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doi:10.2204/iodp.proc.337.102.2013

Downhole logging

Logging data provide measurements of in situ properties in the borehole. Logging data at various depths of investigation into the formation are therefore complementary to centimeter-scale core studies and >10 m scale seismic images. The wireline logging tool is lowered into the open hole on a multiple-conductor contrahelically armored wireline, and measurements are usually taken from the bottom of the hole upward. For depth correlation purposes, we tried to maintain steady cable tension. Most wireline measurements are recorded continuously while the tool string is moving, except for stationary measurements including fluid sampling, pressure measuring tools, and a seismic array. The measurements are made shortly after the hole is opened with the drill bit and before continued drilling operations that adversely affect in situ properties and borehole stability.

During Expedition 337, open hole measurements in the 17½ inch hole from 511 (below the 20 inch casing shoe) to 1220 m DSF were planned but cancelled to overcome the delay in the operations schedule. Measurements in the 10⅝ inch open hole from 1220 to 2466 m DSF were conducted as planned during logging operations and included natural gamma radiation (NGR), spectral gamma ray, density, neutron porosity, photoelectric effect (PEF), resistivity borehole images from the Formation MicroImager (FMI), laterolog resistivity, spontaneous potential (SP), sonic velocity (P- and S-wave) from the Dipole Sonic Imager (DSI), zero-offset vertical seismic profile (VSP) using the Versatile Seismic Imager (VSI), six-arm calipers, mud resistivity, mud temperature, Nuclear Magnetic Resonance (NMR) logging, in situ fluid analysis and sampling by the Modular Formation Dynamics Tester (MDT), and the Quicksilver probe. NGR and VSI measurements were extended to the cased hole interval (511–1220 m WMSF), and DSI was also measured through the casing pipe from 1070 to 1220 m WMSF.

One of the most significant objectives of the geophysical logging operations was to determine the target depths of fluid sampling by Quicksilver probe. Because the time allowed for fluid sampling is limited for each sampling point, only layers with sufficient permeability and fluid mobility can be candidates and they need to be clearly identified before fluid sampling is attempted. The logging operations sequence was designed for this purpose. A total of five runs were conducted, and the tool strings used are shown in Figure F9:

• Logging Run 1 (Platform Express [PEX]-High-Resolution Laterolog Array [HRLA]-Hostile Environment Natural Gamma Ray Sonde [HNGS]-gamma ray (Fig. F9A) was to identify the lithofacies.

• Logging Run 2 (FMI-DSI-Environmental Measurement Sonde [EMS]-gamma ray) (Fig. F9B) was to examine the detailed features of each layer.

• Logging Run 3 (combinable magnetic resonance [CMR]-gamma ray) (Fig. F9C) was to look for permeable layers.

• Logging Run 4 (MDT-gamma ray) (Fig. F9D) was the formation testing tool with fluid sampling.

• Logging Run 5 (VSI-gamma ray) (Fig. F9E) was to correlate the borehole to the seismic profile.

The gamma ray tool needed to be attached to all runs to adjust the depth of each tool to Run 1.

Wireline logging

Wireline logging system

In general, wireline depth is more precise than drilling depth, which is affected by the stretch of the borehole assembly, and is measured accurately during logging operations with an integrated depth wheel. This device uses tension measurement for the correction of cable stretch and provides calibrated absolute depth values.

Data from each wireline logging tool were recorded in the data logger within the tool and were available for real-time display on board the ship via Schlumberger’s multitasking acquisition and imaging system (MAXIS). MAXIS receives and records an array of data from the downhole logging tools, and the real-time data were used for a quality check of data and processes. The MAXIS real-time log data were displayed remotely on a large monitor in the data integration center on the laboratory management deck via the local network.

During Expedition 337, a strong wireline cable (7-46ZVXXS) was used because of the heavy weight of the MDT module. The conventional passive heave compensator system was used with hydraulic lines to the auxiliary tank and for compensating the extra line, which was connected between the riser and the rig (Fig. F10).

Wireline logging tools

Natural gamma ray tools

The gamma ray tool measures natural radiation emitted by the formation in American Petroleum Institute gamma ray units (gAPI) and is a standard device for identifying lithologic units and data quality checks. Gamma ray tools were used in all runs, and the sensors were included in cartridge or combination tools. Gamma ray tools have a depth of investigation ranging from 24 to 61 cm.

Platform Express

The PEX has advantages in shorter total tool length and higher logging speed against a conventional triple combination (triple combo) tool string (resistivity, density, and porosity). The PEX consists of the Highly Integrated Gamma Ray Neutron Sonde (HGNS) and High-Resolution Mechanical Sonde (HRMS) (Fig. F9A).

The HGNS contains a radioactive source that radiates fast neutrons to the formation and detectors to count slowed neutrons deflected back to the tool. Because the neutrons are slowed primarily by hydrogen atoms in the formation, the measurements are scaled in porosity units in terms of formation water content. The porosity this tool measures includes the volume of intracrystalline water contained by clay minerals.

The HRMS houses the Three-Detector Lithology Density (TLD), which measures density and PEF, and Micro-Cylindrically Focused Log (MCFL) tools and also has a one-arm caliper function. The tool is provided with real-time speed correction and depth matching.

The TLD tool measures the attenuation of a gamma ray flux to determine the density of the formation. The gamma radiation interacts with the energy of the gamma ray photon. This energy decreases progressively, and gamma radiation is absorbed by matter by the PEF. 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. The TLD tool uses three detectors to obtain a high-resolution 8 inch density output. The TLD detector with 2 inch resolution is normally applied to correct for minor well bore changes resulting from the hole condition and mud cake. This logging environment allows the use of the 2 inch detector as a standalone porosity device to improve the visibility and reliability of the TLD curves.

The MCFL tool uses a cylinder electrode on which source electrodes emit a highly focused beam (diameter = 2.5 cm) that rapidly diverges and penetrates the formation as deep as 10 cm. The tool gives information on mud cake and flushed zone resistivity, which are useful for the lithology-density correction of the TLD tool.

High-Resolution Laterolog Array

The HRLA tool provides six resistivity measurements with different depths of investigation. They include mud resistivity and five measurements of formation resistivity with increasing penetration into the formation. The sonde sends a focused current into the formation and measures the intensity necessary to maintain a constant drop in voltage across a fixed interval, providing a direct resistivity measurement. The array has one central (source) electrode and six electrodes above and below it, which serve alternatively as focusing and returning current electrodes. By rapidly changing the role of these electrodes, a simultaneous resistivity measurement at six penetration depths is achieved. The tool is designed to ensure that all signals are measured at exactly the same time and tool position and to reduce the sensitivity to “shoulder bed” effects when crossing sharp beds thinner than the electrode spacing.

Hostile Environment Natural Gamma Ray Sonde

The HNGS tool uses spectroscopic analysis and two bismuth germanate scintillation detectors to determine the concentration of radioactive isotopes. The measurements focus on three common decay chain reactions of radioactive isotopes that are common in natural formation (potassium, thorium, and uranium), with each emitting photons at different energies. The HNGS measures gamma radiation from each decay that are converted to the concentrations of potassium, thorium, and uranium.

The radius of investigation depends on several factors: hole size, mud density, formation bulk density (denser formations display a slightly lower radioactivity), and the energy of the gamma rays (a higher energy gamma ray can reach the detector from deeper in the formation). This tool has a 24 cm depth of investigation and can also be used inside casing.

Formation MicroImager

The FMI provides an electrical borehole image and dip information generated from up to 192 microresistivity measurements. The four electrode flaps attached to the four pads are applied to the borehole wall by caliper arms (Fig. F11). The combination of measuring button diameter, pad design, and high-speed telemetry system produces a vertical and azimuthal resolution of 0.51 cm. This means that the dimensions of a feature larger than this resolution can be identified in the image. The size of features <0.51 cm is estimated by quantifying the current flow to the electrode. The azimuthal coverage of the borehole image is ~60% in the 10⅝ inch hole.

The General Purpose Inclinometry Tool, which integrates both a three-axis inclinometer and a three-axis magnetometer, is associated with the FMI to determine the orientation of its image. It can also provide the geometry of the borehole path. The Pad 1 orientation was recorded as P1AZ.

Dipole Sonic Imager

The DSI tool incorporates both monopole and crossed-dipole transmitters with a hydrophone array. The tool is made up of three sections (acquisition cartridge, receiver section, and transmitter section). An isolation joint is placed between the transmitter and receiver sections to prevent direct flexural wave transmission through the tool body. The transmitter section contains a piezoelectric monopole transmitter and two electrodynamic dipole transmitters perpendicular to each other. To the monopole transmitter, an electric pulse at sonic frequencies is applied to excite compressional and shear wave propagation in the formation. The two dipole transmitters obtain azimuthal shear wave anisotropy and are also driven at low frequency to excite the flexural wave around the borehole. The array of eight receiver stations spaced 15.24 cm apart, which confines the vertical resolution of this tool, provides spatial samples of the propagating wave field for full waveform analysis.

Environmental Measurement Sonde

The EMS measures mud resistivity and temperature along the tool axis to support borehole environmental corrections. Based on six independent caliper gauges, the ovality algorithm provides detailed information on the borehole geometry for more representative environmental correction of imaging tool measurements, improved borehole stress analysis, and more precise cement volume estimation.

Power Positioning Device and Caliper

The Power Positioning Device and Caliper (PPC) tool has a four-arm power caliper that delivers accurate (~1 mm) dual-axis borehole diameter measurements. The caliper log is essential for the processing of other logs and can be used in sedimentological and structural interpretation of the formation. With the powered arms, the PPC tool can centralize or decentralize other tools.

Versatile Seismic Imager

The VSI is a borehole seismic wireline tool optimized for VSPs. It can make multiple shuttles (each containing a three-axis geophone) separated by “hard wired” acoustically isolating spacers. During Expedition 337, we used the VSI with four shuttles with 15.12 m spacing. The acoustic waves were generated by a 750 in³ generator-injector air gun positioned ~6 m below sea level and offset 48.2 m from the borehole (Figs. F12, F13). The VSI was clamped against the borehole wall at 15.12 m intervals, and the air gun was typically fired between 5 and 10 times at each station. The recorded waveforms were stacked, and one-way traveltime was determined from the median of the first breaks for each station, thus providing check shots for calibration of the integrated transit time calculated from sonic logs. Check shot calibrations were required for well-seismic correlation because P-wave velocities derived from the sonic log may differ significantly from the velocities determined by seismic data.

Combinable magnetic resonance

The basic technology behind the CMR tool (also known as an NMR tool) is based on measurement of the relaxation time of the magnetically induced precession of polarized protons. A combination of permanent magnets and directional antennas are used to focus a pulsed polarizing field into the formation (Fig. F14). Then the tool measures the relaxation time of polarized molecules in the formation. By exploiting the nature of the chemical bonds within pore fluids (for hydrogen in particular), the tool can provide estimates of the total porosity, permeability, and bound fluid volume. The vertical resolution was 22.86 cm for high-resolution mode acquisition, and the depth of investigation of the measurement was 3.81 cm into the formation.

Spontaneous potential

SP is the natural difference in electrical potential between an electrode in the borehole and a fixed reference electrode on the surface. A significant deflection in SP was observed at boundaries of permeable beds, and the magnitude of the deflection depended mainly on the salinity contrast between drilling mud and formation water and the clay content of the permeable bed. The SP log was useful for detecting permeable beds and estimating formation water salinity where formation clay content was low.

Shipboard data flow and data quality

The wireline logging data were stored in MAXIS, the Schlumberger data acquisition system, and then an initial data quality check was conducted by field engineers and the logging staff scientist. The data were then processed on board the ship for (1) depth-shifting all logs to the seafloor, (2) environmental corrections specific to individual tools and depth matching, and (3) logging data quality control comparing repeated sections. The FMI data processing steps included data format conversion, inclinometry quality check, speed correction equalization, resistivity calibration, and normalization. During Expedition 337, environmental corrections, depth matching, and image data processing were performed on board the ship by the Schlumberger Data Consulting Service. FMI image data needed to be processed on board the ship using Schlumberger’s GeoFrame (version 4.4) software package and were imported into GMI Imager software for further analysis. After data processing and exchanging to a suitable format (digital log information standard [DLIS], log ASCII standard [LAS], or Society of Exploration Geophysicists format “Y” [SEGY]), the data were delivered to shipboard scientists (Fig. F15).

Logging data quality control

During processing, data quality control was mainly performed by cross-correlation of all logging data. Data quality was assessed in terms of realistic values for the lithology of the drilling interval, repeatability between different passes of the same tool, and correspondence between logs affected by the same formation property. For example, the resistivity log generally showed similar features to the acoustic log, and the conductivity log generally showed inverse response features to the gamma ray log. A short repeat session was acquired after or before the main log to check repeatability. The overall quality of the downhole logging data was evaluated in the context of borehole conditions with drilling parameters. This was because large and irregular borehole shapes reduced data quality for eccentralization/centralization tools, and rough surfaces degraded the contact condition for some tools.

Downhole fluid sampling and measurement

The MDT wireline logging tool allows customization for several downhole modules. During Expedition 337, for the first time in IODP we used the MDT tool (1) to measure pore fluid pressure and permeability, (2) to acquire in situ formation fluid and gas samples, and (3) to analyze geochemical parameters in the formation fluids. The tool configuration included the gamma ray sonde, electric power module, multisample module, IFA module (Composition Fluid Analyzer [CFA]), pumpout module, single-probe module (MRPS) for pressure and permeability tests, Quicksilver probe module (MRPS) for fluid sampling, and hydraulic power module (Fig. F9D).

Downhole pressure and permeability tests

Single-probe pore pressure measurements were made by attaching the probe to the borehole wall and extracting pore fluid (Fig. F16). Pressure inside the sealing zone was recorded with a sampling period of 300 ms during and after pore fluid extraction. For single-probe measurements during Expedition 337, a fluid volume of 5–10 cm³ was extracted at a rate of 30–80 cm³/min. The extracted volume was chosen based on anticipated formation permeability and was adjusted during multiple drawdown tests at a single measurement location. Three probe types are available for single-probe measurements: (1) conventional, (2) large-diameter probe, and (3) large-diameter packer.

For all Expedition 337 deployments, the large-diameter probe was used. We estimated in situ pore fluid pressure from the last pressure recorded during the pore pressure recovery in single-probe tests. During Expedition 337, we used Schlumberger’s standard approach to estimate fluid mobility for single-probe measurements. Pore pressure was drawn down for a specified time (typically 15 s), and then the pressure was allowed to partially or fully recover. The mobility calculated by Equation 3 is dimensionally dependent:

where

• k_D = drawdown permeability (mD),

• µ = viscosity (centipoise),

• q = flow rate (volume of fluid extracted during the drawdown divided by time [cm³/s]), and

• ΔP = difference between drawdown pressure and in situ pressure (often approximated as the final build-up pressure) (Pa).

C is a constant that depends on the probe type (C = 5360, 2395, and 1107 for conventional, large-diameter probe, and large-diameter packer, respectively); C = 2395 for all Expedition 337 deployments.

Fluid sampling

The Quicksilver probe is a sophisticated probe used to sample pristine formation fluid with low contamination of the borehole fluid. The Quicksilver probe has a sample port at the center of the probe and an annular guard port outside the sample port (Fig. F17). The guard port is designed to extract contaminated fluid so that the sample probe can take clearer formation fluid. At the beginning, both probes were opened and sample and guard lines were drained. Once the level of contamination was deemed acceptable during monitoring of the sample and guard lines, the valves of the sample bottles were opened to extract fluid from the formation. The SPMC was used to keep downhole in situ pressure during retrieval of samples for analysis of fluids and dissolved gas (Fig. F18). On board the ship, high-pressure formation fluid was transferred to the SSB using a mechanism similar to the SPMC on the surface and delivered to the science party.

During Expedition 337, high permeability was not expected in the sediment formation. The single probe was used for pretest to avoid damage to the delicate Quicksilver probe pad. The “XX” high-pressure pumpout module was used in order to control the lower pump rate against low-permeability formation. Six Dursan-coated SPMC and six SSBs were used to minimize contamination of the fluid sample from the bottle material. The bottles were flushed with pure water before the operation.

In Situ Fluid Analyzer

During fluid sampling, the IFA and CFA were used to monitor the sampling line and guard line, respectively. Their purpose was to (1) analyze in situ formation fluid in the sample line and (2) know the timing to open/close valves by monitoring contamination. IFA monitoring of the sample line measured resistivity, temperature, pressure, hydrocarbon composition (C₁, C₂, C_3–5, C₆₊), carbon dioxide, 16-channel grating spectrometer, 20-channel filter spectrometer, density, and viscosity. The monitoring properties, range, and accuracy of the IFA are shown in Table T4.

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