| IODP Proceedings Volume contents Search | |||||||||||||
 | |||||||||||||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||||||||||||
|
doi:10.2204/iodp.proc.343343T.102.2013 Logging while drillingDuring Expedition 343, three logging-while-drilling (LWD) and measurement-while-drilling (MWD) tools were deployed by Schlumberger Ltd. under contract with Mantle Quest Japan Company Ltd. LWD tools measure in situ formation properties with instruments that are located in special drill collars immediately above the drill bit. 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 and assure communication between tools. During drilling operations, these measurements are combined with surface rig floor parameters for easier drilling monitoring and quality control (QC). Most LWD data are recorded to 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 2 bits per second (bps) and monitored in real time. The LWD and MWD tools used during Expedition 343 include several of Schlumberger’s VISION series tools, namely geoVISION and arcVISION, in addition to their MWD TeleScope tool. Figure F3 shows the configuration of the LWD/MWD bottom-hole assembly (BHA). The set of measurements recorded from LWD/MWD tools are listed in Tables T1 and T2. Systems and toolsDepth tracking systemLWD data interpretation requires specific and precise depth measurements that connect the logging data to formations in the subseafloor. Because LWD tools record data only as a function of time, the Maxwell surface system that is installed on board the Chikyu has software that records the time and depth of the drill string below the drill floor, as well as the rate of penetration (ROP). Accurate and precise depth tracking requires independent measurements of (1) position of the traveling block and top drive system in the derrick; (2) heave of the vessel because of wave action, swells, or tides; and (3) activity of the motion compensator. These measurements are automatically recorded during drilling. The depth of the drill string and ROP are determined from the length of the BHA and drill pipe and the position of the top drive in the derrick. The configuration of these components is illustrated in Figure F4. Measurement while drilling (TeleScope)MWD was critical to successful drilling during Expedition 343, as it provided real-time two-way communication between LWD tools and the surface and enabled scientists to monitor drilling operations, estimate borehole conditions, and detect a target fault while drilling. The MWD TeleScope tool (Fig. F5) transmits real-time measurement data from the MWD tool and selected LWD tools uphole through the drilling fluid (in a process known as mud-pulse telemetry). A list of the real-time LWD/MWD/recorded mode parameters is given in Table T2. A modulator in the tool generates a continuous 12 Hz pressure wave within the drilling fluid and changes the phase of this signal (frequency modulation) to transmit the data. 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 Maxwell system by the field engineer. The 8¼ inch (21 cm) diameter MWD TeleScope tool was used during Expedition 343. This tool is similar to the PowerPulse but allows data transfer to occur up to 4 times faster by using the Orion II MWD/LWD telemetry platform. In the MWD fluid pulsing system, pulse rates range from 1 to 12 bps, depending primarily on water depth and fluid density. During Expedition 343, pulse rates of 2 bps were applied for MWD operations because of the ultra-deepwater environment. The TeleScope tool acquires operational and drilling mechanics data, including collar rpm, drilling fluid turbine rpm, stick and slip, and axial and torsional vibration. The TeleScope tool also contains a drilling fluid turbine that powers the entire LWD string when drilling fluid is circulated at a sufficient flow rate (between 300 and 580 gallons per minute or 18–37 L/s in the TeleScope tool in the case of Expedition 343). Additional tool specifications appear in Figure F5C. arcVISIONThe arcVISION tool (array-resistivity compensated) (Fig. F5B) measures propagation resistivity. Electromagnetic waves are both attenuated and phase-shifted when they propagate in an electrically conductive medium; 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 tool makes 10 phase shifts and 10 attenuation measurements at 5 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 (Table T3). For a given frequency, the vertical resolutions of phase-shift resistivities, measured at different transmitter-receiver separations, are similar. In addition to the resistivity tools, arcVISION measures natural gamma radioactivity of the formation. The gamma ray sensor has a measurement range of 0–250 gAPI, with an accuracy of 3% corresponding to a statistical resolution of ±2 gAPI at 100 API and ROP of 30 m/h (Table T4). The arcVISION tool also measures the pressure and temperature of the borehole fluid in the annulus, which are converted to equivalent circulating density (ECD) (density of circulating drilling fluid when pumping). Downhole pressure is a crucial parameter for detecting any inflow from the formation into the borehole or obstruction of the borehole because of collapse of the borehole walls, characterized by an increase in pressure. Monitoring of downhole pressure also allows us to detect pressure decreases associated with loss of circulation to permeable formations or faults. geoVISIONThe geoVISION tool (also known as resistivity at the bit or geoVISION resistivity) provides laterolog-type (focused-resistivity type) resistivity measurements of the formation and high-resolution electrical resistivity images of the borehole wall (Fig. F5A). The tool uses two transmitter coils and a number of electrodes to obtain several measurements of resistivity (Bonner et al., 1996):
During Expedition 343, the geoVISION tool sampled image data every 10 s. To maintain appropriate sampling intervals matching the vertical resolution of the measurement and record high-quality data to important target depths, the recommended ROP was reduced to 15–20 m/h, or ~5 cm every 10 s, in the interval below 500 mbsf. The geoVISION tool also contains a scintillation detector that provides an azimuthal total natural gamma ray measurement. 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 an ROP of 30 m/h. Its depth of investigation is between 5 and 15 inches. Gamma ray measurements are acquired at 90° resolution as the geoVISION tool rotates. See Table T5 for further geoVISION specifications. Shipboard data flow and quality checkFor each LWD/MWD operation, two types of data were collected: (1) real-time data that include all MWD data and selected LWD data and (2) LWD data recorded downhole and stored in the tool’s memory. 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 Maxwell system. All the depth-converted data for LWD/MWD are provided in digital log information standard (DLIS) format by the field engineers. After determining the position of the mudline on the gamma ray log, all logging data were depth shifted to the seafloor (LWD depth below seafloor [LSF]). The depth-shifted versions of the LWD/MWD data are available in DLIS format, and the main scalar logs were extracted and converted into log ASCII standard (LAS) files. All files were distributed to the shipboard scientists through the shipboard intranet data servers. Analyses, integration results, and reports produced by the shipboard scientists were archived on the server for further distribution. The geoVISION resistivity image data were processed on board the Chikyu using Schlumberger’s GeoFrame (version 4.4) software package and imported into GeoMechanics International’s GMI and Paradigm’s Geolog software for further structural analysis. Normal data flow is illustrated in Figure F6. Logging staff scientists used drilling control logs to identify the sequence of drilling events and assess any possible impact on data quality. Drilling control logs include drilling surface parameters (e.g., ROP, surface weight on bit [SWOB], hook load [HKLD], and standpipe pressure [SPPA]) and downhole drilling parameters (e.g., collar [bit] rotation [CRPM], hole deviation [HDEVI], radial shock rate [SKR_R], tangential shock rate [SKR_T], shock peak [SHKPK], and stick-slip indicator [SLIP]). Elapsed time of the main geophysical measurements after bit penetration, including annular pressure and temperature logs, were also assessed to identify any anomalous zones. Tool rotation was also checked; it must be higher than null and lower than 250 rpm to allow good quality azimuthal measurements and associated images. In zones of high stick-slip, even if CRPM is set to a typical value of 100 rpm, it can greatly vary locally (and exceed 250 rpm), resulting in lower quality images. Log characterization and lithologic interpretationLWD measurements provide in situ petrophysical information on sediment, rock, and pore fluids while the hole is being drilled. These measurements are sensitive to changes in composition, texture, and structure. Changes in the log response are commonly associated with geological unit boundaries. This section addresses the characterization of LWD measurements and imaging tool response, focusing on dividing the well logs into log units. Once representative petrophysical properties for the log units were defined, they were incorporated in the log-based lithologic units. For Expedition 343, the aim was to provide a preliminary assessment of expected lithostratigraphy from LWD data prior to coring. Log characterization and identification of log units: qualitative analysisThe LWD logs were separated into log units by qualitative examination of the log responses and downhole trends: the variation in log shape and magnitude, peak amplitude and frequency, as well as thorough examination of the image logs. Natural radioactivity, resistivity, and borehole images were the main input logs for determining unit boundaries. In addition to defining and characterizing the log units, the logging team identified any compositional features/variation within each unit and interpreted them in terms of geological features (unit boundaries, transitions, sequences, and likely lithologic composition). Log-based geological/lithologic interpretationAfter log characterization and classification, logs were lithologically and geologically interpreted using a combination of log characteristics and borehole images for Site C0019. Compositionally influenced logs, such as NGR, were used to determine unit-scale to bed-scale lithology. In particular, the identification of sand-rich intervals, clay-rich intervals, or alternating beds of sand and clay was a primary element of the interpretation. Resistivity logs were used to identify lithologies with different physical properties, such as mudstone and chert. Borehole images provided useful information on mesoscopic features, such as bedding, sedimentary structures, bed boundaries, and faults. Ground-truthing of these interpretations was done by correlation with core data from Hole C0019E. Physical properties and hydrogeologyResistivity logs provide information on in situ physical properties at scales of tens of centimeters to tens of meters and are used to infer the porosity and density of rocks in the absence of other measurements. Comparison of the deep and shallow buttons also provides indications on the permeability of the formations. This data set provides important information for the characterization of lithologic units, deformation, states of consolidation, and hydrogeologic conditions. It is also used to correlate core, seismic, and downhole log data. We present the logs as a function of depth and describe their features and variation, taking into account information from the gamma ray log, structural geology, and seismics. Sharp variations in physical properties are of particular interest, as they can be indicators of a fault zone and its activity. We derived an estimated porosity from the resistivity logs and an estimated density from the estimated porosity. The different parameters used to compute these logs were adjusted by comparing them with values measured on recovered cores. Estimation of temperature profileThe downhole temperature (T) was calculated using a constant thermal conductivity (TC) of 1.1 W/(m·K) and a surface heat flow (HF) of 45 mW/m2 as documented in Yamano et al. (2008). The calculation is based on the following law:
where a surface temperature (T0) of 1.3°C was used based on measurements made with temperature sensors (see “Observatory and downhole measurements”) and Z is the depth below seafloor. Estimation of porosity from resistivity logsWe used Archie’s law to derive a porosity (ϕ) log from the resistivity:
where F is the formation factor and m is the so-called cementation factor. The cementation factor depends on rock type and is more closely related to texture than to cementation. This value was calibrated using the core measurements. The formation factor was calculated as
where R is the LWD measured deep resistivity and Rf is the fluid resistivity. We used the deep resistivity measurement here because it has a penetration depth sufficiently large to minimize the effects of seawater invasion. We assumed that the pore fluid was similar to seawater. The formula used to calculate the seawater resistivity (σf) as a function of the temperature (in °C) is described by Bourlange et al. (2003):
Estimation of density from the porosity logAn estimated bulk density (ρb) log was calculated from the estimated porosity log using the following equation:
where
We assumed a constant grain density (ρg) of 2.50 g/cm3 and a constant water density (ρw) of 1.024 g/cm3. This grain density value is in good agreement with measurements made on cores recovered from the deep-sea terrace of the Japan Trench (Sacks, Suyehiro, Acton, et al., 2000). Structural analysis: bedding, fractures, and faultsStructural analysis was performed primarily on geoVISION resistivity images using GMI Imager (GeoMechanics International Inc.) software. This software package presents resistivity image data of the borehole wall as a planar “unwrapped” 360° image. The software also allows visualization of the data in a 3-D borehole view alongside correlative log curve data. The orientation of planar surfaces cutting the borehole (bedding, fractures, and faults) is defined by fitting a sinusoid to the unwrapped image. In identifying bedding, care must be taken to avoid horizontal artifacts caused by problems in data acquisition that appear as sharp horizontal lines. We distinguished fractures and faults from bedding by looking for crosscutting relationships between the faults and fractures and the bedding. Where crosscutting relationships were absent, we looked for contrasting orientations between the faults and fractures and the bedding. The fractures and faults were classified as to whether they were conductive, resistive, or undetermined/unclassified. Where resolved, the thickness of faults and fractures was noted. Rare offsets of preexisting faults or bedding allows some faults to be identified as normal, strike-slip, or reverse. The software records the data and displays plots of these features. Borehole wall failure analysisBorehole breakouts and tensile fractures are two types of drilling-induced borehole wall failure that form when the state of local stress field at the borehole wall exceeds rock/sediment strength (Zoback, 2007). In a vertical borehole, breakouts form along the borehole in the direction of the minimum horizontal stress (Shmin) and perpendicular to the maximum horizontal stress (SHmax). Breakouts are recorded in resistivity images as two parallel conductive vertical features 180° apart. Drilling-induced tensile fractures (DITFs) may form in conjunction with breakouts or independently. DITFs form perpendicular to Shmin, 90° from the azimuth of the breakouts. DITFs occur when the hoop stress, or the stress tangent to the circumference of the borehole wall, exceeds rock tensile strength (in terms of absolute value of stress). Where the tensile strength of sediment is negligible, the occurrence of DITFs is an indicator of tensile hoop stress at the borehole wall. We recorded the orientation and width of breakouts with the available image analysis software (all three borehole images [shallow, medium, and deep] were considered) and determined the maximum and minimum horizontal stress orientations. We compared breakout distribution and width observed and measured on electrical images with lithology derived from all log data and drilling parameters. Finding faultsThe principal goal of this expedition was to sample and instrument the fault zone associated with the Tohoku-oki earthquake. Although the borehole is located so that it crosses the fault zone, we must determine the depth of intersection from analysis of log data. The protocol used for locating this fault is detailed in the following sections. Conductive and resistive faults and fractures and resistive structuresA conductive fracture or fault is indicative of a low-density, fluid/seawater-rich feature that could represent a young fracture or fault. Resistive structures are indicative of a collapsed fabric with lower fluid/seawater content than the surrounding rocks. Either a conductive fracture or a resistive structure could represent something that formed recently and is still dilated (conductive) or has collapsed (resistive). Irregular masses of high conductivity associated with horizontal planar high-conductive intervals have been interpreted as hydrofracture zones (Chang et al., 2010). Patterns of bedding, fractures, faults, and breakoutsChanges in the abundance and orientations of bedding, small-scale fractures and faults, and breakouts may be indicative of a major fault zone in the logged section. Faults, especially weak faults, can produce discontinuities in the fault-parallel components of normal stress that result in changes in the orientation and development of minor structures. Borehole breakout patterns are especially useful in identifying abrupt changes in stress associated with faults (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010), as breakouts provide information on the in situ stress at the time the hole is drilled. In contrast, faults and fractures may record present or past stress conditions. Log curve dataVariations in geophysical properties measured by logging are commonly associated with fault zones. For example, fracturing of the rock can decrease the resistivity because of the greater content of conductive fluid. In addition, faults with large displacement can incorporate or juxtapose contrasting rock types, which can be apparent in resistivity and gamma ray logs. Drilling parametersFaults can be characterized by fluid flow (Vrolijk et al., 1991), which may be sensed by annular pressure while drilling (APWD) and annular temperature while drilling measurements. Torque may also increase in fault zones where material has fallen into the annulus, restricting rotation. Faults create loose material; moreover, flow from fluids can encourage spalling of this material, which when introduced into the borehole can increase torque. |
|||||||||||||
