Previous   |    Next

doi:10.2204/iodp.proc.314315316.115.2009

Data and log quality

Hole C0003A

Available data

Hole C0003A was drilled with LWD-MWD-APWD tools. Similar to Hole C0002A, all MWD-APWD data and a limited set of LWD data were transmitted in real time. As the tools were not recovered (see “Operations”), these real-time data are the only data available for this site. In addition to surface and downhole drilling parameter logs, this data set includes the following main geophysical logs:

• Bulk density (RHOB_DH) and bulk density correction (DRHO_DH) computed downhole with a less sophisticated algorithm than the one normally used at the surface with the memory data;
• Thermal neutron ratio (TNRA_ADN_RT) and thermal neuron porosity (TNPH_ADN_RT);
• Average borehole diameter from the ultrasonic caliper (ADIA);
• Natural gamma ray log (GR) and resistivities (bit [RES_BIT_RT] and ring [RES_RING_RT], shallow [RES_BS_RT], medium [RES_BM_RT], and deep [RES_BD_RT] button) from the geoVISION resistivity (GVR) tool;
• Hole deviation data such as relative bearing (RB_RT), hole azimuth (HAZI_RT), and hole deviation (DEVI_RT); and
• Δt compressional wave transit time (DTCO) and sonic compressional semblance (CHCO) and semblance (DTCO and CHCO) from the sonicVISION tool (see Table T2 in the “Expedition 314 methods” chapter).

Depth shift

As Hole C0003A was jetted-in to 50 m LSF at a reduced pump rate, real-time fluid pulse data were available only while the bit was below 50 m LSF. Therefore, geoVISION data are available from ~50 m LSF (Fig. F5, yellow zone), whereas adnVISION data (sensors located ~27–29 m above) are available from ~22 m LSF (Fig. F5, orange zone). Therefore, the mudline was impossible to identify based on a first response in the gamma ray (GR) log and so was picked at 2481.5 m DRF. The depth-shifted version of the main drilling data and geophysical logs in Hole C0003A is given in Figure F5. Figure F6 presents the time-depth relationship linking the time (Fig. F4) and depth version (Fig. F5) of the data in Hole C0003A.

Logging data quality

Figure F5 shows the quality control logs for Hole C0003A real-time LWD data. After the hole was jetted-in to 50 m LSF, the target ROP of 35 m/h was generally achieved to 525.5 m LSF, where loss of the BHA stopped drilling operations (see “Operations”). Below 50 m LSF, SPPA slightly increased from 15 to 18 MPa. CRPM also progressively increased from 30 to 50 rpm, then 80 rpm, and stabilized at 100 rpm below 190 m LSF. Average annular pressure and ECD followed the normal trend of increasing with depth with moderate jumps at pipe connections. Even though not perfectly calibrated (negative temperature value above 280 m LSF), annular temperature (ATMP_MWD) shows a normal increase with depth until the tool became stuck and pump flow increased.

Based on previous experience in Hole C0002A, drilled with a similar ROP, time after bit (TAB) measurements were ~5 min for resistivity and gamma ray logs (geoVISION tool), except in short depth intervals corresponding to pipe connections. TAB measurements for density and neutron porosity logs are ~45–60 min. The real-time acoustic caliper (ADIA) log, which gives the average diameter of the LWD borehole, is the best indicator of borehole conditions. The ECAL_RAB caliper is a resistivity-derived caliper. In comparison to the ADIA, it has the advantage of providing a caliper reading shortly after the hole has been drilled, but its physical foundation (resistivity derived) is less reliable than the acoustic reading (relying only on drilling fluid sound velocity). The ADIA caliper should read a value of 8.5 inches. (21.6 cm) for a perfect in-gauge hole but instead showed values slightly >9 inches for the depth interval from 30 (first reading) to 75 m LSF and slightly <9 inches (22.9 cm) for the depth intervals 155–390 and 460–525.6 m LSF. ADIA exceeds 10 inches in a low–gamma ray zone (sandy formation; 75–155 m LSF). ADIA also shows a complex washout/​bridge pattern between 390 and 460 m LSF that is not concomitant to any change in gamma ray value (lithology indicator) and was therefore interpreted as the result of a fault zone. Hole deviation never exceeded 3°. LWD tools experienced moderate radial and tangential shocks. The highest shocks (SHKPK) correspond to the lower section of the sand-rich interval (110–155 m LSF) where the stick-slip indicator (STICK) gradually increased to 450 m LSF, where it reduced and stabilized to a value of 150 rpm (far below the 250 rpm limit that can impair image quality and therefore also far below the maximum rating of the tool).

The washouts in the interpreted fault zone and, more specifically, in the upper sand-rich interval are associated with major decreases in the bulk density log (RHOB) where bulk density correction (DRHO) could not be fully compensated for this major washout and are therefore underestimated (Fig. F5). Otherwise, a stand-off <1 inch (2.5 cm) between the tool and the borehole wall indicates high-quality density measurements with an accuracy of ±0.015 g/cm³. Comparison between deep button (RES_BD) and shallow button (RES_BS) resistivity values shows that drilling fluid invasion is null or not significant, confirming the short TAB readings.

Because of the loss of the LWD tools, sonic data are limited to real-time downhole automatic picking of compressional arrival time (DTCO) and coherence (CHCO) information. The full waveforms were not available, limiting further processing and detailed quality check. However, interval velocity data derived from the real-time check shot show a surprisingly good fit with the upper limit of V_P data derived from DTCO from 162 to 523 m LSF (see “Log-seismic correlation”).

Top of page   |    Previous   |    Next