A Fracture-Orientation Comparison Between Core-Based and Borehole-Imaging Techniques: Paleomagnetic, Electronic Multishot, and FMI
W. D. Hamilton, David R. Van Alstine, and Joseph E.
Accurate orientation of fractures is crucial to enhanced productivity in low-permeability fractured reservoirs. This study presents results of a rare instance in which three widely different fracture-orientation techniques could be directly compared for orientation accuracy. In a fracture-orientation study of Pardonet/Baldonnel (Upper Triassic) carbonates from the Gwillim field, British Columbia, a 50° to 60° fracture-orientation discrepancy was observed in two consecutive core runs oriented by "electronic multishot" (an electronic version of the conventional "multishot" core-orientation technique) versus Schlumberger's Fullbore Formation Microlmager (FMI). To resolve this discrepancy, the paleomagnetic core-orientation technique was employed as a third fracture-orientation method.
The paleomagnetically-determined fracture orientations agreed within 5° with FMI and confirmed a systematic error in the electronic multishot orientations of 46° in Core 1 and 60° in Core 2. Superimposed on the systematic error was a second-order, oscillatory "torsion error" of ±10°, which probably reflects acquisition of electronic multishot data within a rotating drillstring (unlike conventional multishot where coring is stopped before each "shot").
The paleomagnetic and FMI data reveal that natural and induced fractures have different orientations at this well location in the Gwillim field. Natural fractures dip 72° toward N 45° E, nearly orthogonal to bedding which dips 15° toward S 45° W. In contrast, induced petal fractures strike N 10° E, at a 55° angle to the strike of natural fractures and bedding. These angular relationships suggest there has been a 35° change in the principal horizontal stress direction between the Laramide paleostress field (controlling the strike of natural fractures and bedding) and the present-day in situ stress field (controlling the strike of induced and hydraulic fractures).
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California