Chevron Gulf of Mexico Gas Hydrate JIP Drilling Program
Standard LWD Data Processing
Drilling contractor: Chevron
Logging contractor: Schlumberger
Hole: GC955-H
Expedition: JIP2
Latitude: 27° 00' 02.0707" N
(NAD27)
Longitude: 90° 25' 35.1142" W
Sea floor depth (step in GR log): 6718 ftbrf
Sea floor depth (drillers'): 6721 ftbrf
Total penetration: 8654 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, 4 3/4" collars for MP3) were deployed in hole WR313-H:
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 GC955-H, the first 10 ftbsf were drilled while circulating 200 gpm and rotating 10 rpm. Between 10 and 178 ftbsf, flowrate was increased to 250 gpm and the rotation to 55 rpm, after which point they were increased to 350 gpm and 87 rpm, respectively. At 653 ftbsf, rotary speed increased to 120 rpm. Throughout, the ROP was targeted at 300 ft/hr. Drilling continued relatively smoothly with sweeps every few stands until 1082 ftbsf when ROP was reduced to 180 ft/hr for the target zone of interest. At the same time, drilling fluid was changed to a 10.5 ppg water-based mud in anticipation of more difficult drilling. One hundred feet later, rotary speed increased to 135 rpm in response to torque. At 1520 ftbsf, ROP and rotary speed were reverted back to 300 ft/hr and 120 rpm, respectively. Total depth of 1936 ftbsf was reached at 23h20 on April 25.
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 (-6718 ft). The sea floor depth was determined by the step in gamma ray and resistivity values at the sediment-water interface. In addition, the MP3 tool acoustic slowness and velocity were matched to the same curves from the sonicVision tool.
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