Chevron Gulf of Mexico Gas Hydrate JIP Drilling Program
Standard LWD Data Processing
Drilling contractor: Chevron
Logging contractor: Schlumberger
Hole: AC21-B
Expedition: JIP2
Location:
Latitude: 26° 56' 39.1900" N
Longitude: 94° 53' 35.6216" W
Sea floor depth (step in GR log): 4934 ftbrf
Sea floor depth (drillers'): 4934 ftbrf
Total penetration: 6050 ftbr
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 AC21-B:
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 AC21-B, the first ~10 feet were drilled with circulation and rotation rates of 200 gpm and 10 rpm, respectively. These rates were gradually increased to ~250 gpm and ~53 rpm near 60 ftbsf and maintained to these values until the entire LWD tool string was in the hole (~170 ftbsf). A this point, flow rate was increased to 350 gpm to activate all tools, rotation rate increased to 88 rpm, and the ROP to ~310 ft/hr. Rotation was further increased to ~108 rpm at 220 ftbsf. Drilling continued smoothly with sweeps every couple of stands until 285 ftbsf when ROP was reduced to ~180 ft/hr. Through for the target zone, and the flow rate was set to the minimum necessary for the LWD tools to run, to try reduce the risk of washouts in the sands. All drilling parameters remained generally constant until the total depth of 1116 ftbsf (6050 fbrf was reached at 18h15 on May 4.
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 (- 4934 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).
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