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Publications > Expedition Publications > Logging Summaries

Logging Summaries

IODP Expedition 327:

Juan De Fuca Ridge-Flank Hydrogeology

Expedition 327 Scientific Party

Introduction

    IODP Expedition 327 revisited the Juan de Fuca ridge in order to continue studying fluid-rock interactions in the upper ocean crust, hydrologic properties, connectivity of crustal compartments, ridge-flank circulation, crustal alteration, and geomicrobial processes.

    To that end, Hole U1362A was drilled, logged, cased and completed with an instrumented CORK after permeability tests; Hole U1362B was drilled, cased and completed with an instrumented CORK after a pumping and tracer injection experiment; and an instrument string was recovered and replaced at Hole U1301B.

    The logging program for U1362A was designed to (1) identify suitable intervals for setting the packer elements during hydrologic testing and CORK installation, and (2) continue to quantify crustal lithostratigraphy, alteration and hydrogeologic and petrophysical properties. Wireline was also used with a new electronic release system to deploy the instrument string in Hole U1301B.

     

Logging Tools

    Figure 1. Configuration of the wireline logging string that was run in Hole U1362A. Lengths are in meters.

    A single tool string (Figure 1) wasdeployed in the 9-7/8 hole section in the basement of Hole U1362A: the Logging Equipment Head (LEH-QT) transmitted downhole tension measurements and contained an electrode for generating a qualitative spontaneous potential (SP) curve; the Hostile Environment Gamma Ray Sonde (HNGS) acquired spectral gamma ray data; the Hostile Environment Litho-Density Sonde (HLDS) provided single-arm long-axis caliper and standard formation bulk density measurements; the Modular Temperature Tool (MTT) acquired borehole fluid temperature measurements; the General Purpose Inclinometry Tool (GPIT) provided downhole acceleration data as well as toolstring and borehole orientation data; and the Ultrasonic Borehole Imager (UBI) provided high-resolution borehole images.

 

Logging Operations

    The 9.875 inch open hole section was drilled to 528 mbsf and cleaned with a succession of wiper trips. The hole was not displaced to mud. The drillstring was pulled to surface and made up with a new packer BHA before being run back inside the 10.75 inch casing. The wireline toolstring was then run out of the BHA into casing where the wireline heave compensator (WHC) was started. Surface heave was ~0.9 m pk-pk and the downhole motion (~180% of surface motion) didn't diminish once the WHC was started. The wireline string was then run to bottom while borehole fluid acoustic velocity and temperature were being logged.

    Three logging passes were made at varying speeds using different UBI settings. The first was run at 600 ft/hr using conservative UBI settings in an attempt to acquire usable data over as much of the open hole as possible (hole ID of less than 13 inches was expected over only a few intervals). The second pass was logged using higher-resolution UBI settings at 400 ft/hr over the best hole sections and at 800 ft/hr over the larger hole interval. The third pass was conducted with the highest-resolution UBI settings at 400 ft/hr through the good interval, then 2000 ft/hr thereafter.

    A fourth pass was conducted to investigate a possible tight hole interval. Just before starting the pass, the toolstring was lowered, damaging the extended HLDS caliper extended: the caliper could no longer be closed with surface commands though it continued to read hole diameter. Because of the damaged caliper, the planned WHC testing period was cancelled and the toolstring and was pulled to surface to complete logging operations.

 

Logging Results

    Caliper

    Figure 2. Selected wireline logging measurements from Hole U1362A. All data are from the second logging pass. The mechanical single-arm caliper from the HLDS is displayed. Total spectral gamma ray and computed potassium content are presented. Depth reflects the WMSF scale for logs and the CSF-A scale for core measurements.
    Figure 3. HLDS caliper readings from the three logging passes at Hole U1362A. Undergauge hole is seen during the first pass. During the second pass, the apparent tight interval is shorter and of smaller diameter. No undergauge intervals are seen with the third pass.
    The HLDS caliper identified a consistent near-gauge interval at 447 mbsf and a second, less consistent interval at 417 mbsf (Figure 2). These zones correlate well with two of those predicted from core recovery and drilling mechanics. Because the HLDS caliper has a maximum reach of roughly 18 inches, the hole size in the 14.75 inch rathole is possibly much larger.

    The 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. Where the ultrasonic caliper values appear meaningful, they indicate a nearly circular borehole through the near-gauge sections. The major axis, which represents the direction minimum horizontal stress, lies roughly north-south.

    While the HLDS caliper was generally highly repeatable, a notable anomaly was observed during the first pass: an apparently undergauge 7.7 inch OD interval at 363.5 mbsf (Figure 3). On the second pass, the apparent tight interval had reduced to only 5.5 inch OD and was observed at 376 mbsf. Two new caliper spikes were observed at 359 and 413 mbsf. The third and fourth passes of the HLDS caliper observed no undergauge intervals, nor did the ultrasonic caliper. It's possible that the apparent tight interval represent sloughed material falling down the borehole, or that debris was somehow affecting the mechanical caliper arm.

     

    Natural Gamma Ray

    Figure 4. Total gamma ray in the 9-7/8 inch section of Hole U1362A is strongly influenced by potassium content.. .
    Figure 5.
    1. HLDS caliper and HNGS potassium content vs depth in Hole U1362A.
    2.HLDS caliper and HNGS potassium content correlate poorly over the length of the 9-7/8 inch section, except for the interval from 455 to 470 mbsf.

    Gamma ray measurements repeat well overthe three passes. In the open hole, total gamma ray values are low (2.1 to 7.4 API, typical of basaltic oceanic crust) and are driven by potassium content (Figure 4).

    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 weaker, fractured zones of greater alteration and may be indicative of focused hydrothermal fluid flow. Correspondingly, the intact, in-gauge interval at 455 mbsf has low gamma ray values. Over most of the open hole section, however, HNGS potassium and HLDS caliper correlate inversely (Figure 5). The apparent reduction in HNGS potassium concentration may be due to insufficient correction for borehole size during processing.

    A pronounced increase in HNGS gamma ray values is observed at the casing shoe (Figure 2). This is likely a measure of the casing cement that appears to contain significantly increased concentrations of uranium and thorium relative to background formation.

     

    Density and photoelectric factor

    Wireline density and photoelectric factor (PEF) measurements 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 (Figure 2). Where the hole is near-gauge, HLDS density is consistent with MAD core sample measurements.

    Temperature

    Temperature data were acquired while running into the hole and during the three upward logging passes, revealing a highly repeatable borehole fluid temperature profile (Figure 2). The marked gradient increase at the top of the near-gauge interval may be indicative of more conductive conditions or suggests that seawater flowing into the well isn't mixing below that depth. A short temperature anomaly (a ~0.5C rise followed by a 0.2C drop) is observed 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 14.75 inch rathole section where the hole is beyond the reach of the HLDS caliper.

     

    Ultrasonic Imaging

    Figure 6. Selected UBI ultrasonic images from Hole U1362A

    The ultrasonic borehole images are marred by tool motion (while logging, downhole heave displacement averaged 0.8 m with excursions greater than 1.5 m). Post-logging processing helped mitigate those tool motion effects but they are still pronounced and visible as smears and truncations (Figure 6). Moreover, the UBI sonde head was undersized so no images can be expected where the hole is even moderately out of gauge. Where the hole is near-gauge, certain centimeter- to meter-scale dipping sinusoidal features are observed. The images from the third pass, which were collected using the highest-resolution UBI configuration, discern an interval of pillow basalt lying above a zone of sheet basalt at 458 mbsf.

     

    Spontaneous Potential

    The SP data from the three logging passes repeat very well, exhibit little noise and appear insensitive to hole condition (Figure 2). The SP measurement was developed for wells drilled into sediment formations on land and in Expedition 327 the measurement is impaired by the absence of a proper surface ground, by stray current at the rig, and by the comparable salinities of the drilling fluid and formation waters. More work needs to be done to interpret deepwater SP data.

 

    Stefan Mrozewski: Logging Staff Scientist, Borehole Research Group Lamont-Doherty Earth Observatory of Columbia University, PO Box 1000, 61 Route 9W, Palisades, NY 10964, USA
    Email: Stefan Mrozewski

     


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