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doi:10.2204/iodp.proc.348.102.2015

Logging

LWD and MWD tools sample in situ physical properties and downhole drilling parameters that can be analyzed both in real time by using mud-pulse telemetry and after recovering the BHA and downloading the memory data. For Expedition 348, LWD/MWD acquisition was performed under contract by Halliburton Sperry Drilling Services. Because MWD/LWD data are recorded soon after initial drilling, these measurements are less affected by drill mud formation invasion and disturbance when compared to wireline logging. Time after drilling and exposure time are basic quality control parameters for the analysis and the interpretation of LWD data. Expedition 348 LWD/MWD data were integrated with, and compared to, data sets from cuttings, core, and seismic reflection imaging to constrain structure, lithology, and physical properties.

Two different BHAs were used while drilling the 17 inch diameter Hole C0002N in Run 1 and Run 2 (Fig. F21A, F21B), and another BHA was used for the 12¼ inch diameter Hole C0002P (Fig. F22). The BHA is chosen based on drilling requirements, borehole diameter, and tool availability. Tool specifications and acronyms are shown in Tables T12, T13, T14, and T15.

In Hole C0002N, LWD/MWD data were collected from 872 to 2329.5 mbsf. The LWD/MWD data were acquired in two logging runs using the two different BHAs: Run 1 from 872.5 to 2008.5 mbsf (2840.00–3976.00 m BRT) (Fig. F21A) and Run 2 from 2008.5 to 2329.5 mbsf (3976.00–4297.00 m BRT) (Fig. F21B). In Hole C0002P, LWD/MWD data were collected from 2162.5 to 3058.5 mbsf (4130–5026 m BRT) (Fig. F22). No LWD/MWD data were recorded through the sidetrack kick-off section (3904–4130.5 m BRT) prior to coring. After coring, LWD/MWD recorded reaming of the cored interval (4130.5–4186.0 m BRT) and continued to Hole C0002P total depth.

The LWD/MWD tools recorded a complete data set of geophysical measurements including gamma radiation, annular pressure and mud temperature, resistivity logs and azimuthal resistivity images, compressional and shear sonic velocity, acoustic directional images, and ultrasonic caliper. LWD/MWD tools used in the acquisition of Expedition 348 data included Halliburton’s MWD control unit, electromagnetic wave resistivity (EWR) (Figs. F23, F24), pressure while drilling (PWD), and dual gamma ray (DGR) (Fig. F21A) in the 17 inch Hole C0002N. The azimuthal focused resistivity (AFR) tool (Fig. F25), X-Bimodal AcousTic (XBAT) sonic tool (Fig. F26), and azimuthal gamma ray (AGR) tool were added to the LWD tool string for the Hole C0002P 12¼ inch section. EWR-PHASE4 was only run in Run 1 of Hole C0002N; the EWR-M5 was used for Hole C0002P.

LWD acquisition systems and tools

LWD/MWD equipment contains data memory and battery power within the tool string. Real-time monitoring of drilling and data analysis is performed by transmitting from the tool string to the ship via a modulated pressure wave within the drilling mud (mud-pulse telemetry). Only a subsample of recorded LWD data was sent because of limited bandwidth. Based on the type of information and relevance for formation characterization, specific channels were chosen for real-time transmission. MWD data on drilling parameters such as drilling speed, rate of penetration, and stick-slip indicators were transmitted together with the LWD logs for log quality check during the acquisition. The mud pulser failed at 2260.5 mbsf during drilling of Hole C0002P, ending real-time data transmission and monitoring for the hole. Full resolution logs from each tool’s memory only became available when the BHA was recovered and downloaded. The resistivity imaging data acquired with the AFR tool could not be downloaded immediately due to a failure in the connector that was detected when the tool was retrieved. The AFR tool had to be offloaded and sent to a Halliburton facility for data retrieval and processing, which caused several weeks’ delay before the data became available.

The individual tools that were used during Expedition 348 are described below.

Dual gamma ray

The DGR LWD tool provides a measurement of NGR in API units. Two Geiger-Müller detectors, each with independent counting circuits, create a redundant configuration (Halliburton, 2012a). This dual detection system provides two independent NGR logs, which are processed for best accuracy and precision of the measurement. LWD DGR logs generally produce a better vertical resolution compared to equivalent wireline logs because of slower drilling speeds. Gamma ray logs were corrected for borehole size, mud weight, and mud potassium content.

Azimuthal gamma ray

The AGR sensor measures the NGR activity of the formation. Using two scintillation crystals, this LWD tool has a measurement range of 0–849 gAPI with an accuracy of ±5%. Because the AGR sensor is located only 1.8 ft (0.55 m) from the bottom of the tool (Fig. F21A), it provides a very early indication of changes in lithology while drilling with imaging capability and near-bit positioning.

EWR-PHASE4

This LWD resistivity measurement is based on electromagnetic wave propagation and utilizes a high-frequency induction resistivity sensor. The tool comprises four radio-frequency transmitters and a pair of receivers (Fig. F23). By measuring both the phase shift and the attenuation for each of the four transmitter-receiver spacings, eight resistivity curves with corresponding different depths of investigation are recorded. The measurement range for phase-shift resistivity is 0.05–2000 Ωm. The measurement range for attenuation resistivity is 0.1–100 Ωm (Halliburton, 2012b).

EWR-M5

This LWD resistivity measurement is similar to the EWR-PHASE4 but is optimized by use of data from vibration and pressure sensors. It contains a drill string dynamic sensor, which consists of a triaxial accelerometer to monitor and minimize vibrational noise (Halliburton, 2012c). The tool consists of six transmitters in two sets of three separated by three receivers (Fig. F24). Measurements include 30 unique compensated resistivity sets of both phase shift and attenuation resistivity at 2 MHz, 250 kHz, and 500 kHz. The measurement range for phase-shift resistivity is 0.05–2000 Ωm. The measurement range for attenuation resistivity is 0.1–100 Ωm.

Azimuthal focused resistivity

The AFR tool complements the EWR measurements for high-resolution resistivity images in highly conductive mud and collects electrical images of the formation, omnidirectional and azimuthal laterolog-type resistivity, and at-bit resistivity. Resistivity images are collected using two rows of imaging buttons with two depths of investigation for this 8 inch tool (Fig. F25). Full image coverage is obtained from each row containing three button electrodes separated by 120°. Image resolution is 10 mm for these high-resolution sensors, and the data are acquired in 128 discrete azimuthal bins with 16 bins available in real time for analysis (Halliburton, 2012d). Bedding and fracture orientation can be interpreted from these images, as well as drilling-induced fractures and borehole breakouts, which is used to help estimate the stress field orientation and constrain stress magnitudes.

Pressure while drilling

This LWD tool provides real-time downhole pressure information, including annular pressure and internal pressure measurements by using two high-accuracy quartz gauges (Fig. F24). It also records tool temperature. During mud noncirculation periods (e.g., LOTs and pumps-off phases), the minimum, maximum, and average pressures are recorded and later transmitted when circulation restarts. The tool can be used to detect well flows and kicks (Halliburton, 2012e), as well as equivalent circulating density (see “Introduction and operations”).

X-Bimodal AcousTic

The XBAT tool produces azimuthal sonic and ultrasonic measurements by using four azimuthal transmitters, four azimuthal arrays of receivers (6 receivers per array), and a 4-pinger-axis ultrasonic caliper (Fig. F26). Each transmitter can independently fire either a positive or negative wave. This allows acquisition in monopole, dipole, quadruple, and crossed-dipole modes. We used both the monopole and dipole modes with the source frequency between 2 and 25 kHz. Receivers record full waveform acoustic signals and are isolated from the drill collar to reduce bit noise and mud circulation noise. The ultrasonic caliper determines borehole size and shape with an accuracy of ±3.8 mm, which can also help in the identification of wellbore failures not clearly identified in resistivity image logs.

The XBAT tool has a (manufacturer claimed) measurable limit of compressional slowness range of 40–190+ µs/ft (<1.6–7.6 km/s) and shear slowness range of 60–550+ µs/ft (<0.55–5.1 km/s) (Halliburton, 2013).

Onboard data flow

The LWD tools recorded data at a preset frequency based on logging speed and tool-optimized resolution, providing measurements as a function of time. For standard interpretation and correlation with shipboard sample measurements, the LWD and MWD data need to be referenced to depth below seafloor (mbsf), reported for logging purposes as meters LSF. Halliburton’s integrated logging and drilling surface system, which was installed onboard the Chikyu, was used to record and control the rate of penetration and depth of the drill string at any given time while logging. This was determined using the length of the drill string and derrick top drive position. A crown-mounted motion compensator on top of the derrick helped reduce errors from heave and improved weight-on-bit accuracy.

The real-time data were uploaded to the server every 12 h for initial interpretation by shipboard scientists. Due to the very deep drilling and long bit run times, recovery of the memory data occurred after each run. Data referenced in time were processed to meters BRT. The depth reference was then converted to meters LSF. Data were then distributed in DLIS format, and the main scalar logs were extracted and converted into LAS files.

Data quality assessment

Cross-correlating LWD/MWD data for primary quality assessment included the use of downhole drilling parameters, drilling control logs, and geophysical control logs. The logging staff scientists documented the LWD/MWD operations and converted the raw data received from the Halliburton engineer to the LSF depth scale. Resistivity scalar logs and ultrasonic caliper were used for data quality assessment, permitting analysis of borehole conditions (e.g., caving, washout, bridges, and invasion) for potential effects on logging data. Borehole images were also used to assess borehole conditions. However, regions of high stick-slip affect image quality. Time elapsed after passage of the drilling bit for the main geophysical measurements was also monitored for quality check along with drilling operations. Because of variations in sampling time, all measurements may have insufficient heave compensation and/or unaccounted movements, including bending, shocks, and vibrations of the BHA, creating errors in local depth measurements of up to tens of centimeters.

Real-time quality control

Logging scientists and the logging staff scientist continuously observed the real-time data feed and closed-circuit television feed from the rig floor. This provided an initial quality check on the data and tracking of events (e.g., time off bottom) that could affect the log response. Parameters included observations of sonic log values, resistivity, gamma radiation, annular pressures, torque, weight-on-bit, rate of penetration, and mud volume.

Log and image interpretation

Change in the log response, such as changing values and/or frequency of the signal, are often related to variation of the composition and/or texture of sediment and rock. Therefore, these features were used to define and characterize formation properties. Log units were characterized through both qualitative and quantitative methods.

Lithologic log unit characterization

The geometry of log unit boundaries and bedding information were defined based on scalar LWD logs and included borehole images for Hole C0002P. Trends were analyzed on all the available logs, and rock textures/structures were analyzed using borehole images. Sonic logs and resistivity images helped with textural interpretations. Gamma ray analysis aided in evaluation of composition.

Unit definition used all available LWD/MWD log variations to define distinct geological features and allowed for

• Defining and characterizing each log unit, subunit, and unit boundary;
• Categorizing composition and trends within each unit; and
• Interpreting geological features based on log data.

Lithology, from unit scale to bed scale, was primarily determined from gamma ray logs along with resistivity and sonic logs (for Hole C0002P). Higher or lower gamma radiation is the primary discriminant for clay-rich or sand-rich interval interpretation, respectively. Borehole images helped to characterize geological features such as bedding orientation, faults, fractures, sedimentary structures, bed boundaries, and unconformities. Log units were correlated to this and previous IODP expeditions’ core, cuttings, and seismic data at Site C0002 (Expedition 314 Scientists, 2009; Expedition 332 Scientists, 2011; Strasser et al., 2014b) to further refine the interpretations (see “Lithology”).

Structural interpretation from logs

Structural analysis was performed on AFR images in the lower section using TechLog software (Schlumberger). The azimuthal button resistivity data were displayed unwrapped as 360°-oriented images of the borehole wall for interpretation and dip measurements. As part of the workflow for image analysis, dynamic and static normalization were performed on the resistivity images. Static normalization shows overall change in resistivity in a single borehole, as it displays a color scale covering the entire range of resistivity (e.g., 0.2–200 Ωm) for a single borehole. Dynamic normalization recalculates the displayed color scale range for a specific interval of resistivity and is thus useful for bringing out subtle details in a log such as changes in facies or lithology, natural and drilling-induced fracture resistivity, or borehole breakout width.

Resistivity contrasts in the rock are the basis for the identification and interpretation of geological features on the resistivity images. Dipping planar surfaces are identified as sinusoidal curves of similar contrast in unwrapped AFR images. Dip and azimuth of fractures, faults, and bedding were determined by fitting sinusoids to the image data. Artifacts appear in the processed data due to stick-slip and insufficient heave corrections. Borehole diameter was obtained from the ultrasonic caliper. The borehole size was set to match a constant bit size (12¼ inches) to calculate dip in cases where an independent caliper measurement was missing. This assumption may cause dip overestimates in regions of large borehole diameter, introducing small dip angle errors. This means the reported dips should be viewed as maximum values. AFR depth of investigation was also one of the parameters used for dip angle calculation.

Using the background resistivity as a base, we classed fractures as conductive or resistive, in which we only classified unambiguous fractures. Clear crosscutting or dramatic variation in dip to bedding formed the bases for fracture classification in addition to azimuth orientation and fracture density.

Borehole wall analysis

Stress orientation within the borehole can, in principle, be determined by using both borehole breakouts and drilling-induced tensile fractures (DITFs). The vertical stress (S_v), two horizontal principal stresses (S_hmin and S_HMAX), and fluid pressures are considered to control the circumferential stress azimuthal around the borehole. Borehole breakouts form when the maximum circumferential stress exceeds the formation compressive strength. In a vertical well, breakouts appear in resistivity images as parallel and vertical conductive features 180° apart from each other in the direction of S_hmin. The minimum circumferential stress arises in the direction of S_HMAX where DITFs form if the effective circumferential stress becomes negative (tensional). DITFs appear as vertical pairs of cracks 180° apart if the borehole axis is aligned with the vertical stress but could form en-echelon patterns of inclined cracks if the borehole axis is deviated from the vertical (Zoback, 2007).

Interpretation of shallow, medium, and deep button resistivity images provided the orientation of breakouts and DITFs, integrated with sonic data to constrain the results. Resistivity images were oriented to give measured azimuths and widths of breakouts along with DITFs in true azimuth for estimation of horizontal principal stress direction. The ultrasonic caliper provided a 3-D borehole shape image used to assess breakouts and/or borehole elongation and ellipticity.

Integration with lithologic interpretations helped determine variations in the formation’s strength, stress, and/or pore pressure. Comparison with MWD drilling parameters assisted in analysis of borehole stability, mud pressure surges, and formation strength.

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