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doi:10.2204/iodp.proc.324.107.2010

Downhole logging

Operations

A wiper trip was completed throughout the open hole, and the RCB bit was released at the bottom of the hole using the mechanical bit release before the start of the wireline logging operations. The hole was displaced using 90 bbl of barite mud, and the drill pipe was set at a depth of 4184 m DRF. Logging operations in Hole U1350A consisted of two attempts to deploy one tool string and took place in deteriorating weather with initial ship heave conditions of ~2 m, which gradually changed to ~4 m peak to peak heave and wind gusts of up to 56 kt. Downhole logging operations began at 0510 h on 19 October 2009 and were concluded at 0030 h on 20 October after the tool string was rigged down.

Tool string deployment

HNGS-APS-HLDS-DITE

The wireline tool string deployment consisted of a 29 m long triple combo tool string that included a logging equipment cable head, digital telemetry cartridge, Hostile Environment Natural Gamma Ray Sonde (HNGS), Hostile Environment Natural Gamma Ray Cartridge, Accelerator Porosity Sonde (APS), Litho-Density Sonde Cartridge, Hostile Environment Litho-Density Sonde (HLDS), digital telemetry adapter, and Digital Dual Induction Tool model E (DITE). The tool string was lowered to 3600 m DRF at a speed of ~2200 m/h. At this depth, the head tension decreased dramatically and the cable speed was reduced to ~90–120 m/h to avoid potential damage to the wireline. The slow progress required pumping pressure down the drill pipe to aid the descent and 10–15 strokes/min at ~100 psi was pumped to help push the tool string down. Initial improvement was observed, but at ~4000 m DRF, the progress slowed again and the ship's heave increased to ~3 m peak to peak, which significantly increased tension changes both at the surface and downhole. At this depth, the normal tool weight of 1200 lb could not be picked up, and the tool weight in fluid was ~300–500 lb, dipping to below 0 lb with heave. At this point, a decision was made to pull out of the hole to check the cable and tools as a precaution and to consider rigging down and pumping with high pressure with the tools out of the hole. The radioactive source was removed, the tool string was rigged down, and the high-pressure lines were attached to the top of the drill pipe. An initial pressure of 1000 psi was applied and the pressure dropped to 800 psi, potentially indicating a restriction had cleared. Pump pressure was increased to 1600 psi and 2500 strokes of seawater was pumped for ~30 min to make sure any potential mud was cleared of the pipe.

HNGS-HLDS-DITE

The second attempt to deploy the tool string was performed without the APS to reduce the potential drag from the APS bowspring. This wireline tool string was 22 m long. The radioactive source was installed, and tools were run in the hole by 1400 h on 19 October. During rig up, the weather conditions deteriorated to ~56 kt winds and 4 m peak to peak heave. The second tool string descent slowed again as the head tension decreased at ~3600 m DRF. Pump pressure was used again, and initially it appeared to help the tool string's descent. The increased heave conditions during this descent caused surface tension fluctuations of 2000–3000 lbf, with a maximum of slightly >8000 lbf and small fluctuations on the downhole head tension. From 4000 to 4122 m DRF (Fig. F48), the head tension was an average of 0 lbf, and after the pumps were turned off, the head tension decreased to between –200 and –300 lbf. Several possibilities were discussed on the cause of the problem with no clear answer for what was causing the apparent decrease in downhole head tension. The heave and tension fluctuations made going into open hole a very risky decision, particularly with the pipe potentially being able to sever the wireline if the wireline had slacked conditions near the end of the pipe. Because time was running out and without knowing for sure whether the low-tension problem was caused by a heavy mud buildup inside pipe or a possible logging head load cell problem, it was decided terminate the logging operations.

The tool string was brought back to surface, the radioactive source was offloaded, and the individual tools were safely rigged down without any note of physical damage to tools or wireline. The bottom-hole assembly (BHA) and pipe were later brought to surface and no anomalies were noted. Preliminary assessments of the downhole data show a significant decrease in head tension beginning at ~3200 m DRF and having significant negative values below 4000 m DRF (Fig. F48). The cable head was tested postcruise and the results show temperature- and pressure-dependent problems caused an erroneous decrease in head tension.

Preliminary results

The downlog inside the BHA recorded seafloor at 4074 meters below rig floor, which is 7 m deeper than the drillers seafloor depth, and ~27.7 m of the shallow sediments (Fig. F49). The gamma ray measurements in the shallow sediments show an anomaly from seafloor to ~25 m wireline matched depth below seafloor. The contributions to this anomaly are mainly an increase in Th and a smaller contribution from U (Fig. F49).

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