At Borehole Image Specialists, we have been expanding our efforts to investigate subsurface structure by incorporating a look at archival data. The ancestor of the borehole imager was the dipmeter, which was developed by Schlumberger in the late 1920s. The dipmeter was invented to determine subsurface structure.
“As long as oil migrates updip, it would seem there is nothing more fundamental in oil exploration than determining which way is up.” (Goetz, 1992).
Dipmeters were used up until the mid-1980s as a way of assessing subsurface structural orientation. The dipmeter was able to measure 3 points from a dipping bed within the borehole. While this proved to be a workable solution to help determine structure, it sometimes failed if there was a problem with any one of the arms. A 4th arm was added which gave an increased level of accuracy. This technology evolved into modern borehole image logs, which supply a much greater degree of detail.
Thousands of paper printouts of dipmeter logs are contained in the deep archives of Oil and Gas E & P companies and in data repositories like the Denver Earth Resources Library (DERL). These datasets, largely ignored – even forgotten, represent a vast potential reservoir of basin knowledge. We have a great deal of experience teasing information out of these old archival records.
In order to be efficiently analyzed, paper printout data needs to be properly digitized. This is accomplished by a new generation of scanners that permit the correct orientation of curves and tadpole features at high resolution.
The diagram at left shows SHDT Dipmeter data from a 4 arm, 8 electrode tool. Although it provides a much clearer signal than from a 3-arm dipmeter, it is still difficult to analyze and interpret when compared with modern image log data. Nevertheless, sufficient resolution is present to permit the identification of bedding contacts.
Once digitized, the print-out data must be correctly oriented so that the spatial geometry of mapped bedding features will be accurate. In order to accomplish this, 4 pieces of information are required: Pad 1 Azimuth, Relative Bearing, Borehole Deviation, and Borehole Azimuth. On the diagram at right, the Orientation curve track plots 3 of these curves. The last of the 4 (Borehole Azimuth) can be calculated from the other 3. This capability is designed into dipmeter and borehole image software packages.
Digitized archival dipmeter data can be interrogated using modern software to establish the spatial geometry of bedding. Borehole elongation, which results from stress anisotropy, can be used to establish the trajectory of the minimum and maximum horizontal stresses acting on the borehole. In the absence of other data, present-day stress information can be a crucial piece of information for proper well placement and stimulation design.
The diagram at left displays a lower-hemisphere, equal-area (Schmidt) stereonet plot showing contoured poles-to-planes and superimposed rose petals showing the compass direction of bedding dips. A histogram is below the stereonet. It is scaled from 0° to 90° in 10° increments and shows the distribution of bedding dip angles. The tadpole plot on the right displays the depth and geometry of bedding picks from the dipmeter data plotted against the gamma response (scaled 0 to 200 API). Tadpoles plot at a specific dip angle (vertical lines scaled 0° to 90° in 10° increments) and point in the compass direction of the dip.
Here the lower part of the dipmeter data yielded many more bedding contact picks. Although there is a wide range of dip directions and dip angles, an overall average bedding dip trend is to the NNW at 349°.
The dip azimuth walkaway (or vector) plot here is used to establish broad trends in bedding dip character. The diagram is constructed by aligning bedding tadpoles head to tail starting at the bottom of the dipmeter data. Large changes in bedding dip direction are usually indicative of post-depositional structural deformation (folding or faulting) that has disrupted the normal position of bedding contacts near the horizontal. Sometimes, deflections of bedding dip direction are created by large-scale sedimentary processes, such as sequence boundaries or periods of non-deposition or erosive removal (unconformities).
In this instance, the minor wiggles in the walkaway plot are likely to reflect the lower angle bedding dips. The overall trend of the walkaway diagram is to the NNW.
It has been known for some time, that significant differences in the magnitudes of the minimum and the maximum horizontal compressive stresses (stress anisotropy) are mirrored in directional changes in borehole diameter. Specifically, boreholes drilled in rock with stress anisotropy commonly develop an elliptical cross-section that results from breakage, or spalling, of the rock material in the vicinity of the minimum horizontal compressive stress. The degree of ovality is affected by mud weight, pore pressure, and the magnitude of the stress differential.
Departure of a borehole from a circular cross-section is measured by calipers – information that is included with most dipmeter (and borehole image) data sets. By mapping caliper variation, the direction of the maximum elongation can be established and is taken to indicate the orientation of the minimum horizontal compressive stress. The maximum horizontal compressive stress is, by definition, orthogonal to this direction.
In the examples shown here, the elongation direction is given by the compass orientation of the black caliper breakout tadpoles on the previous diagram. When plotted on a stereonet, these data form a tight cluster that reveals a minimum horizontal compressive stress direction oriented NW-SE (red arrow). The maximum horizontal compressive stress (blue arrow) is 90° away.
Between 670’-705’ the calipers in the far right track indicate 1½” of separation. The second track from the right is the Caliper Size presenting mirror image calipers scaled 10-6 inches. These help to visualize the caliper differential of the breakouts. The Caliper Breakouts track on the (far right) shows the compass direction of the hole elongation.