LWD Image Data Processing
The best known of the Logging While Drilling (LWD) image logs are the Resistivity At Bit (RAB) images (from the geoVISION tool), which are generated as the tool rotates while drilling, providing 360° data coverage of the borehole wall. In addition to the RAB resistivity images, some of the other LWD measurements, made by the RAB, EcoScope, and adnVISION tools provide equivalent images. These logs include density, photoelectric effect, volumetric photoelectric effect, borehole radius, and tool standoff. The adnVISION images are scaled manually and not dynamically normalized. Images in GIF format are provided in both full and 100 m intervals for all data, except for borehole radius and tool standoff images that have full depth intervals only, due to lower sample interval of these images.
The adnVISION images are generally of good quality, and the borehole features such as dips and breakouts are inter-comparable among the LWD images.
The following table provides a summary of the images available, their sampling rate, and the codes used in the GeoFrame processing and online database:
| Property | LDEO-BRG Code | GeoFrame Code | Sample Interval (cm) | Number of Radial Bins | Tool |
|---|---|---|---|---|---|
| Density | RHOB |
ROSI |
3.048 |
16 |
adnVISION |
| Photoelectric Effect | PEF |
PESI |
3.048 |
16 |
adnVISION |
| Volumetric Photoelectric Effect | VPE |
USI |
3.048 |
16 |
adnVISION |
| Borehole radius | Radius |
DCRA |
15.24 |
16 |
adnVISION |
| Tool Standoff | STOF |
UOIM |
15.24 |
8 |
adnVISION |
In addition to the adnVISION, the geoVISION tool also collected image data at both Holes U1378A and U1379A. Unfortunately, problems were encountered during geoVISION data acquisition in both holes. While natural gamma ray and button resistivity data were acquired correctly in Hole U1378A, the azimuthal system that collects tool orientation data did not work properly. At Hole U1379A, the time-depth processing usually performed onboard could not be run because of an improper recording of time by the geoVISION tool clock. The geoVISION data acquired in Holes U1378A and U1379A were sent Schlumberger Houston processing center to be corrected, but efforts to address the data acquisition problems were not successful.
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.
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 a tool, 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 25-50 m/hr range, and the rotation is typically around 60 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 Techlog software at LDEO, 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.
For further questions about the logs, you may direct your questions to:
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-3182
E-mail: Tanzhuo Liu
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia