Drilling Induced Fracture (DIF) Characterization and Stress Pattern Analysis of the Southern McMurdo Sound (SMS) Core, Vitoria Land Basin, Antarctica ABSTRACT Ezer Patlan Academic Affiliation, Fall 2008: Graduate Student, University of Texas at El Paso RESESS Summer 2008 Science Research Mentor: Dr. Terry A. Wilson and Cristina Millan Writing and Communication Mentor: Kelly Carroll There is a significant lack of data about present day stress fields in Antarctica. Stresses provide valuable information data about the forces acting on the plates. In Antarctica, stresses may be related to ridge forces such as rifting and/or uplifting, to ice loading/unloading related processes, or both. This project involves the study of drilling induced fractures from core recovered in the Victoria Land rift basin of Antarctica. Drilling induced fractures form ahead of the drill bit during drilling from stress imbalances due to the removal of excess weight pressure around the rock. Because horizontal stresses strike parallel to the planes made by drilling induced fractures, they can be used to measure modern day stress fields. Whole core images obtained during core logging by digitally scanning the outside of the core are stitched into longer intact intervals. Drilling induced fractures in the core are picked in order to obtain their azimuth. Magnetically oriented acoustic images of the inside of the drill hole are then compare side by side with the stitched whole core images and visually scanned for matching features. Once the same set of fractures is found in the core and the borehole is then possible to rotate to core images to match the orientation of the borehole image. This will result on a core image with all the fractures in that interval re-oriented to true north. This final orientation of drilling induced fractures in the core will thus provide the direction of maximum horizontal compressional stress in this area. This work was performed under the auspices of the Research Experience in Solid Earth Science for Students (RESESS) program. RESESS is managed by UNAVCO, with funding from the National Science Foundation and UNAVCO. RESESS partners include the Significant Opportunities in Atmospheric Research and Science Program, the Incorporated Research Institutions for Seismology, the United States Geological Survey, and Highline Community College.
1. Introduction There is a significant lack of data about present day stress fields in Antarctica and about the geological nature of these stresses. It is important to study stresses because they can provide valuable information about forces acting on the plates and, therefore, the dynamics of plate tectonics. In Antarctica, stresses may be related to ridge forces such as rifting and/or uplifting, to ice loading/unloading related processes, or to both. This project involves the study of drilling induced fractures from core recovered in a rift basin in the Victoria Land Basin of Antarctica, which is located in the westernmost branch of the Ross Sea rift system (Figure 1). Drilling induced fractures form during drilling as a result of in situ crustal stresses surrounding the rock, as well as stress imbalances caused by removal of excess loading pressure (Kulander et. al., 1990). Drilling induced fractures in the rock indicate the forward direction of the drill bit, an element at the end of a drill pipe that actually does the cutting. Because these fractures may extend sidewise beyond the column to be drilled, they may also be present in the borehole walls, which ultimately provide the means to extract contemporary horizontal stresses and fault regimes at the regional scale for the Antarctic continental interior. Figure 1: Map Antarctica with area of study enlarged at right. Note location of SMS drill hole used in this study. There are four types of DIF: petal, centerline, petal-centerline, and core edge fractures (Figure 2). Petal fractures are fractures that initiate at the edge of the borehole and curve smoothly downward toward the center of the core (Bell, 1996). Centerline fractures propagate ahead of the drill bit as a continuous fracture that vertically bisects the core. Petal-centerline fractures are petal fractures that grow to become a centerline fracture or that join with a centerline fracture. Core edge fractures are smooth curved fractures located along the edge of the core. RESESS 2008, Ezer, 2
Figure 2: Cartoon of three major types of drilling induced fractures and the direction of minimum and maximum horizontal stresses (from Yatschenko, 2008). Data recorded during core logging operations will be used to locate the depth, orientation relative to pre-assigned arbitrary north and type of drilling induced fractures. Core photos will aid in characterizing the different types of drilling induced fractures (Figure 3). Whole core images obtained during core logging by digitally scanning the outside of the core will be used to stitch the core into longer core intervals. These will then be visually scanned in order to pick the individual drilling induced fractures that will be later used for core re-orientation and, ultimately, to measure the contemporary horizontal stresses. Borehole Televiewer (BHTV) images, acoustic images of the inside of the borehole walls, will be used to re-orient the stitched core intervals to true north (Paulsen et. al., 2000). Geophysical software packages such as CoreBase and WellCAD will be used to stitch, orient, analyze, describe and characterize drilling induced fractures. RESESS 2008, Ezer, 3
Figure 3: Digital photos of core-edge (left), petal and petal-centerline fracture (right). 2. Methodology The appropriate intervals used in this study, identified from the log sheets prepared during core logging operations, are intact core intervals that contain abundant drilling induced fractures. An intact core interval is a continuous interval of core in which run breaks the length of core brought out to the surface after each drilling episode and fractures within the core can be fitted together to resemble the way it was before being brought out to the surface. Breaks or fractures within the core, where material is too broken up or missing, or where there is evidence of internal core rotation cannot be fitted together and are not, therefore, suitable for use in this study (Figure 4). Figure 4: Characteristics of an intact core interval (from Paulsen et al., 2000) RESESS 2008, Ezer, 4
Digital images of the whole core are never longer than 1 meter but core intact intervals can be any length in SMS an intact interval up to 75 meters long has been identified. The individual scanned whole core images need first to be stitched together to the length of the identified intact interval. This is done with a special software called CoreBase. This software can be used to join core run breaks, to fix any depth inconsistencies that may have been introduced during image scanning and to fix the red scribe line-an arbitrary north drawn along the whole core length during logging operations against which all fracture azimuth orientations are measured-angle misfits across fractures (Figure 5). Once a whole length is stitched the software WellCAD is used to pick the orientation of the drilling induced fractures by drawing a best fit sinusoid to each fracture; from this sinusoid the software calculates a down dip and a dip azimuth for each fracture; note that this orientation is not relative to true north at this point. Because the drilling induced fractures in the core propagate into the drillhole walls, it is then possible to find the same fractures in both the stitched and the BHTV images (Jarrard et. al., 2001). The tool used to record these images is magnetically orientated to north and therefore its images are already properly oriented. The stitched and picked images will be then compared side by side with the BHTV images and then visually scanned in order to find the core and borehole wall fractures that are the same (Figure 6). Once the same set of fractures is found in the core and the borehole is then possible to rotate to core images to match the orientation of the borehole image. This will result on a core image with all the fractures in that interval re oriented to true north. This final orientation of drilling induced fractures in the core will thus provide the direction of maximum horizontal stress in this area (Bell, 2003) (Figure 2). Figure 5: Left: mismatch across fracture. Right: matched fracture. RESESS 2008, Ezer, 5
Figure 6: Left: picked DIF in the core image. Right: BHTV image orientated to true north. 3. Discussion & Results Whole core scan images from the Southern McMurdo Sound (SMS) core were stitched into 6 intact core intervals ranging from 5 meters to ~24 meters in length. From these intervals a total of thirteen best fit sinusoids from petal, petal-centerlines and core edge fractures were picked to obtain fracture orientation. Table 1 shows the depth and orientations of the fractures that were pick in the unoriented core intervals. A visual representation of the orientation of the picked fractures can be done with a stereonet (Figure 7). A problem introduced during BHTV downhole data acquisition the internal compass used for orientating was strongly biased by magnetization resulting in an internal drift of +/- 40 degrees away from north have hindered the efforts for orientation of these intervals to true north. RESESS 2008, Ezer, 6
Intervals Bottom Depth Bottom Depth (mbsf) Fracture Fracture Strike Dip N, E, S, W (mbsf) (mbsf) 753.92-761.34 753.86 753.91 141.53 79.29 W 755.45 755.50 329.73 67.92 E 760.40 760.45 163.60 80.97 W 778.57-784.31 778.77 778.82 337.08 81.35 E 837.64-851.98 845.81 845.86 94.58 81.36 W 919.16-926.32 925.98 926.03 119.26 66.11 W 1036.80-1049.13 1041.93 1041.98 54.97 21.91 E 1044.13 1044.18 63.33 79.23 E 1100.44-1117.15 1100.55 1100.60 72.28 33.03 E 1108.22 1108.27 248.23 20.04 W 1108.91 1108.96 146.40 29.57 W 1111.85 1111.90 87.43 25.64 E 1114.99 1115.04 52.90 16.97 E Table 1: Unoriented core intervals with DIF depth s and orientations. Figure 7: Stereonet represents the orientation of the thirteen DIF s in the intact core intervals. RESESS 2008, Ezer, 7
4. Conclusion Whole core scanned images from the Southern McMurdo Sound drill core of Antarctica were successfully stitched into six intact core intervals, but acquisition errors on the BHTV data impeded a final true north orientation of these intervals at this time. Reprocessing of the BHTV data to correct the 40 degree error will allow proper true north orientation of core images and, therefore, the direction of maximum horizontal stresses acting in the Antarctic continental interior. RESESS 2008, Ezer, 8
REFERENCES Bell, J.S., 1996. In situ stresses in sedimentary rocks (Part 1) Measurement techniques. Geoscience Canada 23 2, pp. 85 100. Bell, J.S., 2003. Practical methods for estimating in situ stresses for borehole stability Applications in sedimentary basins, Journal of Petroleum Science and Engineering, 38, British Columbia, Canada, pp. 111-119. Jarrard, R.D., Buecker, C.J., Wilson, T.J. And Paulsen, T.S., 2001. Bedding dips in the CRP-3 Drillhole, Victoria Land Basin, Antarctica, In: Studies from the Cape Roberts Project, Ross Sea, Antarctica: Scientific Report on CRP-3, Cape Roberts Project, Antarctica, Terra Antartica, 8(3), 167-176. Kulander, B. R., Dean, S. L. & Ward, B. J. 1990. Fractured core analysis: interpretation, logging, and use of natural and induced fractures in core, Methods in Exploration Series, 8, American Association of Petroleum Geologists, Tulsa, USA. Paulsen, T., Wilson, T., Moos, D., Jarrard, R., and Wilson, G., 2000. Orientation of CRP2A core, IN: Barrett, P.J. and Ricci, C.A. (eds.), Studies from the Cape Roberts Project, Ross Sea, Antarctica: Scientific Report of CRP-2/2A, Terra Antartica, V. 7: 271-278. Yatsenko, A., 2008. Tectonic Implications of AND-1B and AND-2A Borehole Data, McMurdo Sound Region, Antarctica, The University of Utah, Salt Lake City, Ms. Thesis. RESESS 2008, Ezer, 9