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doi:10.2204/iodp.proc.319.102.2010

Logging

Logging data provide measurements of in situ properties in the borehole. Logging data with various depths of investigation into the formation depending on the tool are therefore complementary to centimeter-scale core studies and >10 m seismic images (Expedition 314 Scientists, 2009). We used the following logging methods during Expedition 319:

• Wireline logging: the logging tool is lowered into the open well bore on a multiple-conductor contra-helically armored wireline, and measurements are usually taken from the bottom of the hole upward. For depth correlation purposes, tension on the cable is maintained as constant as possible. Most wireline measurements are recorded continuously while the tool string is moving except for certain fluid sampling and pressure-measuring tools and a seismic array.

• LWD: measurements are taken with self-contained tools near the bottom-hole assembly (BHA). Data are recorded in memory while drilling the hole. MWD data are transmitted in real time to the surface by mud pulses, together with a few measurements made from the LWD tools.

During Expedition 319, wireline logging, LWD, and MWD were conducted in both riser and riserless holes. Measurements taken by LWD included azimuthal resistivity images and laterolog resistivity-at-the-bit (RAB). MWD measurements included rate of penetration (ROP), downhole torque, inclination/orientation of the hole, weight on bit (WOB), gamma ray emissions, and annular pressure while drilling (APWD) (collected only during 26 inch riserless hole drilling). Measurements from wireline logging included density/porosity, photoelectric factor (PEF), resistivity images from the Formation MicroImager (FMI), laterolog resistivity and SP, natural gamma and spectral gamma, sonic velocity (P- and S-waves), various types of calipers, mud resistivity, and temperature. In addition, the MDT was used to measure pore pressure, permeability, and stress. A walkaway VSP and zero-offset VSP were conducted using the Versatile Seismic Imager (VSI).

Logging while drilling and measurement while drilling

During Expedition 319, MWD tools were used in three holes: C0009A, C0010A, and C0011A. Figure F3 shows the configuration of the LWD/MWD BHA, and the set of measurements recorded from the tools are listed in Table T2.

MWD tools measure downhole drilling parameters, gamma radiation, and annular pressure and assure communication between tools. During drilling operations, these measurements are combined with surface rig floor parameters for drilling monitoring (e.g., WOB, torque, etc.) and quality control. The APWD sensor is included with the MWD sensors for safety monitoring and provides measurements of downhole pressure in the annulus, which are also converted to equivalent circulating density (ECD; density of the circulating drilling fluid when pumping). Downhole pressure and ECD are crucial parameters used to detect any inflow from the formation or obstruction (increase in APWD and ECD) or loss of circulation caused by permeable formations or faults or overbalancing of drilling mud (decrease in APWD).

The key difference between LWD and MWD tools is that LWD data are recorded into downhole memory and retrieved when the tools reach the surface, whereas MWD data and a selection of LWD data at lower resolution are transmitted through the drilling fluid by means of a modulated pressure wave (mud pulsing or fluid pulse telemetry) at a rate of 6 bits per second (bps) and monitored in real time at the surface. The term LWD is often used more generically to cover both LWD and MWD measurements, as the MWD tool is required during any LWD operation to provide communication between the LWD tools and the surface.

The LWD tools are battery powered and use erasable programmable read-only memory chips to store the logging data until they are downloaded. LWD tools take measurements at evenly spaced time intervals using a downhole clock installed in each tool. The depth tracking system on the surface monitors time and drilling depth. After drilling, LWD tools are retrieved and the data downloaded from each tool to a computer. Synchronization of the surface and downhole clocks allows merging of time-depth data (from the surface system) and downhole time-measurement data (from the tools) into depth-measurement data files, which then undergo further processing and analyses by the onboard logging scientists.

LWD/MWD systems

Depth tracking systems

The Schlumberger integrated drilling evaluation and logging (IDEAL) surface system for MWD tools records the time and depth of the drill string below the rig floor. MWD operations require accurate and precise depth tracking and the ability to independently measure and evaluate the position of the traveling block and top drive system in the derrick and the motion of the ship from waves/swells and tide action. The length of the drill string (combined length of the BHA and the drill pipe) and the position of the top drive in the derrick are used to determine the depth of the drill bit and ROP. The system configuration is illustrated in Figure F4A. A hook-load sensor measures the weight of the load on the drill string and can be used to detect whether the drill string is in-slips or out-of-slips. When the drill string is in-slips (i.e., the top of the drill string is hung on the rig floor by the "slip" tool and is detached from the top drive), motion from the blocks or motion compensator will have no effect on the depth of the bit and the drawworks encoder information will not augment the recorded bit depth. The heave of the ship will still continue to affect the bit depth whether the drill string is in-slips or out-of-slips.

The rig instrumentation system measures and records heave and motion of the active compensator's cylinder. The Chikyu uses a crown-mounted motion compensator (CMC) (Fig. F4A), which is installed on the top of the derrick to reduce the influence of heave on the drill string and to increase the accuracy of the bit weight measurement. The CMC is united with the crown block, which is a stationary pulley, and absorbs tension by moving the crown block up and down according to the ship's up and down motion. The crown block movement is absorbed by the change in the position of the horizontally overhung pulley even though the length of cable changes between the drawworks and the deadline anchor.

Onboard data flow and quality check

For each LWD/MWD operation, two types of data are collected: (1) real-time data that include all MWD data and selected LWD data and (2) LWD data that have been recorded downhole and stored in the tool's memory (Fig. F5). Data are originally recorded downhole at a preset sampling interval, and no depth information is recorded in the tool. The depth-referenced version is obtained after merging the time (downhole) with the time-depth relationship recorded on the surface by the IDEAL system. For LWD and MWD geoVISION tools, both time (log ASCII standard [LAS] format) and depth (digital logging interchange standard [DLIS] format) versions of the data are generated.

MWD and annulus pressure tools

The 8.25 inch (21 cm) diameter MWD PowerPulse and TeleScope tools serve a similar function; the TeleScope is a newer generation tool. They transmit data by generating a continuous mud wave within the drilling fluid and by changing the phase of this signal (frequency modulation) to convert relevant bit words representing information from various sensors (Fig. F6A), which is compressed and coded digitally in pressure pulses. Drilling fluid pulses are recorded on two pressure transducers mounted on the standpipe manifold (SPT1) and the gooseneck of the standpipe (SPT2) where they are automatically decoded and uncompressed using the horizon signal processing module (HSPM) and the IDEAL system by the field engineer (Fig. F6C). In the MWD fluid pulsing system, pulse rates range from 1 to 12 bps, depending primarily on water depth and fluid density. During Expedition 319, pulse rates of 3 bps were achieved for MWD-APWD operations and 6 bps for LWD.

MWD measurements are made using paired strain gauges, accelerometers, and lateral shock sensors near the base of the MWD collar. A list of the main MWD parameters is given in Table T2. During LWD operations, the mud pulse system also transmitted a limited set of data from the geoVISION LWD tool to the surface in real time.

LWD geoVISION tool

The geoVISION resistivity tool is based on RAB technology (Table T3) (Anadrill-Schlumberger, 1993). It provides resistivity measurements and electrical images of the borehole wall, calibrated in a homogeneous medium. In addition, the geoVISION tool provides a total gamma ray measurement (Fig. F6B).

The geoVISION tool is connected directly above the drill bit and uses the lower portion of the tool and the bit as a measuring electrode. This allows the tool to provide a bit resistivity measurement with a vertical resolution just a few centimeters longer than the length of the bit (28 cm; Fig. F3C). A 1½ inch (4 cm) electrode is located 102 cm from the bottom of the tool and provides a focused lateral resistivity measurement (ring resistivity) with a vertical resolution of 2–3 inches (5–7.5 cm). The characteristics of ring resistivity are independent of where the geoVISION tool is placed in the BHA, and its depth of investigation is ~7 inches (17.8 cm; diameter of investigation ≈ 22 inches). In addition, button electrodes provide shallow-, medium-, and deep-focused resistivity measurements as well as azimuthally oriented images, which can reveal information about formation structure and lithologic contacts. The button electrodes are ~1 inch (2.5 cm) in diameter and located on a clamp-on sleeve. The buttons are longitudinally spaced along the geoVISION tool to render staggered depths of investigation of ~1, 3, and 5 inches (2.5, 7.6, and 12.7 cm). Multiple depths of investigation allow quantification of invasion caused by drilling fluid and fracture identification (drilling induced and natural). Vertical resolution and depth of investigation for each resistivity measurement are summarized in Table T3. Drilling fluid resistivity and temperature are also measured (Schlumberger, 1989) for environmental correction of the resistivity measurements. The gamma ray sensor has a range of operability of 0–250 gAPI and an accuracy of ±7%, corresponding to a statistical resolution of ±3 gAPI at 100 API and ROP of 30 m/h. Its depth of investigation is between 5 and 15 inches.

The tool's orientation system uses Earth's magnetic field as a reference as the drill string rotates, thus allowing both azimuthal resistivity and gamma ray measurements. The azimuthal resistivity measurements are acquired with a ~6° resolution around the borehole, whereas gamma ray measurements are acquired at 90° resolution as the geoVISION tool rotates.

Wireline logging

During wireline logging at the riser site (Hole C0009A), a passive heave compensator system was used by switching hydraulic lines to the auxiliary tank and compensating the extra line, which is connected between the riser and rig (Fig. F4B). A compensating line (a fixed-length steel wire) is connected between the risers and the rig using special heavy-duty compensating sheave. Then the existing ship compensating system is disconnected by switching the hydraulic line from the rig heave compensating system to the auxiliary tank.

Wireline logging tools

The wireline logging tool strings and runs are shown in Figures F12 and F8 in the "Site C0009" chapter, components of the tools are shown in Figure F7, and the set of measurements recorded from the tools are listed in Table T4.

Total and spectral gamma ray tools

The gamma ray tool passively measures the natural gamma radiation emitted by the formation. The main natural radioactive sources are ⁴⁰K and isotopes of Th and U. All gamma ray tools give the total gamma ray emission in units of gAPI. Gamma radiation is mainly dependent on lithology, and gamma ray logs offer good repeatability. Gamma ray tools used in this expedition had a depth of investigation ranging from 24 to 61 cm (Table T5).

Hostile Environment Natural Gamma Ray Sonde

⁴⁰K and isotopes of the decay chain of radioactive isotopes of Th and U emit photons at different energies. The Hostile Environment Natural Gamma Ray Sonde (HNGS) tool uses spectroscopic analysis to determine the concentration of radioactive ⁴⁰K (in weight percent), Th (in ppm), and U (in ppm). The HNGS tool also measures U-free gamma ray emission (in gAPI, also called computed gamma ray emission). The HNGS uses two bismuth germanate scintillators for gamma ray detection. The tool response is affected by the tool standoff (distance between the sensor and the borehole wall) and the weight and concentration of bentonite or KCl within the drilling mud (KCl may be added to the drilling mud to prevent hydrous clays from swelling and obstructing the well). The spectral analysis filters out gamma ray energy below 500 keV to reduce sensitivity to bentonite and KCl in the drilling mud and to improve measurement accuracy. Environmental corrections are usually made during the data processing. This tool has a 24 cm depth of investigation and can also be used inside casing.

Platform Express

The Platform Express (PEX) tool string combines a Highly Integrated Gamma Ray Neutron Sonde (HGNS), a Three-Detector Lithology Density (TLD) tool and a Micro-Cylindrically Focused Log (MCFL) tool (Fig. F7) and is usually coupled with an azimuthal resistivity measurement or an induction image tool. TLD and MCFL are integrated in the single pad of the High-Resolution Mechanical Sonde (HRMS). In the Hole C0009A configuration, the supplementary electrical tool is the High-Resolution Laterolog Array (HRLA). The tool is provided with an accelerometer that enables real-time depth correction and accurate depth matching of the various sensors.

Highly Integrated Gamma Ray Neutron Sonde

Neutrons interact differently with matter depending on their energy. Fast neutrons (E > 10 keV) are scattered elastically, primarily by H atoms. They can also induce inelastic scattering, in which case the excited atoms release gamma rays. If the neutron energy becomes small enough (thermal neutron, E < 0.1 eV), they can be absorbed by the medium by thermal capture. The most efficient absorbers are Cl, B, and H. Thermal capture also releases gamma rays. The HGNS contains an Am-Be radioactive source that bombards the formation with fast neutrons (>10 keV). These neutrons are slowed by scattering and then captured. Epithermal neutron detectors quantify elastic scattering, and thus the H index and a porosity estimate are generated. Gamma ray detectors and thermal neutron detectors document the thermal capture.

The measurement by the HGNS sensor is dependent on hydrogen content, and hence water content. We note that clay-rich rocks contain intra-crystalline water. The measured value is also affected by water-based mud; therefore the tool is pressed against the borehole wall to reduce this effect.

Three-Detector Lithology Density tool

The TLD tool assesses the density of the formation by measuring the attenuation of a gamma ray flux. The gamma radiation will interact with matter according to the energy of the gamma ray photon. Gamma rays are emitted by the ¹³⁷Cs source at 622 keV. At that energy, photons will first interact with matter by Compton scattering. Their energy then decreases progressively, and gamma radiation is absorbed by matter by the photoelectric effect. The number of scattered gamma rays that reach the detectors is directly related to the number of electrons in the formation, which is related to bulk density. Porosity may also be derived from this bulk density if the grain density is known or assumed.

The TLD tool also measures the PEF caused by absorption of low-energy gamma rays. The PEF depends on the atomic number of the elements in the formation and is essentially independent of porosity; therefore it is an indicator of the chemical composition of the formation. PEF values can be used in combination with HNGS curves to identify different types of clay minerals. For the HNGS, gamma rays are partly adsorbed by the drilling mud. Therefore the gamma ray detectors are mounted on a shielded skid, which is pressed against the borehole wall by a hydraulically activated centralizing arm to produce good coupling. Both density correction and caliper measurement of the hole are used to check the contact quality. The TLD tool has a ¹³⁷Cs source and three detectors: a backscattering detector, a short spacing detector, and a long spacing detector (Fig. F7).

Micro-Cylindrically Focused Log tool

The MCFL tool includes a cylinder electrode on the same pad as the TLD tool. The source electrodes emit a highly focused beam (2.5 cm) that rapidly diverges and penetrates the formation as deep as 10 cm. The tool gives information on the mud cake resistivity, which is useful for the lithodensity correction of the TLD tool (Fig. F7E).

High-Resolution Laterolog Array

The HRLA combines four electrodes to provide five independent, actively focused, depth- and resolution-matched measurements to resolve true formation resistivity (R_t) in thinly bedded and deeply invaded formations in addition to a shallowest mode (Mode 0) for mud resistivity. Modes 1 to 5 have increasing depths of investigation. The supplementary mode helps improve the mud invasion profile and correct the raw resistivity data to retrieve the true formation resistivity. It is less affected by shoulder beds than traditional laterolog measurements because of the active focusing and multifrequency operation, together with the symmetric tool design. In addition, the tool employs software focusing to improve the accuracy of true resistivity estimates through advanced 2-D inversion processing.

Environmental Measurement Sonde

The Environmental Measurement Sonde (EMS) combines a six-arm caliper, mud resistivity measurements, and mud temperature measurement. The caliper measurement of borehole diameter helps to identify washouts (large increase in hole radius) and is used for quality control and correction of other logs. Calipers also enable identification of drilling-induced breakouts as stress orientation indicators. The temperature data record the combination of the temperature profile and the drilling-induced temperature perturbation. If the EMS is run multiple times within the hole with the hole kept undisturbed, changes in the temperature profile may be used to quantify the drilling-induced temperature disturbance.

Formation MicroImager

The FMI provides high-resolution resistivity images of the borehole wall. The FMI generates an electrical image of the borehole from 192 microresistivity measurements. The electrodes are located on four pads and four attached flaps that are applied to the borehole wall by caliper arms (Fig. F7B). An applied voltage causes a current to flow from each focused electrode button on the lower pad through the formation to the electrode on the outer cartridge housing. The depth of investigation is ~2.5 cm, and the electrode buttons produce images with a vertical resolution of 5 mm. The quality of the image enables assessment of rock composition and texture, structure, and fluid content.

The borehole wall coverage by the resistivity image ranges from 80% for an 8.5 inch hole to <50% for a 12.25 inch hole (Hole C0009A). The General Purpose Inclinometry Tool (GPIT), which integrates both a three-axis inclinometer and a three-axis magnetometer, accompanies the FMI to orient its image. It can also provide the geometry of the borehole path. The center of Pad 1 is oriented by the P1AZ parameter in the log data file.

Sonic Scanner

The Sonic Scanner is a new generation of acoustic measurement tool and is a successor to the Dipole Sonic Imager (DSI). In addition to axial and azimuthal measurements, the Sonic Scanner makes a radial measurement to probe the formation for near well bore and far-field slowness. The depth sensitivity is equal to two to three times the borehole diameter. Other attributes of the Sonic Scanner include (1) a wide-frequency spectrum ranging from 300 Hz to 9 KHz, (2) a longer azimuthal array (five more receiver stations and 2 ft longer than DSI), (3) a borehole-compensated monopole with long (11–17 ft) and short (1–7 ft) spacing, and (4) cross-dipole acquisition. The Sonic Scanner provides accurate radial and axial measurements of stress-dependent properties near the borehole. These data are converted into (1) P- and S-wave velocities, (2) anisotropy of propagation, (3) Stoneley wave velocity, and (4) cement bond quality.

The configuration of the Sonic Scanner receivers produces a long azimuthal array (i.e., 8 azimuthal receivers at each of the 13 stations; Fig. F7C). With the two near-monopole transmitters straddling this array and a third transmitter beyond, the short- to long-monopole transmitter-to-receiver spacing combination provides a radial monopole profile.

Versatile Seismic Imager

The VSI is an array of seismometers clamped to the casing to receive seismic waves from air guns (see "Downhole measurements").

Data flow and processing

Data for each wireline logging run were recorded and stored digitally and monitored in real time using the Schlumberger MAXIS 500 system (Fig. F5). Onboard logging processing included (1) depth-shifting all logs relative to a common datum, (2) corrections specific to individual tools, and (3) quality control.

FMI image data were processed onboard the Chikyu using Schlumberger's GeoFrame (version 4.3) software package and imported into Geomechanics International (GMI) Imager software for further analysis. The processing steps included conversion of data format, inclinometry quality check, speed correction equalization, resistivity calibration, and normalization. Sonic Scanner data were processed at the Schlumberger Data Consulting Service in Tokyo with their in-house software and the results were used by the Science Party.

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