Chevron Gulf of Mexico Gas Hydrate JIP Drilling Program
Standard LWD Data Processing
Drllling contractor: Chevron
Logging Contractor: Schlumberger
Hole: WR313-G
Expedition: JIP2
Location:
Latitude: 26° 39' 47.4841" N
(NAD27)
Longitude: 91° 41' 01.9404" W
Sea floor depth (step in GR log): 6615 ftbrf
Sea floor depth (drillers'): 6614 ftbrf
Total penetration: 10200 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-G:
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 WR313-G, the first 173 ft were drilled while circulating 48 gallons per minute (gpm). The flowrate was increased to 385 gpm but fluctuated over the next 200 ft, including a 64 ft-long section where it dropped low enough to
impact MWD turbine power. Only 15 rpm when the bit penetrated seafloor, collar rotation became 45 rpm by the time the geoVISION entered the hole and increased to 90 rpm at 181 ftbsf. The rate of penetration, ROP, ranged from 350
to over 1000 ft/hr until 240 ftbsf when it was toggled back to roughly the target rate of 160 ft/hr. At 1650 ftbsf, it became necessary to backream each stand and drilling became very difficult despite the increase in seawater sweeps. Rotary speed was increased to 120 rpm in response to torque, and the string would occasionally pack off despite multiple backreams per stand. A major packoff at 2630 ftbsf (stalling the rotary and requiring 140klbf of overpull) prompted a switch to 10 pound per gallon (ppg) water-based mud.Weighting up further to 10.5 ppg at 2984 ftbsf, the rest of the hole was drilled incident-free. Total depth of 3585 ftbsf was reached at 18h45 on April 20 and the hole was displaced with 12 ppg mud.
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 (- 6614 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. The original geochemical data were processed at Schlumberger to yield weight percentages of 9 major elements, including Titanium and Gadolinium (expressed as ppm).
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