A. R. Vásquez and M. Q. Sanchez, VVA Consultores
D. R. Van Alstine and J. E. Butterworth, Applied Paleomagnetics, Inc.
W. Poquioma, Maraven, S.A.

This study provides an important test of core-based versus downhole logging-based methods for determining the present-day direction of maximum horizontal in situ stress (sHmax) and the orientation of natural fractures. This study also provides a direct comparison test of 3 different core-orientation and fracture-orientation techniques: (1) the paleomagnetic core-orientation technique developed by Applied Paleomagnetics, Inc.; (2) “electronic multishot” scribed core; and (3) a combined acoustic/resistivity downhole logging tool (STAR).

Results of this 3-way comparison test are as follows:

The average strike (125°) of 4 paleomagnetically-oriented induced fractures in cores from Well X agrees with the average perpendicular (131°) to 53 wellbore “breakouts” identified by the acoustic/resistivity tool to within 6°. This provides a good check on the accuracy of the paleomagnetic and acoustic/resistivity orientation data. The paleomagnetically-oriented induced fractures and acoustic/resistivity-oriented wellbore breakouts are consistent in indicating a sHmax direction near 128°.


1. The acoustic/resistivity tool, however, grossly underestimated the true fracture density in this well. The acoustic/resistivity tool detected only 17 fractures in the entire Well X log (5267-6360=1093 ft) and detected virtually no fractures over an interval in which we identified 41 natural fractures in 86 feet of core. Moreover, all fractures identified by the acoustic/resistivity tool are from the same fracture phase, whereas at least two different fracture phases can clearly be recognized in the core.
   
   
2. All 17 fractures detected by the acoustic/resistivity tool strike NW/SE and represent what we identified as “Set 1” fractures in the core. The average strike of Set 1 fractures, oriented paleomagnetically and by the acoustic/resistivity tool, is within a few degrees of being perpendicular to the strike of a nearby major fault. This provides an independent “geologic check” on fracture-orientation accuracy, besides providing clues to the stress field responsible for Set 1 fractures.
   
   
3. The acoustic/resistivity tool lacked sufficient resolution, however, to detect that the Set 1 fractures, as observed in the core, form a clearly-defined, bimodal distribution of conjugate shear fractures with a dihedral angle of 20° and with opposite senses of dip. In the paleomagnetically-oriented core, Set 1a fractures exhibit an average strike of 144° and average dip of 83° to the SW; Set 1b fractures exhibit an average strike of 124° and average dip of 81° to the NE. The 17 fractures identified by the acoustic/resistivity tool exhibit an average strike of 128° and average dip of 75° toward the SW.
   
   
4. The acoustic/resistivity tool completely failed to identify another fracture phase clearly present in the core which we designate “Set 2” fractures and which form an orthogonal set. Most (12/14=86%) Set 2 fractures are Set 2a fractures, exhibiting an average strike of N 8° E, which is within a few degrees of being perpendicular to another major fault trend in the vicinity of Well X.
   
   
5. Of the fractures present in the core, open natural fractures preferentially include the more southerly-striking (144°) Set 1a fractures and nearly due-south-striking Set 2a fractures. The optimum trajectory for a deviated well near Well X, therefore, would be perpendicular to 144° (if designed to maximize intersection of open Set 1a fractures) or perpendicular to 160° (if designed to “bisect” the open Set 1a and open Set 2a fracture patterns). These trajectories are 9° to 25° more clockwise of an optimum trajectory based solely on the average strike (135°) of all Set 1 fractures, which was the only fracture set detected by the acoustic/resistivity tool.
   
   
6. In this study, good agreement (within 5° to 10°) between paleomagnetic and acoustic/resistivity orientations in contrast to poor agreement (discrepancies of 20° to 70°) between paleomagnetic and electronic-multishot orientations indicates orientation problems with electronic multishot.
   
   
7. In Core 2, the electronic-multishot tool failed completely, so no comparison was possible. In the other three cores, electronic-multishot exhibited systematic counterclockwise bias relative to the paleomagnetic/acoustic/resistivity orientations: -17° bias in Core 3, -70° bias in Core 4, and -21° bias in Core 5.
   
   
8. The boundary between Cores 3 and 4 represents a rare instance in which core can be perfectly reconstructed across a core-run boundary, providing an additional check on the accuracy of the electronic-multishot orientations. Across this core-run boundary, the -17° systematic electronic-multishot error in Core 3 “turns into” the -70° systematic electronic-multishot error in Core 4. Thus, electronic-multishot disagrees with itself by 53°.