S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Energy Systems Engineering (에너지시스템공학부) Theses (Master's Degree_에너지시스템공학부)
Discrete Element Modeling of Anisotropic Mechanical Behaviors for Transversely Isotropic Rock
개별요소법을 이용한 횡등방성 암석의 역학적 이방성 모사
- 공과대학 에너지시스템공학부
- Issue Date
- 서울대학교 대학원
- Discrete Element Method ; Smooth Joint Model ; Transverse Isotropy ; Wellbore Stability ; Hydraulic Fracturing
- 학위논문 (석사)-- 서울대학교 대학원 : 에너지시스템공학부, 2013. 2. 민기복.
- Discrete Element Model is now recognized as a powerful tool for simulating mechanical behaviors of rock. In particular, bonded particle model has been successfully applied in emulating the elastic modulus, Poissons ratio and strength parameters of rock by controlling the micro parameters of DEM. Although the consideration of anisotropy is important in many rock mechanics applications, DEM still remains as a tool for modeling an isotropic rock. In this study, DEM modeling was performed in order to represent the transversely isotropic rock such as shale and schist. The Smooth Joint Model was assigned to represent the bedding planes in order to form the transversely isotropic rock. DEM model was verified using the transformation of compliance tensor in transversely isotropic rock containing one set of fractures. Verification with respect to the strength anisotropy also resulted in good agreement with analytical model. It is demonstrated that the microparameters of smooth joint model were shown to contribute significantly to the anisotropic mechanical behaviors. Particle size sensitivity analysis was carried out and it was confirmed that using sufficiently high resolution is one of the key factors that can lead to more accurate DEM modeling. The bonded particle model with smooth joints model can provide a competent tool for simulating the elastic modulus of transversely isotropic rock as long as it maintains a reasonable resolution.
Three different types of transversely isotropic rock were reproduced as DEM models and the properties of Asan gneiss, Boryeong shale and Yeoncheon schist (Cho et al., 2012) were used as a reference for transversely isotropic rock modeling. To determine the microproperties of smooth joint model, the equivalent continuum model was used in order to obtain the normal and shear stiffness of weak plane. Bedding planes and foliations are generally smooth so the dilation angle (ψ) was regarded as 0o. It is reasonable to choose the tensile strength of smooth joints as the value less than Brazilian tensile strength with the inclined angle of 0o. Finally, cohesion and friction coefficient were estimated through iterative process. The strength and elastic modulus of transversely isotropic model varied with respect to the orientation of the smooth joints and the overall trends were similar to the laboratory experiments in Asan gneiss, Boryeong shale and Yeoncheon schist. BTS of the transversely isotropic model with high resolution seems to have good agreement with the laboratory results, however, it tended to be higher than actual value and this has been identified as the inherent limitations. In spite of the issues such as higher tensile strength in strong rock, the DEM model of transversely isotropic rock seems to model the strength and elastic behavior of the transversely isotropic rock to a reasonable extent.
After failure, fractures on DEM model were compared with broken rock specimen and the failure mechanism was analyzed. Based on the experimental study on layered rock (Abbass and André, 2010), the portion of layer fracture and central fracture length on Boryeong shale was measured and it was analyzed that the strength and failure mechanism containing high portion of layered fractures were dominated by the shear and /or tensile strength of the layers. By counting the number of micro tensile/shear cracks in DEM model, failure patterns were investigated.
This discrete element modeling in anisotropic rock is expected to pave the way for wide variety of engineering applications such as hydraulic fracturing for shale gas production and wellbore stability analysis.