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doi:10.2204/iodp.proc.314315316.112.2009

Logging while drilling

During Expedition 314, six LWD and MWD tools were deployed under the contract by the Global Ocean Development Inc. with Schlumberger Drilling and Measurements Services. LWD surveys have been successfully conducted during previous Ocean Drilling Program (ODP) and IODP expeditions on the JOIDES Resolution with various tools of different generations, focusing on density, porosity, resistivity, gamma ray, and sonic velocity measurements. ODP Leg 196 was the last use of LWD in the Nankai Trough off Cape Muroto, ~200 km southwest of the current locations (Mikada, Becker, Moore, Klaus, et al., 2002). During Expedition 314, the first operation of a ground-breaking complex drilling project with multiple expeditions over several stages, scientists conducted LWD at all NanTroSEIZE Stage 1 sites using the most advanced tools in scientific ocean drilling history.

LWD and MWD tools measure different parameters. LWD tools measure in situ formation properties with instruments that are located in special drill collars immediately above the drill bit. The LWD and MWD tools used during Expedition 314 include several of Schlumberger’s VISION series tools, namely geoVISION, adnVISION, sonicVISION, and seismicVISION, in addition to MWD and annular-pressure-while-drilling (APWD) tools. Figure F1 shows the configuration of the LWD-MWD bottom-hole assembly (BHA), and the set of measurements recorded from LWD-MWD tools are listed in Tables T1, T2, and T3.

LWD measurements are made shortly after the hole is opened with the drill bit and before continued drilling operations adversely affect in situ properties and borehole stability. Fluid invasion into the borehole wall is also reduced relative to wireline logging because of the shorter elapsed time between drilling and taking measurements. MWD tools measure downhole drilling parameters (e.g., collar rotation) and annulus pressure and assure communication between tools. During drilling operations, these measurements are combined with surface rig floor parameters for easier drilling monitoring (e.g., weight on bit, 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 (collapse of borehole walls), characterized by increases in APWD and ECD, or loss of circulation caused by permeable formations or faults, characterized by a 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 are transmitted through the drilling fluid within the drill pipe by means of a modulated pressure wave (mud pulsing) at a rate of 6 bps (bits per second) and monitored in real time. The term LWD is often used more generically to cover both LWD and MWD type measurements, as the MWD tool is required during any LWD operation to provide communication between the LWD tools and the surface.

The LWD equipment is battery powered and uses erasable/programmable read-only memory chips to store the logging data until they are downloaded. The LWD tools take measurements at evenly spaced time intervals using a downhole clock installed in each tool and are synchronized with a depth tracking system on the rig that monitors time and drilling depth. After drilling, the LWD tools are retrieved and the data downloaded from each tool to a computer. Synchronization of the uphole and downhole clocks allows merging of the time-depth data (from the surface system) and the downhole time-measurement data (from the tools) into depth-measurement data files. The resulting depth-measurement data are transferred to the processing systems in the Downhole Data Processing Room for the Logging Staff Scientist and systematically distributed to the data servers for the science party to interpret. Data flow is described in the next section.

Systems and tools

Depth tracking system

LWD tools record data as a function of time. The Schlumberger integrated drilling and logging (IDEAL) surface system records the time and depth of the drill string below the rig floor. LWD operations aboard the D/V Chikyu 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, heave of the vessel by the action of waves/​swells and tides, and action of the motion compensator. 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 rate of penetration. The system configuration is illustrated in Figure F2.

A hook-load sensor is used to measure 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) and motion from the blocks or motion compensator will not have any effect on the depth of the bit, the drawworks encoder information does not augment the recorded bit depth. It is clear when the drill string is out-of-slips (i.e., the drill string is connected to the top drive and is free from the rig floor). 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 used by the drillers measures and records heave and the motion of the cylinder of the active compensator among many other parameters at the rig floor. The Chikyu uses a crown-mounted motion compensator (CMC) (Fig. F2), which is installed on the top of the derrick to reduce the influence of heave on the drill string and to raise 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 hull’s up and down motion. When the crown block oscillates, the difference 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.

Measurement-while-drilling (PowerPulse) and annulus pressure tools

The MWD tool is the most basic but most important tool for operation data. This tool allows real-time two-way communication between LWD tools and the surface. MWD tools have previously been deployed during ODP Legs 188 (O’Brien, Cooper, Richter, et al., 2001), 196 (Mikada, Becker, Moore, Klaus, et al., 2002), and 204 (Tréhu, Bohrmann, Rack, Torres, et al., 2003) and IODP Expeditions 308 (Flemings, Behrmann, John, et al., 2006) and 311 (Riedel, Collett, Malone, et al., 2006). During Expedition 314, the Schlumberger MWD PowerPulse tool was used in combination with the APWD tool for pilot hole drilling and with LWD tools in other applications (Fig. F1).

MWD data are transmitted by means of a pressure wave through the fluid within the drill pipe (fluid pulse telemetry). The 6¾ inch (17 cm) diameter MWD PowerPulse tool operates by generating a continuous mud-wave transmission 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. F3A). The data are compressed and coded digitally in pressure pulses that are sent up the well through the drilling fluid. Figure F3B illustrates the MWD fluid pulse telemetry system and a representative pressure wave. Drilling fluid pulses are recorded on two pressure transducers (signal pressure transducers) mounted on the standpipe manifold and the gooseneck of the standpipe where they are automatically decoded and uncompressed by the surface equipment. With the MWD fluid pulsing system, pulse rates range from 1 to 8–12 bps, depending primarily on water depth and fluid density. During Expedition 314, pulse rates of 3 bps were achieved for MWD-APWD operations (pilot holes) and 6 bps for the LWD holes.

The MWD parameters transmitted by fluid pulses include tool status information, vibrations, shocks, and tool stick-slip for continuous monitoring of the drilling operation. The latter 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 T1. Tables T4 and T5 list typical telemetry frames sent in real time, showing measurements recorded using the MWD-APWD (PowerPulse) tools and their update rates. These data are transmitted to the surface. The comparison of MWD drilling parameters with rig instrumentation system data and ship heave information is used to improve drilling control and monitor any inflow or loss of circulation during drilling.

During LWD operations, the mud pulse system also transmitted a limited set of geophysical data from the adnVISION, geoVISION, sonicVISION, and seismicVISION LWD tools to the surface in real time. These scientific measurements include gamma ray values, resistivity, bulk density, neutron porosity, compressional velocity (P-wave), and seismic waveforms. Measurement parameters from each LWD collar were updated at rates corresponding to 15 cm to 1.5 m depth intervals, depending on the initialized values and rate of penetration (ROP) of the tool (Tables T4, T5). The combination of all these data confirmed the operational status of each tool and provided real-time logs for identifying lithologic contacts and potential shallow-water flow or overpressured zones. The real-time data also became the only source of information in cases when the data recorded in memory could not be retrieved (e.g., Site C0001 adnVISION data and all data for Site C0003).

adnVISION tool

The adnVISION tool is similar in principle to the older compensated density neutron tool (Anadrill-Schlumberger, 1993; Moore, Klaus, et al., 1998). The density section of the tool uses a 1.7 Ci ¹³⁷Cs gamma ray source in conjunction with two gain-stabilized scintillation detectors to provide a borehole-compensated density measurement (Table T6). The detectors are located 5 and 12 inches (12.7 and 30.48 cm) below the source (Fig. F4). The number of Compton scattering collisions (change in gamma ray energy by interaction with the formation electrons) is related to the formation density (Schlumberger, 1989).

Returns of low-energy gamma rays are converted to a photoelectric factor (PEF) value, measured in barns per electron. The PEF value depends on electron density and therefore responds to bulk density and lithology (Anadrill-Schlumberger, 1993). PEF value is also particularly sensitive to low-density, high-porosity zones.

The density source and detectors are positioned behind windows in the blade of 8¼ inch (25.9 cm) integral blade stabilizer. This geometry forces the sensors against the borehole wall, thereby reducing the effects of borehole irregularities and drilling. Neutron logs are processed to eliminate the effects of borehole diameter, tool size, temperature, drilling mud hydrogen index (dependent on mud weight, pressure, and temperature), mud and formation salinities, lithology, and other environmental factors (Schlumberger, 1994). The vertical resolution of the density and photoelectric effect measurements is ~6 and 2 inches, respectively.

For measurement of tool standoff and estimated borehole size, a 670 kHz ultrasonic caliper is available on the tool. The ultrasonic sensor is aligned with, and located just below, the density detectors. This sensor has an accuracy of ±0.1 inch and a vertical resolution of ~6 inches. In this position the sensor can also be used as a quality control for the density measurements.

Neutron porosity measurements are obtained using fast neutrons emitted from a 10 Ci americium oxide-beryllium (AmBe) source. Hydrogen quantities in the formation largely control the rate at which the neutrons slow down to epithermal and thermal energies. The energy of the detected neutrons has an epithermal component because much of the incoming thermal neutron flux is absorbed as it passes through the 1 inch drill collar. Neutrons are detected in near- and far-spacing detector banks, located 12 and 24 inches (30.48 and 60.96 cm), respectively, above the source (Fig. F4). The vertical resolution of the tool under optimum conditions is ~12 inches (34 cm). The neutron logs are affected to some extent by the lithology of the matrix rock because the neutron porosity unit is calibrated for a water-saturated sandstone environment (Schlumberger, 1989).

The azimuthal measurement from the adnVISION tool is not reliable in wells with low deviation (<10° inclination). In such environments, an average or maximum value should be used instead. Data output from the adnVISION tool include apparent neutron porosity (i.e., the tool does not distinguish between pore water and lattice-bound water), formation bulk density, and photoelectric factor. The density logs graphically presented here have been “rotationally processed” to show the average density that the tool reads while it is rotating. In addition, the adnVISION tool outputs an inferred density caliper record based on the standard deviation of density measurements made at high sampling rates around the circumference of the borehole. The measured standard deviation is compared with that of an in-gauge borehole and the difference is converted to the amount of borehole enlargement (Anadrill-Schlumberger, 1993). A standoff of <1 inch (2.54 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 (Anadrill-Schlumberger, 1993).

Logging while drilling is a challenging environment for formation density measurement. The measurement is strongly affected by tool motion and influenced by the drilling fluid composition. The net effect is formation density and PEF measurements that can be inaccurate or misleading. The accuracy of the measurements can be greatly improved by acquiring data in both depth and azimuthal dimensions, assembling these data into a two-dimensional image, and selecting the density measurements least influenced by borehole effects from this image. In LWD tools, and in particular the adnVISION tool, azimuthal data are acquired most economically from one set of sensors swept around the borehole by the rotation of the drill string.

The image-derived density algorithm uses the compensated density image to compute a single compensated density. It identifies which sectors at each depth level provide the highest quality measurements and computes a density measurement based on those sectors. It essentially automates what a skilled log analyst does when interpreting a density image. The algorithm consists of the following three steps:

1. Quality factor computation. For each depth level and sector, the short- and long-spaced densities and volumetric PEF are used to compute a quality factor. The quality factor is based on qualitative expectations and an empirical choice of parameters. Larger quality factors represent more accurate density measurements.
2. Toolpath identification. As a function of depth, the centroid of the region of high-quality measurements defines a toolpath. The toolpath can be thought of as the path of closest approach of the tool to the formation. This path is computed from the quality factor at each depth level by a partial Fourier decomposition.
3. Density calculation. The density is computed at each depth level by averaging the bulk density over four sectors centered on the toolpath. Fractional sectors are accounted for by linear interpolation. These steps are described in detail in Radkte et al. (2003).

By construction, this algorithm yields the highest quality density and PEF measurements possible. This technique has several other advantages: it is computed only from density sensor data, it is immune to the statistical bias and limited applicability of maximum density approaches, and the toolpath serves as a powerful quality control indicator.

geoVISION tool

The geoVISION resistivity tool is based on resistivity-at-the-bit (RAB) technology, which was designed to provide real-time at-bit resistivity data. This explains why numerous geoVISION measurement acronyms include RAB in their name (see Table T3). The geoVISION resistivity tool provides resistivity measurements and electrical images of the borehole wall, calibrated in a homogeneous medium. In addition, the geoVISION tool contains a scintillation counter that provides a total gamma ray measurement (Fig. F5).

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. 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. These images can then reveal information about formation structure and lithologic contacts. The button electrodes are ~1 inch (2.5 cm) in diameter and reside 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). The spacing provides multiple depths of investigation for quantifying invasion profiles and fracture identification (drilling induced versus natural). Vertical resolution and depth of investigation for each resistivity measurement are shown in Table T7. For environmental correction of the resistivity measurements, drilling fluid resistivity and temperature are also measured (Schlumberger, 1989).

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

The geoVISION tool collar configuration is intended to run in 8½ inch (22 cm) and 9 inch (25 cm) diameter holes depending on the size of the measuring button sleeve. During Expedition 314, we used an 8½ inch diameter bit and an 8¼ inch diameter button sleeve for the geoVISION tool. This resulted in a minimum standoff between the resistivity buttons and the formation, giving higher quality images.

sonicVISION tool

The sonicVISION sonic-while-drilling tool delivers real-time interval transit time data for compressional waves. The available measurement range is ~40–230 µs/ft (1.3–7.6 km/s), depending on mud type, but intensive processing was sometimes required to obtain reliable sonic velocity measurements in the relatively slow formations drilled during Expedition 314. In real-time LWD operations, the sonic processing parameters are conventionally set at the surface before the tool is run in the hole. The real-time projection log and labeling with quality control log affect the real-time slowness and quality of the data. This results in the possible mislabeling of arrivals (especially for slow formations) and limited confidence levels, as only the end result of downhole processing is seen uphole in the real-time log. In general, during Expedition 314 the real-time sonic traveltimes were spurious and unreliable; however, full waveform data are recorded in memory. Advanced onboard postprocessing extended the range of measurement to near the mud velocity, a key feature for achieving the scientific objectives of this cruise, including log-seismic ties. Additional quality control is performed using automatic stationary measurements made during a pipe connection. In this less noisy environment, the tool is able to take a station measurement that is sent uphole, when pumping resumes, for further quality control of the real-time log (Fig. F6).

The wideband frequency measurement and high-power transmitter have been improved from the older tool. Wideband frequency measurements reduce aliasing and eccentralization effects, improve formation signal strength, and allow measurement of shear and Stoneley waves. The transmitter outputs large amounts of wideband power to effectively couple more energy into various formation types, ultimately improving the signal-to-noise ratio and, therefore, measurement quality and hole size range. Extended battery life, large memory capacity, and fast dump speed significantly enhance the sonicVISION tool’s reliability and functionality. Standard memory life is 140 h at 10 s acquisition, and this is easily doubled. In addition, standardized “planning” software allows the engineer to easily optimize the tool configuration for the well being drilled.

In shallow unconsolidated formations where the compressional velocity approaches or is below the fluid (mud) velocity, it is difficult to directly measure the formation slowness with the refracted wave because the energy of the refracted wave is too weak. When the compressional slowness is larger than the mud slowness, a significantly low frequency (several kilohertz) source is needed to measure the formation slowness (Wu et al., 1995). However, if the compressional slowness is close to the mud slowness but still smaller than the mud slowness, the formation slowness can be extracted with processing a leaky compressional (“leaky-P”) mode excited by a wideband source. Tichelaar and Luik (1995) and Valero et al. (1999) discussed the compressional slowness processing in such conditions for wireline sonic data. They processed a leaky-P mode by applying a lower frequency band-pass filter to attenuate fluid modes and enhance formation arrivals. The leaky-P mode consists of multiple reflected and constructively interfering compressional waves traveling in the borehole fluid (Paillet and Cheng, 1991). The leaky-P mode is dispersive, such that at lower frequencies the slowness asymptotically approaches the formation compressional slowness and at higher frequency to the mud slowness. Because of this dispersive effect, the slowness estimated by the nondispersive semblance processing (Kimball and Marzetta, 1984) is greater than the true P-wave slowness. Therefore, correction for dispersion is needed in order to obtain the true formation compressional slowness.

Standard LWD-sonic measurements are operated with a frequency band of ~11 kHz (Aron et al., 1997). However, in the case of a very slow formation, it is difficult to obtain the compressional slowness using a standard source because the energy of the fluid arrivals dominates those of the leaky-P mode. It is necessary to expand the source spectrum of the monopole transmitter to lower frequencies to excite the leaky-P mode. Therefore, wide frequency band data acquisition is required to excite a leaky-P mode. Both the semblance processing and the dispersive analysis with Prony method (Ekstrom, 1995) clearly showed the existence of dispersive leaky modes.

The modeling of leaky-P dispersion has to take into account a realistic tool structure for a wide range of borehole and formation parameters. Based on these modeling results, a correction table for the dispersion biases is established and dispersion correction applied to obtain formation compressional slowness. This procedure was applied to the ODP logs from Leg 196 Hole 1173B and Leg 130 Hole 808I and compared with the core slownesses measured on the previous leg (Mikada et al., 2002). The result of the dispersion-corrected LWD sonic processing showed good agreements with these core velocity measurements (Goldberg et al., 2005). A similar procedure was applied by the Schlumberger Data Consulting Service (DCS) specialist for onboard processing of sonic data.

seismicVISION tool

The seismicVISION LWD system delivers time- and depth-velocity information to provide interval velocity. The seismicVISION tool, which contains a processor and memory, receives seismic energy from a conventional air gun suspended from a crane on the drillship. After acquisition, the seismic signals are stored and processed downhole, and check shot data and quality indicators are transmitted uphole in real time by connection with the MWD pulse system. Waveforms are recorded in the tool memory for further processing after a bit trip. Refer to “Log-seismic correlation” for more information (Fig. F7).

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