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Rock stress orientation from borehole breakouts and its correlation to drill parameters and geology: results from boreholes KFM01A and KFM05A of the Forsmark site investigation, Sweden
2007 (English)Student paper otherStudent thesis
Abstract [en]

Knowledge of stress orientation is crucial for the understanding of many processes in the Earth's crust such as tectonic development, earthquake occurrence, and fluid transport along faults. In the Forsmark site investigation: this knowledge plays an important role for engineering decisions with respect to borehole stability and the siting of the future repository for nuclear waste. Breakouts are zones of failure of the borehole wall in response to high compressive tangential stresses. The failures elongate the borehole cross-section from the original circular shape and are observed by geometrical logging tools, for example borehole televiewer (BHTV). Borehole breakouts are reliable indicators of the orientation of the maximum horizontal stress [e.g. Bell and Gough, 1979: Zoback et al., 1985]. The borehole breakout method is a stress method that provides continuous information on stress in intervals where borehole breakouts occur, which makes this method unique in comparison to other existing methods. In other words, this method provides information on the continuation of the stress field along a borehole. The orientation of borehole breakouts can be measured using mechanical (three-, four- and six-arm caliper), acoustic (BHTV) or electrical resistivity (e.g. Formation MicroScanner (FMS) and Formation MicroImager (FMI)). Optical logging tools such as BIPS and borehole cameras can be used to view borehole breakouts. While BHTV and FMS/FMI tools provide excellent data for breakout analysis, borehole cameras, BIPS, and caliper tools are known to have poorer quality. At the Forsmark Site Investigation, the borehole geometries have been logged using BHTV and BIPS. This work presents stress orientation data derived from a set of BHTV logs for the core-drilled, approximately 1 km deep, boreholes KFM01A and KFM05A. The two boreholes were drilled by the Swedish Nuclear Fuel and Waste Management Co. (SKB) as part of their site investigation program for storage of nuclear waste in hard rock in Forsmark, Southeast Sweden. The study has the following objectives: (1) determine the downhole orientation of horizontal stresses: (2) correlate borehole breakouts with Measurement While Drilling (MWD) parameters: (3) study the influence of lithology and structures on rock stress orientation: and (4) identify the zones of rock continuum. Two different types of breakouts were recognized from the amplitude log with both shallow and deep failure depths. The most dominant occurrence is of the shallow failure type which is observed in both boreholes KFM01A and KFM05A. This study cannot categorically state the full characterization of identified breakouts but previous studies have inferred that they could originate as a result of the differential rock strength, the depth of the borehole and the state of stress [Plumb, 1989]. In borehole KFM01A, 33 borehole breakouts with a combined length of 253 m have been observed from 111 to 1001 meters borehole length (mbl). They suggest a mean orientation of the maximum horizontal stress of about 141„b8„a N. In borehole KFM05A, 14 borehole breakouts have been observed between 188 and 996 mbl. They have a combined length of 32 m and a mean maximum horizontal stress orientation of about 159„b33„aN. According to the World Stress Map quality ranking scheme, boreholes KFM01A and KFM05A are of B- and D quality [Zoback, 1992]. MWD parameters that were used for this analysis were the rotation pressure vs. depth, water pressure vs. depth and the feed speed vs. depth. It was observed that MWD parameters are sensitive to fractures and any kind of opening in the rock as they reveal changes in rock¡¦s mechanical and physical properties of the borehole wall. The variations in the strength of the rock and fracture occurrence in the borehole show changes in the water pressure and also rotation pressure values. Borehole breakouts are associated with changes in the rock quality or rock strength, fractures, mechanical properties and change in lithology. Sections marked as regions without breakouts gradually show minimal changes in MWD parameters and in the rock¡¦s mechanical and physical properties within the depth interval as compared to sections with borehole breakouts that show significant variations in their parameters. Therefore, MWD parameters and borehole breakouts are dependent on the mechanical properties of the rock mass, lithology and the physical properties as well. MWD systems are not absolute rock recognition systems: however, with proper interpretation, changes in rock formations and properties can be inferred. The observed breakouts in both boreholes KFM01A and KFM05A start within the first 100 m of the core drilled part and this has a significant implication that the horizontal stresses are even high at shallow depth intervals. The fact that the orientation has been very much uniform in borehole KFM01A predicts that the rock domain here have not been influenced much by the lithology and structures which are prominent in the borehole. For borehole KFM05A, the influence of lithology and structures in the stress orientation was observed at depth intervals between 350 ¡V 450 m, 600 ¡V 750 m and 900 ¡V 1000 m. The prominent fractures at these depth intervals result from the gradual changes from one rock units to another and deformation zones that have been detected. It validates that the stress field is not continuous but with influence of previously existing structures on the prevailing stress field in the rock mass. Borehole KFM01A shows consistency in the minimum stress orientation values along the entire borehole length and this signifies zones of rock continuum without much variance in the rock¡¦s properties: lithology and structures. Borehole KFM05A shows inconsistency in the horizontal stress orientation and intervals with stress re-orientation suggests that there are zones of rock discontinuum which infers that the lithology and structures have a role to play in the changes in stress orientation.

Place, publisher, year, edition, pages
Keyword [en]
Technology, Borehole breakouts, drill parameters, geology
Keyword [sv]
URN: urn:nbn:se:ltu:diva-51797ISRN: LTU-PB-EX--07/056--SELocal ID: 8fb5ed07-8dc5-44f3-a9a0-6568fce1e5d0OAI: diva2:1025161
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Project work, other
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Student paper other
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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