In-situ stress magnitude and core disking

• Author / Creator

  Lim, Seong Sik

• Accurate measurement of in situ stress using surface-drilled deep boreholes is a
  challenge in high stress regimes in which both horizontal stresses exceed the vertical
  stress. For such stress regimes core damage and core disking is often observed and
  Accurate measurement of in situ stress using surface-drilled deep boreholes is a challenge in high stress regimes in which both horizontal stresses exceed the vertical stress. For such stress regimes core damage and core disking is often observed and these observations were used to constrain the stress state.
  
  Digital image analysis was used to examine the characteristics of microcracks and volumetric strain measurement technique used to quantify stress-induced microcracks in granite-cored samples, obtained in the depth range from ground surface to 1000 m. The results indicate that at depths of less than 200 m, the dominant mode of microcracks can be classed as naturally occurring. The volume of stress-induced microcracks was found to increase linearly with sampling depth with the proportion of grain-boundary, intragranular and transgranular microcracks remaining relatively constant. Moreover it was observed that most of the stress-induced transgranular microcracks formed in a plane perpendicular to the core axis.
  Disked cores from boreholes drilled from underground excavations in massive unfractured granite at AECL’s Underground Research Laboratory, where the stress magnitudes are known with confidence, were used to establish a relationship between core disk thickness and the stress magnitude. Relationships were established three disk thickness categories; (1) thin (t/D<0.2), (2) medium (0.2<t/D<0.4) and (3) thick (0.4<t/D<2.2) and partial disking. The data suggests that core disking initiates when the maximum principal stress normalized to the tensile strength is 6.5. Stress path analyses indicated that tensile stress controlled the onset of disking.
  
  Three dimensional numerical analyses were carried out to determine the distribution of tensile stresses in the vicinity of the advancing drill bit. A methodology was developed to examine the spatial distribution of the maximum, minimum, and average, maximum tensile stress. A criterion based on the Averaged Maximum Tensile Stress (AMTS) was found to give good agreement with the thickness of field core disks. This approach was then used to establish general core disking nomograms using site specific geometry, the Brazilian tensile strength, and the AMTS. The approach was applied to two sites and found to be in agreement with field observations.

• Subjects / Keywords

  • Microcrack
  • In situ stress
  • Core disking

• Graduation date

  Spring 2013

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3VW43

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.