Mode I and Mode II Fracture Toughness of Montney Shale under Confining Pressure
Date
2019-01-22
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Abstract
This thesis investigated the geomechanical properties of Montney shale using uniaxial compression, triaxial compression, Brazilian, semi-circular bending (SCB), straight-notched disk-bending (SNDB) and double shear (DS) tests. The volumetric strain method (VSM), crack volumetric strain method (CVSM), axial strain response method (ASR), and lateral strain response method (LSR) along with the crack axial and lateral strain method (CALSM) were employed to determine the crack closure and crack initiation stresses under uniaxial and triaxial compression. The LSR and ASR methods depend heavily on the initial portion of the stress-strain curve. Thus, at least two or three methods should be used when determining the crack stress thresholds. The Mode I fracture toughness under confining pressure was measured using SNDB samples with crack length to thickness ratios of about 0.2 and 0.4, respectively. The application of confining stress might induce tensile crack at the top of the SNDB sample with crack length to thickness ratio around 0.4. The tensile stresses achieved around the crack tip in confined SNDB tests under crack initiation load are close to the tensile strength of the sample and not affected by confining pressure. The Mode I fracture toughness of Montney shale is independent of confining pressure. The Mode II fracture toughness and shear fracture energy of Montney shale were measured using DS tests. The shear fracture energy values determined from post-peak portion of the axial force-displacement curve are always higher than those converted from the Mode II fracture toughness. However, with increasing confining pressure, the amount of difference decreases. Under confining pressures ranging from 0 MPa to 30 MPa, the Mode II fracture toughness and shear fracture energy increase with confining pressure. For confining pressure greater than 30 MPa, the double shear sample tends to behave ductile, evidenced by the multiple shear fractures branched out from the main shear fracture. Consequently, the double shear test is only suitable for determination of the Mode II fracture toughness of rock with confining pressure below 30 MPa.
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Montney shale, Mode I fracture toughness, Mode II fracture toughness, Fracture energy, Confining pressure, SNDB, Double shear
Citation
Chen, Q. (2019). Mode I and Mode II Fracture Toughness of Montney Shale under Confining Pressure (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.