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Insights from scientific riser operations

Expedition 319 marked the first riser drilling in IODP history. As noted above, this allowed us to conduct several scientific operations for the first time in IODP, including measurement of in situ stress, permeability, and pore pressure using the MDT tool, real-time analysis of mud gases, and analysis of cuttings for sedimentological, biostratigraphic, chemical, structural, and physical property data. Here, we briefly discuss some of the key insights gained from each of these new endeavors.

Stress, permeability, and pore pressure from MDT and LOT measurements

During Expedition 319, we deployed the MDT wireline logging tool in riser Hole C0009A to measure in situ formation pore pressure, formation permeability (often reported as mobility = permeability/viscosity), and in situ stress magnitude at several isolated depth intervals. This was the first time that the tool had been used in IODP drilling because it is currently not usable with the small-diameter riserless drill pipe used by IODP. We conducted nine single probe tests to measure formation pore pressure and fluid mobility and three dual packer tests (one drawdown test to measure formation hydraulic properties and two hydraulic fracturing tests to measure the least principal stress magnitude) (cf. Fig. F9). Successful deployment of this tool to measure in situ pore fluid pressure and stress magnitude deeper within the accretionary prism and in the vicinity of major fault zones in future riser holes will constitute a major breakthrough in understanding subduction zone fault mechanics and is a critical part of the NanTroSEIZE program. However, there are limitations in using the MDT tool successfully that warrant consideration, most notably that pore pressure and permeability measurements may be unreliable in low-permeability formations (k < ~10–15 m2) because the time required for pressure equilibration can greatly exceed the potential deployment time. Expedition 319 operations and results also highlight the value of obtaining FMI or other borehole imaging data prior to running the MDT in order to select measurement targets that will yield meaningful data.

Sampling and analysis of cuttings and mud gas

We collected samples from the riser drilling mud, including cuttings, mud, and mud gas, during riser drilling at Site C0009. The results of shipboard analyses will provide guidance for future IODP riser drilling by serving as a blueprint for handling, treatment, and analysis approaches and by demonstrating the types and quality of data that can be obtained from these materials, especially for poorly lithified shallow sediments and sedimentary rocks. In addition to basic lithological description and microfossil analysis to define ages, for sufficiently lithified cuttings (deeper than ~1000 m MSF in ~2.5 Ma claystones at Site C0009) we were able to document deformation features preserved in cuttings, measure physical properties including porosity and density, and measure composition by XRD, XRF, and carbonate analyses. We also conducted several experiments on cuttings and core samples to investigate the sensitivity of shipboard measurements to cuttings processing techniques (including the composition of fluid used for washing and the soaking time), to the cuttings size fraction(s) used for measurements, and to different drilling modes and mud compositions.

One concern in using drilling mud and cuttings for scientific analysis centered on the ability to obtain intact cuttings from shallow (<2000 m MSF) sediments and sedimentary rock and on uncertainty about the material's depth of origin. Comparisons between cuttings (collected from 5 m depth intervals) and wireline log data at Site C0009 indicate a likely depth uncertainty of ~10 m, but mixing during mud ascent and/or cavings from uphole may occur over larger distances. A second concern was that cuttings may be preferentially preserved from particular lithofacies (i.e., more lithified or cohesive claystones) and thus provide a biased sampling of the formation. Initial results suggest that this is likely the case. We conclude that sedimentological and geochemical data from cuttings (at least over the depth range we encountered) are useful for some applications, including the definition of overall lithofacies from relative abundance of lithic fragments, mudstone cuttings, and disaggregated sands, or the characterization of mudrock chemistry, provenance, and compositional variations. Data from cuttings may be less meaningful for other applications such as quantitative grain size assessment, detailed lithofacies characterization, chemical analyses if there are effects from interaction with drilling mud, and physical property measurements. This emphasizes the importance of collecting complementary data sets (e.g., spot cores for calibration and log data) to supplement cuttings analysis.

Based on comparison between cuttings, cores, and logs, we also noted that the absolute values for some measurements on cuttings samples do not accurately reflect formation properties, although trends in these data sets show good agreement with equivalent log or core data in some cases. This is particularly true for physical properties and some geochemical analyses. For example, the high pH of the drilling mud likely affects carbonate, calcium, and carbon content values and as a result also generates errors in relative abundance of mineral phases defined by bulk XRD. However, variation in carbonate content downhole does not correlate with mud pH variation. This chemical contamination correlates with the time the cuttings were exposed to drilling mud in the borehole. Similarly, porosity values from cuttings are anomalously high, both with respect to their depth of origin and in comparison to log and core data; the discrepancy varies with sample handling and washing procedures and with soaking time. Thus it appears that the absolute values of compositional and physical property data from cuttings are unlikely to be accurate, but overall downhole trends might be reliable.

We also monitored mud gas chemistry in real time throughout riser drilling operations to document the composition and concentration of gas released from the formation as it was drilled. This method, used previously during International Continental Scientific Drilling Program drilling (e.g., Erzinger et al., 2006), was used in scientific ocean drilling for the first time during Expedition 319. One example of the value and reliability of these data comes from comparison of the drilling mud gas, cuttings, and wireline logging data in Subunit IIIB (Figs. F5, F8). Increased mud gas methane concentrations are clearly correlated with increased wood content in cuttings; gas concentrations are also tightly correlated with several intervals of low VP/VS and Poisson's ratio observed in sonic velocity logs (Fig. F8). Because pore water geochemical analyses are not possible on cuttings and are difficult in lithified or low-porosity core samples, mud gas analysis offers a promising approach for characterizing formation fluids in future deeper riser drilling. These data are especially important for understanding hydrologic and geochemical processes associated with faulting and fluid flow.

Observatory installation and preparation

Dummy run

The dummy run test in Hole C0010A simulated operational procedures for installation of planned permanent future observatories. The permanent observatories include three major components: bottom-hole instruments, tubing to provide structural support for downhole cables and hydraulic lines, and a circulation obviation retrofit kit (CORK) wellhead (Becker and Davis, 2005), which lands at the casing hanger, seals the borehole, and suspends the instruments below (Fig. F12A, F12B). In this test, we successfully confirmed operational procedures to make up the bottom-hole instruments and lower them into the water, which went smoothly. We were initially concerned that the weak surface of the strainmeter might hit and be damaged by the guide funnel below the rotary table as the tubing below the strainmeter entered the ocean and was exposed to the Kuroshio Current. However by adjusting the length of tubing prior to assembly we were able to solve this problem by trial and error.

The instruments were subjected to significant vibration on the drill string as the Chikyu drifted to the site for reentry because of the Kuroshio Current. This resulted in damage to the accelerometer and loss of a seismometer and strainmeter during the two dummy runs. Before the experiment, vibration on the drill string was acknowledged as a potential issue but not emphasized as the primary concern. The tubing and the instrument string are much weaker than drill pipe, and therefore vibration was amplified in these weak sections through resonance. Effects were so strong that some vital parts of instruments (such as hinges and pivots in the seismometers) may not maintain their performance after installation in the hole, even if we could modify design of the instrument carrier to maintain its integrity during installation. In addition, we did not test the entire planned instrument string, which will include a CORK head and >500 m of hydraulic tubing and cables. This experience necessitates a more complete evaluation of the observatory design and installation. Evaluation and modifications are necessary to address several issues, including: (1) sensor integrity under vibration, (2) resonance effects and stress in sections of the observatory under vibration, and (3) tolerance of downhole cable and hydraulic lines to stress and vibration. Acceleration data from the dummy run test in this expedition will provide valuable information for these evaluations.

There are also other options for installation of long-term observatories that should be considered. The smart plug installed in this expedition (see "Temporary monitoring system") offers one encouraging option for intermediate to long-term emplacement of robust but simple retrievable observatories. In a modified smart plug incorporating a seismometer, continuous seismic observations for a period of 2 y would be possible in conjunction with pressure and temperature observations. Such an installation would still require evaluation of effects from vibration on seismometers and pressure gauges because, like the bottom-hole instrument string for the dummy run test, the smart plug is run into the borehole using drill pipe. Another option would be to separate the tasks of lowering sensors into the hole from lowering the observatory to the seafloor in order to reduce effects from the ocean current-induced vibration. In this case, an observatory package that houses bottom-hole sensors and a downhole cable wound in a winch could be landed on the reentry funnel of the borehole, avoiding severe vibration of sensors and cables. The sensors could then be lowered to the bottom of the hole by wireline using the winch. Such an observation system has been developed for logging (Amitani et al., 2002) but is not used routinely in boreholes. Installation of cabled sensors into a borehole by controlled wireline has also been achieved (Stephen et al., 2003). In this case, the downhole cable was payed out under the seafloor station before reentry. This has the risk of damaging the cable by heave of the ship when the cable is lowered into the borehole. For any of these options, it is necessary to implement risk assessment in each stage of deployment.

Temporary monitoring system

After LWD, casing, and the dummy run test at Site C0010, we suspended the hole by installing a sensor package (smart plug) attached below a retrievable casing packer (Fig. F12C, F12D). The smart plug is a robust, retrievable, stand-alone instrument package designed with relatively short lead time in order to make use of suspended boreholes prior to final observatory installation. Although it is relatively simple, the smart plug at Site C0010 represents the first long-term monitoring in the NanTroSEIZE project and the first observatory element installed by the Chikyu. If successful, it will provide another tool for long-term hydrologic and/or thermal monitoring in ocean boreholes and short-term data at this site.

For the installation at Site C0010, the retrievable casing packer was placed above two screened casing joints that provide hydraulic communication with the megasplay fault zone (Fig. F12C). In this configuration, the smart plug will monitor pore pressure and temperature within the megasplay fault, and as a reference will also record the hydrostatic pressure (overlying ocean) (Fig. F12C). The hole completion relies on cement at the base of the casing shoe and in the annulus in the lower part of the hole, and on collapse of soft sediment and thrust wedge material against the casing over the ~400 m of annulus above the screens, to achieve hydraulic isolation from the sediments above and below, respectively. Upon recovery of the instruments (planned for 2010 or 2011), we will assess the efficacy of hydraulic isolation by comparison of the fault zone and hydrostatic pore pressure time series and by the response of the fault zone pore pressure to tidal loading (e.g., Wang and Davis, 1996). We also anticipate conducting a CBL as part of future operations to define the top of cement and to constrain the extent of formation collapse against the casing above the screens.

Despite strong ocean currents (up to 4.5 kt and persisting to a depth of several hundred meters below sea level), the smart plug was successfully run to the wellhead and set inside the casing. However, even for installation of this relatively simple and short sensor package, we encountered problems with the Kuroshio Current; upon running the drill pipe to the surface, the running tool sheared off from the drill pipe at the 3½ inch tubing connection, presumably as a result of VIV in the water column. One key difference between the smart plug and many previous hydrologic observatory installations in ODP (e.g., Becker and Davis, 2005) is that data cannot be downloaded from the sensor package until it is retrieved with the packer using a drillship. Thus, we cannot assess whether damage to the electronics or pressure sensors was sustained during running to the wellhead or hole reentry prior to instrument recovery.