Chevron Gulf of Mexico Gas Hydrate JIP Drilling Program
GVR Image Processing Report
The LWD (Logging While Drilling) GVR tool maps the electrical resistivity of the borehole wall at three depths of penetration. Because the tool is rotating while drilling, its three electrodes (one for each penetration depth) provide 360° data coverage of the borehole wall. These data are displayed as an electrical image of the formation in either gray or color scale. The purpose of this report is to describe the images from the Chevron Gulf Mexico Gas Hydrate JIP Drilling Program and the different steps used to generate them from the raw GVR measurements. The GVR tool also takes total gamma radiation and resistivity logs, which are presented with the 'standard' data.
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
Azimuth Reference (P1AZ): 225.1°
The images are of good quality, and are fairly repeatable between the deep, medium and shallow GVR images. There are differences between the deep, medium, and shallow images that appear to be unrelated to hole diameter (DCAV).
Image
Processing
Processing
is required to convert the electrical current in the formation, emitted by the
GVR button electrodes, into a gray or color-scale image representative of the
resistivity changes. This is achieved through two main processing phases, the
first shortly after the data is downloaded from the tool by the Schlumberger
engineer, and the second post-cruise at LDEO-BRG.
1) Azimuthal
orientation and conversion to depth
The main processing steps are performed using Schlumberger's 'Ideal' software package by the Schlumberger LWD engineer, just after the raw data is downloaded from the tool. An azimuth and a depth are assigned to each measurement based on measurements of the pipe orientation and position at the rig floor. The resistivity measurements are assigned to 56 radial bins (each 6.4° wide). A full 360° revolution of resistivity data is sampled every 10 (or 20) seconds, therefore the data density in terms of depth depends upon the rate of penetration (ROP) into the formation – the slower the penetration, the more densely sampled the formation will be. For this hole, the ROP was targeted at 300 ft/hr until 1082 ftbsf when it was reduced to 180 ft/hr for the target zone of interest. At 1520 ftbsf, the ROP was reverted back to 300 ft/hr.
The
GVR tool does not move with a constant velocity down the hole: new sections of
drill pipe have to be added every 10 m and ship heave is never completely
compensated. This means that there will often be repeat measurements for one
particular depth in the borehole. The measurement that is used is the first one
taken at a particular point, before the borehole has had time to deteriorate.
The
effects of ship heave are sometimes apparent as horizontal discontinuities in
the image. They exist because it can be difficult, with a long drill string, to
accurately determine the depth of the bit based on measurements on the rig
floor.
The
GVR data is output from the Ideal software as a depth-indexed DLIS file.
2)
Image Normalization:
The DLIS file is loaded into the
Schlumberger GeoQuest GeoFrame software at LDEO-BRG, where the depth-based
image for each depth of penetration (shallow, medium, and deep) is normalized
both statically and dynamically.
In 'static
normalization', a histogram equalization technique is used to obtain the maximum
quality image. In this technique, the resistivity range of the entire interval
of good data is computed and partitioned into 256 color levels. This type of
normalization is best suited for large-scale resistivity variations.
The image can be enhanced when it is
desirable to highlight features in sections of the well where resistivity
events are relatively subdued when compared with the overall resistivity range
in the section. This enhancement is called 'dynamic normalization'.
By rescaling the color intensity over a smaller interval, the contrast between
adjacent resistivity levels is enhanced. It is important to note that with
dynamic normalization, resistivities in two distant sections of the hole cannot
be directly compared with each other. A 6-ft normalization interval is used.
The normalized images are shifted to a
sea-floor reference and converted to gif files using in-house software. They
are presented in the downhole log online database. The image is displayed as an
unwrapped borehole cylinder. A dipping plane in the borehole will be displayed
as a sinusoid on the image; the amplitude of this sinusoid is proportional to
the dip of the plane. The images are oriented with respect to the north, hence
the strike of dipping features can also be determined.
For further information or questions about the processing, please contact
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-3182
E-mail: Tanzhuo Liu