Logging-While-Drilling Data Processing

Science operator: CDEX-JAMSTEC

Hole: C0002S

Expedition: 358

Location: Nankai Trough (NW Pacific Ocean)

Latitude: 33° 18.0507' N

Longitude: 136° 38.2029' E

Logging-while-drilling date: February 2 - 9, 2019

Sea floor depth (as seen on logs): 1967.5 m LRF

Total penetration:  4901.5 m LRF (2934 m LSF)

 

Logging Tools

 

The downhole logs were recorded using the LWD/MWD (Logging-While-Drilling/Measurement-While-Drilling) system, which employs instruments that are part of the drill string itself. The advantages of this technique include being able to log in formations that would not provide a stable hole for wireline logging (e.g. the upper section of sedimentary formations) and logging a hole immediately during drilling, so that it is in good condition and largely free of wash-outs. The data is recorded in the tool's memory and downloaded when the drill string returns to the rig floor. A small subset of the data is transmitted up to the ship during drilling. The following data was processed by the logging specialists and the Schlumberger engineer aboard the Chikyu, with additional processing performed onshore by Schlumberger Information Solution. In order to meet the format constraints of the LogDB online database, some additional data reduction has been performed by personnel at the Borehole Group of the Lamont-Doherty Earth Observatory.

 

The following LWD/MWD services were employed in Hole C0002S :

 

LWD

arcVision ((phase and attenuation resistivity, gamma ray, annulus temperature and pressure, equivalent circulation density)

MicroScope (Gamma ray, resistivity, resistivity Images)

SonicScope (velocity)

TeleScope (drilling parameters)

seismicVision (seismic data)

 

MWD

TeleScope (torque, drilling weight on bit)

 

Multiple runs were carried out at Hole C0002S:

 

LWD/MWD tools
Run
Top Depth (m WMSF)
Bottom Depth (m WMSF)
Casing Depth (m WMSF)
Notes
Telescope
1
~2802
Real Time only
arcVision/MicroScope/SonicScope/TeleScope/seismicVision
2
2772
2933
~2802

Drilling went smoothly in hole C0002S.The first run acquired only MWD data in real tim, without downhole torque or weight on bit data. The second run acquired a full suite of LWD/MWD logging data.

 

During run 2 the rate of penetration was stable at about 2 m/hr. The high ROP observed between 2876 and 2880 LSF is an artifact due to tripping after an orientation survey.

 

Processing

 

Depth shift: The original have been depth shifted to the sea floor (- 1967.5 m) by the logging scientists aboard the ship using Schlumberger's Techlog software. This value matches the water depth used in the C0002P hole drilled during expedition 348 and hole C0002Q drilled in expedition 358. The sea floor depth was originally determined by the step in gamma ray and resistivity values at the sediment-water interface. The sonic waveforms were depth-shifted by personnel at the Borehole Group of the Lamont-Doherty Earth Observatory.

Gamma Ray data processing: The Gamma Ray logs have been environmentally corrected for bit size (8.5 in) and mud properties by the Schlumberger engineer aboard the ship.

 

Acoustic data: The data was processed onhore by Schlumberger personnel at Schlumberger Information Solution, who provided both compressional and shear wave outputs. The data was acquired in three modes: monopole, monopole-low frequency, and quadrupole mode. Compressional and shear data were obtained only between about 2802 and 2884 mLSF from the monopole and quadrupole mode data. However, the compressional velocity values are generally lower than in Hole C0002P in the 2836-2869 mLSF interval; this is believed to be caused by the pervasive drilling damages occurring in the proximity of Hole C0002S, casued by previous logging operations. Below 2869 mLSF, the hole is considered to be undamaged and the compressional velocities are consistent with C0002P and are therefore more reliable. Only spotty shear velocities could be computed and they are of questionable quality.

 

Sampling rates: The data was acquired with the following sampling rates:

 

Attenuation and Phase Resistivity (arcVision): 15.24 cm

Button Resistivity (MicroScope): 3.04 cm

Computed e-caliper (Microscope): 3.04 cm

Computed e-caliper (arcVision): 15.24 cm (published curve not available; recomputed at LDEO)

Gamma ray (arcVision): 15.24 cm

Miscellaneous (arcVision, TeleScope): 15.24 cm

Slowness/Velocity (SonicScope): 5.08 cm

Sonic waveforms (SonicScope): 5.08 cm

Temperature / Pressure (arcVision): 15.24 cm

 

IMPORTANT: Because Hole C0002S is a sidetracked hole, there is a difference between the measured depth (MD) and the true vertical depth (TVD). Because the difference is small (~1-2 m in total in expedition 358 Holes C0002Q and C0002S), the scientists decided to use the the measured depth instead of the true vertical depth for all the measurements reported in the IODP Proceedings. Accordingly, we have used the measured depth in the files included in the LogDB online database.

Quality Control

 

During the processing, quality control of the data is performed by inter-comparison of all logging data to ensure that reasonable values are returned for expected lithology types and that features on the logs reflect true formation characteristics and are not artifacts. The best data are acquired in a circular borehole. A good data quality indicator is usually given by the caliper measurement of the borehole diameter. In Hole C0002S, however, none of the tools deployed provided a direct measurement of the borehole diameter. Borehole size (e-caliper) was computed from the MicroScope data by Schlumberger, using an inversion technique. A second e-caliper was calculated onboard from the arcVision resistivities using Techlog but it was missing from the dataset provided on the CDEX web site. It was recomputed at LDEO using the Arcwizard module in Techlog; the output is similar to that published in the site report of the IODP Proceedings (Figure 75) but is not identical. The MicroScope e-caliper (HD_MI6) is only available below 2836 mLSF. According to the IODP Proceedings, both the MicroScope and the arcVision-derived calipers are in good agreement below 2836 mLSF, while the latter seems to indicate a borehole as large as 30 inches above 2836 mLSF. The MicroScope e-caliper shows values of 11-12 inches between 2836 and 2869 mLSF and 9.5-10 inches below 2869 mLSF, suggesting that the hole conditions improved with depth.

 

Additional information about the drilling and logging operations can be found in the Site Expedition Report and in the Methods section, Proceedings of the International Ocean Discovery Program, Expedition 358.

 

For questions about the logging operations and the processing performed onboard, please contact:

 

Yukari Kido

E-mail: ykido@jamstec.go.jp

 

For database-related questions you may contact:

 

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia

 

Tanzhuo Liu

Phone: 845-365-8630

Fax: 845-365-3182

E-mail: Tanzhuo Liu