Chevron Gulf of Mexico Gas Hydrate JIP Drilling Program
Standard LWD Data Processing
Drilling contractor: Chevron
Logging contractor: Schlumberger
Hole: GC955-Q
Expedition: JIP2
Location:
Latitude: 27° 00' 07.3484" N
Longitude: 90° 26' 11.7156" W
Sea floor depth (step in GR log): 6564 ftbrf
Sea floor depth (drillers'): 6567 ftbrf
Total penetration: 8078 ftbrf
The logs were recorded using the LWD/MWD (Logging-While-Drilling/Measurement-While-Drilling) technique, which allows the acquisition of open-hole logs using 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 after it is drilled, so that it is in good condition and largely free of wash-outs.
The following LWD/MWD services (6-3/4" collars) were employed in hole GC955-Q:
LWD EcoScope (resistivity, density, porosity, pressure, temperature, gamma ray, and geochemistry)
LDW geoVision ( resistivity and gamma ray)
LDW sonicVision (acoustic velocity)
LWD MP3 (acoustic velocity)
LWD PeriScope (resistivity)
In hole GC955-Q, the first 64 ftbsf were drilled while circulating 200 gpm and rotating 14 rpm. Between 64 and 171 ftbsf, flowrate was increased to 250 gpm and the rotation to 52 rpm, after which point they were increased to 350 gpm and 105 rpm, respectively. The first 190 ftbsf were drilled with an erratic rate of penetration, ROP, that averaged roughly 600 ft/hr. After that the ROP was targeted at 400 ft/hr. Drilling continued smoothly with sweeps every couple of stands until 1000 ftbsf when rotary speed was increased to 135 rpm in response to drilling conditions. At 1238 ftbsf, the ROP was reduced to 180 ft/hr for the target zone of interest. At the same time, drilling fluid was changed from seawater to 10.5 pound per gallon (ppg) water-based mud. When making a connection at 1516 ftbsf, shortly after real-time measurements had indicated the bit had reached hydrate-rich formation, a gas bubble was visually observed at the wellhead by the ROV. Suspecting a gas kick, it was ordered to kill the well with 13.0 ppg mud. The total depth of 1516 ftbsf was reached at 00h04 on April 27.
Standard processing was performed by personnell at the Borehole Research Group of the Lamont Doherty Earth Observatory. Processing of the acoustic and geochemical data was performed by Schlumberger personnel.
Depth shift. The original logs have been depth shifted to the sea floor (- 6564 ft). The sea floor depth was determined by the step in gamma ray and resistivity values at the sediment-water interface.
Gamma Ray data. Processing of the data is performed in real-time onboard by Schlumberger personnel. Gamma Ray is measured as Natural Gamma Ray: the GR from the geoVision tool has been corrected for hole size (bit size), collar size, and type of drilling fluid.
Density data. The 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 boreholes with irregular or elliptical shape. This statistical 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.
Neutron Porosity data. The neutron porosity measurements have been corrected for standoff, temperature, mud salinity, and mud hydrogen index (mud pressure, temperature and weight).
Resistivity data. The geoVision resistivity is sampled every 1.2 inches; the Ecoscope attenuation and phase resistivities are sampled every 0.5 ft.
Acoustic data. The acoustic data from the sonicVision and MP3 tools were processed by Schlumber to yield a good quality delay time. The velocity has been calculated from this value.
Geochemical data. Add something here.
The quality of
the data is assessed by checking against reasonable values for the logged
lithologies, by repeatability between different passes of the same tool, and by
correspondence between logs affected by the same formation property (e.g. the
resistivity log should show similar features to the sonic velocity log).Data acquired in a circular in-gauge borehole are generally free of artifacts. An enlarged borehole can affect the logs, particularly density and porosity. The average density caliper (DCAV) and the image derived density correction (IDDR) measurements provide an indication of data quality.
Additional information about the drilling operations can be found in the expedition report.
For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Crisitna Broglia
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-3182
E-mail: Tanzhuo Liu