Logging-While-Drilling Data Processing
IODP logging contractor: USIO/LDEO
Location: Costa Rica Pacific Margin - CRISP (equatorial NE Pacific)
Latitude: 8° 35.5415 ' N
Longitude: 84° 4.6313' W
Logging-while-drilling date: March 18-19, 2011
Sea floor depth (driller's): 537 m DRF
Sea floor depth (logger's): 532 m LRF (from adnVISION)
Total penetration: 992 m LRF (455 m LSF)
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 LWD/MWD tools were deployed in Hole U1378A:
adnVISION 675 (density, porosity, caliper)
arcVISION (resistivity, gamma ray, temperature, pressure)
geoVISION 675 (RAB resistivity and gamma ray)
TeleScope 675 (MWD tools, power and real-time transmission)
All tools are 6 3/4 in. in diameter in a BHA with 8 1/2 in. bit and 6 1/2 in. drill collars.
The drilling of hole U1378A proceeded smoothly in the top 300 m, with a rate of penetration of about 25-30 m/hr. Drilling slowed down in the 300-455 m LSF interval, where the rate of penetration dropped to about 13 m/hr. At 455 m LSF the high standpipe pressures and backflow observed during the connections indicated hole-cleaning problems. The bit was lifted and the hole was reamed to improved circulation; after several hours of attempts to deepen the hole it was decided that progress was too slow and drilling stopped. Unfortunately, problems were encountered during geoVISION data acquisition. While natural gamma ray and button resistivity data were acquired correctly in Hole U1378A, the azimuthal system that collects tool orientation data did not work properly. The geoVISION data were sent Schlumberger Houston processing center to be corrected, but efforts to address the data acquisition problems were not successful.
Depth shift: The original logs have been depth shifted to the sea floor (- 532 m). The sea floor depth was determined by the step in the density log from the adnVision tool. Note that this differs 5 m from the seafloor depth tagged by the drillers at 537 m DRF.
Gamma Ray data processing: Processing of the data is performed onboard by Schlumberger personnel after downloading of the data from the tool. Gamma Ray data is measured as Natural Gamma Ray (GR): the GR from the geoVISION and arcVISION tools has been corrected for hole size (bit size), collar size, and type of drilling fluid.
Neutron porosity data processing: The neutron porosity measurements have been corrected for standoff, temperature, mud salinity, and mud hydrogen index (mud pressure, temperature, and weight).
The porosity data are available with two sampling rates: 0.1524 m (0.5 ft) and 0.0305 m (1.2 in).
Density data processing: Density data have been processed to correct for the irregular borehole using a technique called 'rotational processing', which is particularly useful in deviated or enlarged borehole with irregular or elliptical shape. This method measures the density variation while the tool rotates in the borehole, estimates the standoff (distance between the tool and the borehole wall), and corrects the density reading. Density data are available with two sampling rates: 0.1524 m (0.5 ft) and 0.0305 m (1.2 in). A porosity curve (DPHI) has been calculated from the IDRO curve using a matrix density of 2.65 g/cc and a fluid density of 1.025 g/cc. This is included in the composite plot available online.
Resistivity data: The geoVISION resistivity is available with two sampling rates: 0.01524 m (0.5 ft) and 0.03048 m (1.2 in) sampling rate. The ARC resistivity is sampled with a 0.1524 m (0.5 ft) sampling rate.
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; this is particularly true for the density tool, which uses clamp-on stabilizers to reduce standoff and to ensure proper contact with the borehole wall. A data quality indicator is given by the hole diamtere, measured by the density caliper (DCAV) and by the ultrasonic caliper measured along four axes (1-5,2-6,3-7 and 4-8); the calculated average of the ultrasonic measurements is given by the UCAV (ultrasonic caliper average). Another quality indicator is represented by the density correction (DRHO and IDDR).
Additional information about the drilling and logging operations can be found in the Operations and Downhole Measurements sections of the expedition report, Proceedings of the Integrated Drilling Program, Expedition 334. For further questions about the logs, please contact:
E-mail: Tanzhuo Liu
E-mail: Cristina Broglia