Poroelastic Calibration for Hydromechanically Coupled Numerical Discontinuum Models with Application in Low Permeability Hard Rock

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Date
Authors
Kennedy, Madison
Keyword
Poroelasticity , Discrete Element Method , Numerical Modelling , Geomechanics
Abstract
Societal demands require the design of underground structures in challenging ground conditions such as densely populated areas with sensitive infrastructure, deep mines, and for the storage of radioactive waste in deep geological repositories. Due to the risk associated with these types of challenging conditions a greater understanding of the geological processes and engineering/ground models is required to produce safe realistic designs. To obtain a better understanding of the ground’s response the poroelastic response of the rock and flow through the intact rock can be incorporated into numerical models. This thesis provides a process to calibrate grain based poroelastic discontinuum numerical models which simulate the elastic response and fluid flow through low permeability hard rock. In this thesis, methods for obtaining the poroelastic parameters from laboratory samples of rock are explored. Alternate methods of obtaining preliminary poroelastic parameters using conventional/ standard rock mechanics laboratory tests and values from literature are provided for preliminary use where data is limited. A parametric analysis was performed to determine the optimal loading configuration for the numerical apparatus. The results indicated that a loading configuration where both platens were simultaneously loaded at a rate of less than 0.2 mm/s provided resultant values with error less than 1% and an efficient transfer of stress through the sample. The proposed process for the poroelastic calibration of laboratory scale numerical samples involves the selection of the solid grain properties and the calibration of the drained response, undrained response and permeability. The grain properties are determined based on the mineralogy and size of the mineral grains of the rock. The drained calibration consists of calibrating the contact shear and normal stiffnesses to obtain the target drained resultant elastic parameters which are obtained from laboratory tests. The undrained response is calibrated by adjusting the intergranular aperture to obtain the undrained resultant elastic parameters. Finally, the permeability is calibrated by adjusting the model permeability factor to obtain the target permeability at high and low confining stress. Cobourg limestone is used as an example to demonstrate the parameter selection and calibration process.
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