Chevron Gulf of Mexico Gas Hydrate JIP Drilling Program

Logging-While-Drilling Data Processing

JIP drilling contractor: Chevron

JIP logging contractor: Schlumberger-Anadrill

Hole: AT14-1

Expedition: 1

Location: Atwater Valley (Gulf of Mexico)

Latitude: 27°56' 15.4" N (NAD27)

Longitude: 89°16'15.3" W (NAD27)

Sea floor depth (as seen on logs): 1312.5 mbrf

Total penetration: 286.6 mbsf

Logging Tools

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 Atwater Valley 14-1:

LWD EcoScope (ARC resistivity, density, porosity, pressure, temperature, and gamma ray)
LDW adnVision (ADN density, porosity, and caliper)
LDW geoVision (GVR resistivity and gamma ray)
MWD = Measurement-While-Drilling (azimuth and shock)
LWD proVision (NMR magnetic resonance, porosity, and bound fluid volume)

A list of the tool and log acronyms is available at http://brg.ldeo.columbia.edu/data/brg/ghp/ghp-database_documents/jip-acronyms.html.

In Hole Atwater Valley 14-1, the rate of penetration was approximately 15-28 m/hr, with a bit rotation average of 80 rpm. The well was initially drilled with only seawater as the drilling fluid, but as the drilling progressed periodic sweeps of attapulgite-based drilling mud were used to sweep and stabilize the hole.

A controlled spud in drilling protocol was used at the Atwater sites in an attempt to collect high quality GVR data near the surface.  This involved drilling at a low mud flow rate and with a limited penetration rate; in these conditions, the turbine-powered tools including the LWD Vision/EcoScope and MWD/MWD ProVision, did not operate.  These tools operated correctly after the mud flow rate was increased at approximately 25 mbsf.

Processing

Processing was performed by personnel at the Borehole Research Group of the Lamont-Doherty Earth Observatory.

Depth shift: The original logs have been depth shifted to the sea floor (-1312.5 m). The sea floor depth was determined by the step in gamma ray and resistivity values at the sediment-water interface.

Resistivity data: The GVR resistivity is sampled every 0.03048 m (1.2 in).

Nuclear Magnetic Resonance Data: The proVISION tool data was processed onshore by Schlumberger-Anadrill and returned to the drill ship during the cruise.

Quality Control

The quality of the data is assessed by checking against reasonable values for the logged lithologies, by repeatability between different measurements of the same property, and by correspondence between logs affected by the same formation property (e.g. the resistivity log should show similar features to the density 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), the density correction (DRHO), and the image derived density correction  (IDDR) measurements provide an indication of data quality.

The NMR porosities need to be interpreted with caution. The magnetic resonance porosity may be underestimated in clay-rich formations that have small pores with fast T2 relaxation times. Murray et al. (2006) note that this underestimation of porosity is most likely to take place when the NMR echo spacing is greater than 200 microseconds. The proVISION tool is limited to a minimum echo spacing of 800 microseconds, and the NMR porosities measured by this tool may be underestimated.

Additional information about the drilling operations can be found in the cruise report.

For further questions about the logs, please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia

BIBLIOGRAPHY

Murray, D. R., R. L. Kleinberg, B. K. Sinha, M. Fukuhara, O. Osawa, T. Endo, and T. Namikawa, 2006, Saturation, Acoustic Properties, Growth Habit, and State of Stress of a Gas Hydrate Reservoir from Well Logs, Petrophysics, 47(2), p. 129-137.