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Downhole logging

The Expedition 334 downhole logging program was designed to complement the core sample record by measuring continuous in situ profiles of physical properties such as bulk density, porosity, resistivity, and NGR. In addition to these formation properties, downhole logging provides oriented images of the borehole wall that are useful in determining the directions of bedding planes, fractures, and borehole breakouts. With the conventional technique of wireline logging, downhole measurements are taken by tools lowered in a previously drilled borehole. Wireline logging has had limited success in deep holes in unconsolidated or fractured clastic sequences because these holes tend to be unstable after drilling. It may be difficult to lower wireline tools in an unstable borehole, and hole irregularity can compromise the quality of the measurements. Using LWD, downhole measurements are taken by instrumented drill collars in the BHA near the drill bit. Hence, LWD measurements are made shortly after the hole is drilled and before the adverse effects of continued drilling or coring operations. LWD has been successful in previous scientific drilling expeditions to convergent margins, such as Barbados (ODP Legs 156 and 171A), Costa Rica off Nicoya Peninsula (ODP Leg 170), and Nankai Trough (ODP Leg 196 and IODP Expeditions 314, 319, and 332) (Shipley, Ogawa, Blum, et al., 1995; Moore, Klaus, et al., 1998; Kimura, Silver, Blum, et al., 1997; Mikada, Becker, Moore, Klaus, et al., 2002; Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009; Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010; Kopf, Araki, Toczko, and the Expedition 332 Scientists, 2011). LWD was selected as the logging technique for Expedition 334. The LWD equipment used during this expedition was provided by Schlumberger Drilling and Measurements under contract with the Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG).

Logging-while-drilling tools

LWD tools are supplemented by a measurement-while-drilling (MWD) tool that is located in the midst of the LWD tools in the BHA and measures downhole drilling parameters and well bore direction. The MWD tool also transmits a limited LWD data set by acoustic telemetry to the surface for real-time monitoring. Complete LWD data are recorded into downhole computer memory and retrieved when the tools are brought to the surface. The term LWD is often used generically to cover both LWD- and MWD-type measurements, tools, and systems, and we follow this convention here.

LWD tools are powered by batteries or a drilling fluid turbine and store logging data in nonvolatile memory chips. The tools take measurements at regular time intervals and are synchronized with an acquisition system on the drilling rig that matches time with drilling depth. Drilling depth is determined using a drawworks encoder that measures the vertical motion of the top drive. After drilling, the LWD tools are retrieved and their data downloaded. The Schlumberger Maxwell logging system merges time-depth data (from the surface system) and the downhole time-measurement data (from the tools) into depth-measurement data files. Data files are then transferred to LDEO-BRG onshore for further processing. Processing includes depth shift to the seafloor, corrections to certain logs, documentation (with an assessment of log quality), and conversion of the data to ASCII format for the conventional logs and to GIF for the images. Schlumberger GeoQuest’s GeoFrame software package is used for most of the processing. The data are transferred back to the ship within a few days of logging and made available in ASCII and DLIS formats through the shipboard IODP logging database.

The Schlumberger LWD tools used during Expedition 334 were the geoVISION 675 (near-bit electrical resistivity, resistivity images, and NGR), the arcVISION 675 (annular borehole pressure, resistivity, and NGR), the adnVISION 675 (neutron porosity and azimuthal measurements of ultrasonic caliper and of bulk density), and the MWD TeleScope 675 (drilling mechanics data and real-time telemetry). All of these tools had a 6¾ inch (17.1 cm) diameter and were located above an 8½ inch (21.6 cm) drill bit. Some tools had stabilizers to centralize the collars and keep measurement sensors near the borehole wall. Figure F16 shows the configuration of the LWD BHA, with the depth of the measurements relative to the bit, and Table T7 lists the principal measurements recorded by each tool. The measurement principles of these LWD tools are described below. More detail on the physical principles and methods of downhole logging is given by Ellis and Singer (2007).

geoVISION tool

The geoVISION resistivity tool (also known as resistivity at the bit) provides laterolog-type resistivity measurements of the formation and high-resolution electrical resistivity images of the borehole wall. The tool uses two transmitter coils and a number of electrodes to obtain several measurements of resistivity (Bonner et al., 1996):

  • Bit resistivity: the lower transmitter coil generates a current that flows through the bit and into the formation, returning to the drill collar far up the tool string. By measuring the axial current through the bit for a given voltage, resistivity near the bit is determined by Ohm’s law.

  • Ring resistivity: the upper and lower transmitter coils produce currents in the collar that flow out of the tool at the ring electrode. In a homogeneous medium, these currents flow perpendicular to the tool. In a heterogeneous formation, this radial current flow is distorted and the current pattern generated by the upper and lower transmitters is adjusted to focus current flow into the formation. A high-resolution resistivity measurement is taken by measuring the amount of current leaving the tool at the 4 cm thick ring electrode.

  • Button resistivity: the same focusing process used in measuring the ring resistivity is applied to determine the resistivity at three 1 inch (2.5 cm) button electrodes. Button resistivity measurements made as the tool rotates in the borehole are stored and processed to produce a resistivity image of the borehole wall. The button electrodes measure resistivity at three depths of investigation and thus generate three resistivity images: shallow, medium, and deep. The tool uses the geomagnetic field to orient the resistivity images to magnetic north.

During Expedition 334, the geoVISION tool sampled image data every 5 s and the resistivity images were computed at 1 inch (2.5 cm) intervals. To maximize the spatial resolution of the images, the recommended rate of penetration is 2.5 cm every 5 s, or 18.3 m/h. Images with adequate spatial resolution, however, can be acquired at lower drilling rates, trading off resolution for drilling time.

The geoVISION tool also contains a scintillation detector that provides an azimuthal NGR measurement. The NGR log is indicative of clay content in a clastic sedimentary sequence because sands typically have a relatively low content of radioactive elements compared to clay minerals.

adnVISION tool

The adnVISION tool (azimuthal density neutron) measures bulk density, neutron porosity, and borehole diameter (Evans et al., 1995). As the tool rotates, it acquires data in azimuthal sectors around the borehole. The density section of the tool uses a 137Cs gamma ray source and a near and a far scintillation detector that provide a borehole-compensated measurement. The density source and detectors are positioned in the fin of an 8.5 inch (20.6 cm) stabilizer. This geometry forces the sensors against the borehole wall, thereby reducing the effects of borehole irregularities. Whereas high-energy Compton scattering of gamma rays is a function of the bulk density, returns of scattered low-energy gamma rays also depend on the atomic number and are converted to a photoelectric effect (PEF) measurement that is sensitive to lithology. In a clastic sequence, PEF will be lower in sand-rich intervals (the PEF of quartz is 1.8 b/e) and higher in intervals containing clay, mostly because of Fe in clay minerals (Fe has a high PEF of 31.2 b/e) (Ellis and Singer, 2007).

Neutron porosity measurements are obtained by emitting high-energy fast neutrons from an americium oxide beryllium (AmBe) source and measuring low-energy epithermal and thermal neutrons in near- and far-spacing detectors. Hydrogen nuclei have a mass close to that of neutrons and are most efficient in slowing the fast neutrons emitted by the source. The neutron log estimates porosity from the density of hydrogen nuclei and gives an accurate porosity in clean formations (e.g., quartz sands) where almost all hydrogen is in formation waters. Conversely, hydrogen in clay mineral hydroxyls contributes to the slowing of neutrons and results in an overestimate of porosity in shales (Ellis, 1986).

The adnVISION tool measures tool standoff and borehole diameter with an ultrasonic caliper. In addition, the adnVISION computes an azimuthal density caliper based on the differences in density determined by the near and far detectors, which have different sensitivities to the standoff between the tool and the borehole (Labat et al., 2002). A standoff of <1 inch (2.5 cm) between the tool and the borehole wall indicates good borehole conditions, for which the density log values are considered to be accurate to ±0.015 g/cm3. The azimuthal density measurements are processed to obtain full-coverage images of bulk density and borehole radius. The images display 8 or 16 azimuthal measurements of density and borehole radius (from density and ultrasonic measurements).

arcVISION tool

The arcVISION tool (array resistivity compensated) measures propagation resistivities. Electromagnetic waves are attenuated and phase-shifted when they propagate in an electrically conductive medium, and the degree of attenuation and phase shift depends on the resistivity of the formation (Bonner et al., 1995, 1996). Phase-shift resistivity has relatively high vertical resolution and a shallow depth of investigation, whereas attenuation resistivity has lower vertical resolution and a greater depth of investigation. The dual-frequency (2 MHz and 400 kHz) array of coils in the arcVISION makes 10 phase-shift and 10 attenuation measurements at five transmitter-receiver separations of 16, 22, 28, 34, and 40 inches (40.6, 55.9, 71.1, 86.4, and 101.6 cm), which correspond to several depths of investigation. For a given frequency, the vertical resolutions of phase-shift resistivities measured at different transmitter-receiver separations are similar. The arcVISION also measures the NGR of the formation and the pressure of the borehole fluid in the annulus (the space between the drill string and the borehole wall). During Expedition 334, the annular pressure measurement was monitored while drilling for safety (see below).

TeleScope tool

The TeleScope MWD tool transmits data uphole through the fluid in the drill pipe in a process known as “mud-pulse telemetry.” A modulator in the tool generates a continuous 12 Hz pressure wave within the drilling fluid and changes the phase of this signal to transmit bit words encoding various measurements made by the MWD tool or by other LWD tools in the BHA. Two pressure transducers attached to the standpipe (one near the top and a second near the bottom) on the rig floor acquire the pressure signal that is then decoded by the Schlumberger surface software. The MWD real-time data transmission rate is adjustable, depending primarily on water depth and drilling fluid density, and was 6 bits/s during Expedition 334.

In addition to transmitting uphole selected measurements from the other LWD tools, the TeleScope acquires operational and drilling mechanics data, including collar rotation per minute, drilling fluid turbine rotation per minute, stick and slip, and axial and torsional vibration. The TeleScope also contains a turbine that powers the entire LWD string when drilling fluid is circulated at a sufficient flow rate (~300 gal/min, or 18.9 L/s, in the TeleScope tool used during Expedition 334).

Gas monitoring with real-time LWD data

During Expedition 334, LWD logs were acquired in the first hole drilled at each site. As these holes were drilled without coring, the LWD data had to be monitored to detect gas entering the well bore. This LWD monitoring procedure substitutes the IODP standard of using gas ratio measurements made on cores for hydrocarbon safety analysis (Pimmel and Claypool, 2001). The LWD monitoring protocol used during Expedition 334 was similar to protocols used during previous IODP expeditions where LWD holes were drilled before coring, such as IODP Expeditions 308 (Gulf of Mexico hydrogeology) and 311 (Cascadia margin gas hydrates) (Flemings, Behrmann, John, and the Expedition 308 Scientists, 2006; Riedel, Collett, Malone, and the Expedition 311 Scientists, 2006).

The primary measurement used for gas monitoring was annular pressure while drilling, measured downhole and transmitted to the surface in real time by the arcVISION LWD tool. Free gas in the borehole lowers the borehole fluid density and decreases the pressure. The monitoring procedure consisted primarily in monitoring variations of annular pressure while drilling over a baseline hydrostatic pressure trend. A sustained drop in pressure greater than a specified threshold required drilling to stop and circulation of a full volume of the borehole annulus while monitoring pressure. If the pressure remained static and equal to the hydrostatic pressure trend, drilling could be resumed. If the pressure was lower than hydrostatic, the protocol required killing the hole with weighted mud and abandoning the hole.

The threshold pressure drops requiring attention were defined from a pressure decrease chosen to ensure that gas flow in the well could be killed with weighted mud. The threshold pressure drops were 30 psi (0.207 MPa) in the 0–100 mbsf interval, 35 psi (0.241 MPa) in the 100–200 mbsf interval, and 50 psi (0.345 MPa) in the 200–1000 mbsf interval.

Pressure decreases caused by gas flow into the borehole may also be preceded by a pressure increase as the result of acceleration of fluids in the annulus (e.g., Aldred et al., 1998). The protocol required close monitoring of the annular pressure if pressure increases over the hydrostatic trend were observed. Such a pressure increase could be the result of the aforementioned precursor or be induced by drilling (e.g., mud sweeps or packing off of cuttings). A drilling-induced pressure increase would be resolved by cleaning the hole, whereas the gas flow precursor event would be followed by a pressure decrease, leading to appropriate response as in the procedure above.

Gas flow into the borehole should cause a sustained decrease in annular pressure. There could also be occasional, brief pressure drops that are not caused by gas entry but are induced by drilling (e.g., pressure drops caused by flushing cuttings out of the hole). These transient pressure drops did not require the preventive actions described above.

Interpretation of LWD data

Log characterization and logging units

LWD logs provide in situ petrophysical information on the rock formation and interstitial water while the hole is being drilled. These measurements are sensitive to changes in formation properties such as composition, texture, and structure. Compositionally influenced logs such as NGR and PEF were used to estimate lithologic variation with depth. Borehole images provided useful information on geological features such as bedding, sedimentary structures, bed boundaries, unconformities, fractures, and faults. The characterization of logging data allows the borehole to be zoned into distinct logging units based on intervals of different log responses that are commonly associated with lithostratigraphic units. For Expedition 334, the aim was to provide a preliminary assessment of formation properties and lithostratigraphy from LWD data prior to coring.

Structural and breakout analyses from borehole images

The LWD tools used during Expedition 334 collected full-coverage borehole images of electrical resistivity, NGR (geoVISION), bulk density, and borehole radius from density and ultrasonic measurements (adnVISION). These images are typically plotted as an unwrapped cylindrical borehole wall and are referred to true north using a magnetic compass in the tools (Fig. F17). The resistivity images have the highest spatial resolution and can be used to map bedding planes, sedimentary structures, and fractures.

On unwrapped images taken in a vertical borehole, horizontal planes (e.g., bed boundaries) show up as horizontal straight lines, whereas dipping planes (e.g., fractures) as sinusoidal curves (Fig. F17). The dip direction is the azimuth of the lowest point in the sinusoidal curve, and the dip angle is tan–1(h/d), where h is the amplitude of the sinusoidal curve and d is the borehole diameter. The interpretation of a large number of dipping planes can be aided by software that lets the user fit sinusoidal curves to features in the images (e.g., the BorView module in Schlumberger GeoQuest’s GeoFrame software package).

Resistivity image data were displayed as statically and dynamically normalized images. Static normalization displays the image with a color range covering all resistivity values for the entire logged interval; this displays absolute changes in resistivity throughout the borehole and is useful for identifying lithologic or facies changes. Dynamic normalization scales the color range for resistivity values in a moving depth window (1 m), highlights small-scale, subtle variations in resistivity, and is commonly used for detailed identification of fractures and formation structures.

Borehole breakouts are subvertical hole enlargements that form on opposite sides of the borehole wall by local drilling-induced failure caused by nonuniform stress. In a vertical borehole, the breakout direction is parallel to the minimum principal horizontal stress orientation and perpendicular to the maximum principal horizontal stress orientation (Bell and Gough, 1983). Therefore, borehole breakouts are key indicators of the state of stress in the subsurface. LWD images of resistivity and borehole radius clearly display borehole breakouts as two parallel, vertical bands of low resistivity or large borehole radius 180° apart. Borehole breakouts were mapped using GeoFrame software, and further quantitative analysis can be conducted postexpedition.

Core-log-seismic integration

To correlate core and log data acquired at depth with seismic reflection measurements that are a function of traveltime, a depth-traveltime relationship must be determined at each drill site. This relationship can be estimated with synthetic seismograms, which are computed by convolving a seismic impulse function with reflection coefficients obtained from contrasts in P-wave velocity and density. These velocities and densities may be measured in situ with downhole logs or on cores in the physical properties laboratory. Synthetic seismograms can be calculated using seismic interpretation packages. For example, IESX, part of the Schlumberger GeoFrame software suite, allows interactive adjustment of the depth-traveltime relationship until a good match is achieved between reflectors in the synthetic seismogram and in the measured seismic data. While P-wave velocities were not measured by the suite of LWD tools deployed during Expedition 334, velocities measured on cores and LWD densities could be combined to estimate reflection coefficients. Should the quality of the shipboard core and LWD measurements be sufficient, synthetic seismograms can be produced postexpedition.