compensator (WHC). The chosen configuration reduced tool motion by ~50% from surface heave. After completion of WHC testing, logging down resumed at 5000 ft/h. It was stopped at 5100 mbrf to calibrate the High-Resolution Laterolog Array (HRLA) resistivity tool for in situ temperature. Calibration was performed while the tool was lowered to a total depth of 5165 mbrf (1520 mbsf), indicating that no fill had accumulated at the bottom of the hole.

After starting logging upward at 900 ft/h, the caliper of the Hostile Environment Litho-Density Sonde (HLDS) did not open immediately and the first caliper reading was only at ~5132 mbrf (1487 mbsf). Considering the position of the caliper in the tool string, ~12 m of caliper reading was missed. It was decided not to attempt a routine repeat pass, to avoid the risk of not being able to open the caliper a second time and not be able to record a complete profile of the hole size.

At a depth of 4850 mbrf (1205 mbsf), the logging speed was increased to 3600 ft/h to complete the caliper log.

The uplog was completed after entering the bit, and the tools were brought back to the surface without incident. However, the Modular Temperature Tool (MTT) started showing erratic readings at the end of the uplog. Because temperature measurements provided by the Enhanced Digital Telemetry Cartridge (EDTC) were in good agreement with the MTT, it was decided not to run the MTT on the second tool string.

During rig-down, it was observed that the three bowsprings of one of the centralizers were worn out, and the three springs were replaced with new ones. Unlike the replaced springs, the new ones had an inside rubber padding designed to prevent erosion.

Second deployment: FMS-sonic

After running into the hole, the tool could not be lowered below 3885 mbrf. With the bit at 3864 mbrf, it was assumed that the hold was at the newly refurbished upper centralizer and the tools were brought back to the surface. No sign of any damage was observed, and it was decided that the new springs were just stiff and that we should be able to go through. As a precautionary measure, one of the springs of the lower centralizer that was also showing signs of wear was also replaced at this time.

This second attempt was stopped at the same depth, and after more attempts within safe operating parameters, we were not able to make any progress or retrieve the tool. After ~90 min without progress, it was decided to use the Kinley fishing set to recover the tool string.

The procedure was performed very efficiently by the rig crew. After crimping the wire to secure the tools, cutting the wireline, and then recovering it, the drill string was pulled out of the hole, and all logging tools were safely recovered. When the tools arrived at the rig floor, it was confirmed that the upper centralizer was jammed in the fishing landing sub. The rubber padding under the new centralizer bowsprings had made the centralizer too wide to go through. The lower centralizer with only one new spring was able to go through without any trouble, but the upper one with three new springs was too wide, indicating the narrow margin for error. The last logging tools were rigged down at 1130 h on 28 May 2011.

Data quality

The hole size in Figure F97 shows that the bottom was significantly enlarged after the several weeks and bit runs dedicated to cleaning the hole. The hole is irregular in the new section logged below ~1400 mbsf, and the low density and high porosity readings below this depth are a direct consequence of the hole size and the inability of the tool sensors to make proper contact with the formation. As in the several shallower intervals with large hole diameter, density log values are systematically much lower than measurements made on core samples below 1400 mbsf. The decoupling (i.e., offset) between the shallow and deep resistivity logs below this depth is also a consequence of hole size. However, the deep resistivity measurement should not be affected by hole size.

The comparison between the hole sizes during Expeditions 312 and 335 in Figure F98 shows that the hole has changed little after 5 y and despite working the hole for several weeks before logging. Hole enlargements are indicated in the same intervals and have similar extents. This is confirmed by the very good repeatability between the different logs for both expeditions.

The only significant difference between the two sets of logs is in the gamma ray log between ~920 and ~960 mbsf, suggesting that the cement used includes radioactive nuclides detected by the gamma ray sensor. This is shown more clearly in Figure F99, where differences between the two holes are illustrated by showing intervals of hole enlargement and hole reduction between the two expeditions. Intervals where hole size has decreased, presumably because of the cement emplaced when the bit was at ~960 mbsf, mostly coincide with higher gamma ray readings during Expedition 335. The largest difference in the gamma ray logs is between 925 and 934 mbsf, where the hole was the largest and presumably the largest volume of cement was deposited. The increase in hole size above 920 mbsf is probably the result of several days spent trying to pass this depth. The cement reduced the hole size and its roughness between 930 and 970 mbsf, eliminating asperities and allowing the many smooth reentries following the cementing.

In addition to density and porosity measurements in the enlarged hole, the other questionable data are the large temperature excursions recorded by the MTT during the uplog in several places above 850 mbsf (Fig. F97). These excursions were neither recorded on the downlog nor by the EDTC temperature sensor during any of the passes and are likely related to episodic tool failures and not indicative of any borehole fluid temperature anomalies. They were removed from the other figures in this chapter to avoid any erroneous interpretation.

Logging results

Figure F100 shows data recorded in the deeper section of the hole. To compensate for the lack of reliable density and porosity logs in the enlarged hole, we used the relationship defined by Archie (1942) to estimate porosity from the deep resistivity log. For the fluid resistivity necessary in this relationship, we used the fluid resistivity measurement provided by the HRLA. The value used for the tortuosity coefficient (a = 1) and the cementation coefficient (m = 2) in this relationship are generic values (Becker, 1985) that seem appropriate to provide a reasonable agreement with the core measurement. We then used the average grain density measured on core samples for a log-derived continuous density profile from the porosity calculated from the resistivity.

One of the most significant observations in the newly recorded data is a decrease in resistivity with depth, starting within Gabbro 1 (~1420 mbsf) and becoming more noticeable in the deeper dike screens and Gabbro 2. As a resistivity increase with depth was expected in the plutonic section based on the shallower trend and previous observations, these lower resistivity values suggest that the deepest section might be fractured, possibly part of a fault, which could explain some of the difficulties encountered while coring. However, these resistivity values are in the same range as higher in the sheeted dikes complex and could be representative of the actual electric properties of the dike screens.

The ~5 m interval immediately above the lower resistivity trend (1410–1415 mbsf) defines a sharp interval with lower resistivity and higher gamma ray that could encompass a quartz-rich oxide diorite and several occurrences of evolved plutonic rocks observed in Core 312-1256D-214R (Expedition 309/312 Scientists, 2006). The lower resistivity and increased gamma ray in this interval are likely related to higher intensities of alteration observed in these rocks.

Temperature logs

The comparison between the temperature logs recorded by the two temperature tools during Expedition 335 and the temperatures measured during previous expeditions in Hole 1256D (Fig. F101) shows similar trends as the borehole fluid recovers from the disturbance of drilling operations. The maximum temperature recorded (80°C) is well below the predicted equilibrium temperature and shows that the tools were not at risk from excessive temperature.

Several excursions to lower temperatures, in particular around 925 mbsf and at 1060 mbsf, at the top of the sheeted dike complex, coincide with intervals with lower resistivity, indicating more permeable intervals where the formation might have been invaded by drilling fluid and is consequently recovering more slowly from the drilling process. However, the larger hole diameter in these intervals could contribute to the slower thermal rebound. A kick at ~1300 mbsf, also observed during Expedition 312, coincides with lower resistivity and is probably also associated with fluid exchange with the formation. These anomalies will be the object of numerical modeling, which, in combination with other logs, should provide estimates of the permeability in these intervals.

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