Miscellaneous LWD Image Data Processing

 

Drilling and logging operator: CDEX

Expedition:  338

 

Perhaps the best known of the Logging While Drilling (LWD) image logs are the geoVISION Resistivity At Bit (GVR) images (formerly know as RAB images), which are generated as the tool rotates while drilling, providing 360 data coverage of the borehole wall. In addition to the GVR resistivity images, some of the other LWD measurements, made by the GVR and EcoScope tools provide equivalent images. These include density, photoelectric effect, gamma radiation, borehole radius, and tool standoff. In contrast to the GVR resistivity images, the other LWD images are scaled manually and not dynamically normalized (in order to retain the original data units, eg. g/cm3 for density). Otherwise, processing is similar to that applied to the GVR data. Gif images are provided in 100 m intervals for all data, and additionally in 20 m intervals for data sampled at 3.048 cm.

 

The following table provides a summary of the images available, their sampling rate, and the codes used in the GeoFrame processing and online:

 

Property LDEO-BRG Code GeoFrame Code Sample Interval (cm) Number of Radial Bins Tool
Gamma radiation GR GR_RAB_IMG 15.24 56 geoVISION

 

The following table provides a summary of the intervals logged at each hole as well as technical information about the logging-while-drilling.

 

Hole Depth Range (m)

Approx. Rate of

Penetration (m/hr)

Approx. Rotation Rate (rpm) Comment
C0002F 874-2006 15-35 30 - 90 geoVISION
C0012H 58-709 >40 30 - 90 geoVISION
C0018B 38-350 30-45 40 - 90 geoVISION
C0021A 0 -294 35-45 30 - 90 geoVISION
C0022A 38-420 20 - 45 60 - 90 geoVISION

 

Image Processing

 

Processing is required to convert the initial measurements into a gray or color-scale image. 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, just after the raw data is downloaded from the tool. The azimuth of the sensors relative to north is set at the rig floor, and subsequent pipe rotation is tracked during drilling so that the image is oriented correctly. For the measurements made by the density sensor in the EcoScope or ADN tools, a full revolution (360°) of data is sampled every 10 seconds, so the vertical resolution depends upon the rate of penetration (ROP) into the formation - the slower the penetration, the more densely sampled the formation will be. ROP is typically in the 25-45 m/hr range, and the rotation is typically around between 30 and 90 rpm.

 

The depth assigned to LWD data is derived from the known length of pipe and the vertical position of the top drive at the rig floor. After the LWD data is downloaded from the tool, the depth data are merged with it based on accurately synchronized time data. 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 exactly determine the depth of the bit based on measurements on the rig floor.

 

The LWD tools do 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 LWD data is output from the Ideal software as a depth-indexed DLIS file.

 

2) Depth Shift and Image Generation

 

The DLIS file is loaded into the Schlumberger GeoFrame software at LDEO-BRG, where the image for each measurement is shifted so that the depth is relative to sea floor, and output gif and DLIS files are produced. The color scale ranges for the gif images are chosen manually.

 

The image is displayed as an unwrapped borehole cylinder. A dipping plane in the borehole appears 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 North, hence the strike of dipping features can also be determined.

Additional information about the drilling and logging operations can be found in the Operations and Downhole Measurements sections of the expedition reports, Proceedings of the Integrated Drilling Program, Expedition 338.

 

For questions about the online database please contact:

 

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia

 

For questions about the processing of the resistivity images please contract:

 

Tanzhuo Liu

Phone: 845-365-8630

Fax: 845-365-3182

E-mail: Tanzhuo Liu

 

For questions about the logs, please contact:

 

Yoshinori Sanada

E-mail: sanada@jamstec.go.jp

 

Yukari Kido

Email: ykido@jamstec.go.jp

 

Yuichi Shinmoto

Email: shinmoto@jamstec.go.jp