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1. Computational Modeling of Creep Behavior in Shales Induced by Fluid-Rock Interaction

   https://doi.org/10.56952/ARMA-2022-0831  

   Prakash, R. ; Abedi, S. ( January 2022 , 56th U.S. Rock Mechanics/Geomechanics Symposium, Santa Fe, New Mexico, USA, June 2022)

   The chemo-mechanical loading of rocks causes the dissolution and precipitation of multiple phases in the rock. This dissolution and precipitation of load-bearing mineral phases lead to the stress redistribution in neighboring phases, which in turn results in deformational changes of the sample composite. The aim of this study is to investigate the link between microstructural evolution and creep behavior of shale rocks subjected to chemo-mechanical loading through modeling time-dependent deformation induced by the dissolution-precipitation process. The model couples the microstructural evolution of the shale rocks with the stress/strain fields inside the material as a function of time. The modeling effort is supplemented with an experimental study where shale rocks were exposed to CO2-rich brine under high temperature and pressure conditions. 3D snapshots of the sample microstructure were generated using segmented micro-CT images of the shale sample. The time-evolving microstructures were then integrated with the Finite element-based mechanical model to simulate the creep induced by dissolution and precipitation processes independent of the intrinsic viscoelasticity/viscoplasticity of the mineral phases. After computation of the time-dependent viscoelastic properties of the shale composite, the combined microstructure model and finite element model were utilized to predict the time-dependent stress and strain fields in different zones ofmore »reacted shale.

   Determination of viscous behavior of shale rocks is key in wide range of applications such as stability of reservoirs, stability of geo-structures subjected to environmental forcing, underground storage of hazardous materials and hydraulic fracturing. Short-term creep strains in hydraulic fracturing can change stress fields and in turn can impact the hydraulic fracturing procedures(H. Sone & Zoback, 2010; Hiroki Sone & Zoback, 2013). While long-term creep strains can hamper the reservoir performance due to the reduction in permeability of the reservoir by closing of fractures and fissures(Du, Hu, Meegoda, & Zhang, 2018; Rybacki, Meier, & Dresen, 2016; Sharma, Prakash, & Abedi, 2019; Hiroki Sone & Zoback, 2014). Owing to these significance of creep strain, it is important to understand the viscoelastic/viscoplastic behavior of shales.

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2. Characterization of Fracture Process Zone in Short Beam Compression Tests on Barre Granite

   https://doi.org/10.56952/ARMA-2022-0466  

   Garg, Prasoon ; Hedayat, Ahmadreza ; Griffiths, D. V. ( June 2022 , 56th U.S. Rock Mechanics/Geomechanics Symposium)

   Due to rock mass being commonly subjected to compressive or shear loading, the mode II fracture toughness is an important material parameter for rocks. Fracturing in rocks is governed by the behavior of a nonlinear region surrounding the crack tip called the fracture process zone (FPZ). However, the characteristics of mode II fracture are still determined based on the linear elastic fracture mechanics (LEFM), which assumes that a pure mode II loading results in a pure mode II fracture. In this study, the FPZ development in Barre granite specimens under mode II loading was investigated using the short beam compression (SBC) test. Additionally, the influence of lateral confinement on various characteristics of mode II fracture was studied. The experimental setup included the simultaneous monitoring of surface deformation using the two-dimensional digital image correlation technique (2D-DIC) to identify fracture mode and characterize the FPZ evolution in Barre granite specimens. The 2D-DIC analysis showed a dominant mixed-mode I/II fracture in the ligament between two notches, irrespective of confinement level on the SBC specimens. The influence of confinement on the SBC specimens was assessed by analyzing the evolution of crack displacement and changes in value of mode II fracture toughness. Larger levels ofmore »damage in confined specimens were observed prior to the failure than the unconfined specimens, indicating an increase in the fracture resistance and therefore mode II fracture toughness with the confining stress.

   The fracturing in laboratory-scale rock specimens is often characterized by the deformation of the inelastic region surrounding the crack tips, also known as the fracture process zone (FPZ) (Backers et al., 2005; Ghamgosar and Erarslan, 2016). While the influence of the FPZ on mode I fracture in rocks has been extensively investigated, there are limited studies on FPZ development in rocks under pure mode II loading (Ji et al., 2016; Lin et al., 2020; Garg et al., 2021; Li et al., 2021).

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3. Effect of grain orientation on the compressive response of highly oriented MAX phase Ti3SiC2

   https://doi.org/https://doi.org/10.1016/j.msea.2021.140869  

   Zhao, Xingyuan ; Sokol, Maxim ; Barsoum, Michel W. ; Lamberson, Leslie ( January 2021 , Materials science engineering)

   The MAX phases comprise of a group of layered ternary carbides that exhibit unique mechanical properties which bridge the gap between their metal and ceramic constituents. To study the effects of the global grain orientation, Ti, Si and TiC powders were hot pressed to synthesize highly oriented bulk Ti3SiC2. X-ray diffraction (XRD) was used to verify the grain orientation and a Lotgering factor of 0.87 with respect to the c-axis was obtained. Prepared Ti3SiC2 samples have been compressed in two orientations, loading along the c-axis (‖c-axis) and perpendicular to the c-axis (⊥c-axis) at 10− 3 s− 1 using a standard load frame and at 102 s− 1 using a Kolsky (split-Hopkinson) bar. The average compressive strength along the ⊥c-axis orientation was 761 MPa under quasi- static conditions and 987 MPa under dynamic loading, exhibiting a 30% increase on average. The ‖c-axis orientation exhibited no rate dependence in compressive strength; however both orientations exhibited an in- crease of strain at failure under dynamic conditions by over 0.5%, on average. The orientation-dependent failure behavior at different strain rates were examined using high-speed imaging and 2D digital image correlation (DIC) during loading and via scanning electron microscopy (SEM) post-mortem. Results indicate that themore »⊥c-axis fracture surface exhibited a mixture of transgranular and intergranular cracks, kink bands and delaminations, whereas ‖c-axis was limited to a combination of intergranular and transgranular cracks. Such fracture distinc- tions due to the availability (or lack thereof) for kink band formation appear to be responsible for the anisotropic compressive behavior.« less
4. Investigation of Fracture Process Zone in Barre Granite under Mode II Loading

   Garg, P ; Hedayat, A ; Griffiths, D.V. ( June 2021 , 55th US Rock Mechanics/Geomechanics Symposium)

   Fracturing in brittle rocks exhibits a significant nonlinear region surrounding the crack tip called the fracture process zone (FPZ). In this study, the evolution of the FPZ under pure mode II loading using notched deep beam under three-point loading was investigated. The experimental setup included the simultaneous monitoring of surface deformation using the two-dimensional digital image correlation technique to characterize various crack characteristics such as its type and FPZ evolution in Barre granite specimens. Both displacement and strain approaches of the two-dimensional digital image correlation were used to identify the mode of fracture under pure mode II loading. Both approaches showed that the crack initiation occur under mode I despite the pure mode II loading at the notch tip. The displacement approach was used for characterizing the evolution of the FPZ which analyzed the crack tip opening displacement and crack tip sliding displacement to identify the transition between the three stages of FPZ evolution, namely, (a) elastic stage, (b) formation of the FPZ, and (c) the macro-crack initiation. The results showed that the evolution of the FPZ of mode I fracture under pure mode II loading is similar to cases of pure mode I loading of the same rock.
5. Investigating Fracture Network Deformation Using Noble Gas Release

   https://doi.org/10.1155/2021/6697819  

   Gardner, W. Payton ; Bauer, Stephen J. ; Broome, Scott ( May 2021 , Geofluids)

   Vaselli, Orlando (Ed.)

   We investigate deformation mechanics of fracture networks in unsaturated fractured rocks from subsurface conventional detonation using dynamic noble gas measurements and changes in air permeability. We dynamically measured the noble gas isotopic composition and helium exhalation of downhole gas before and after a large subsurface conventional detonation. These noble gas measurements were combined with measurements of the subsurface permeability field from 64 discrete sampling intervals before and after the detonation and subsurface mapping of fractures in borehole walls before well completion. We saw no observable increase in radiogenic noble gas release from either an isotopic composition or a helium exhalation point of view. Large increases in permeability were observed in 13 of 64 discrete sampling intervals. Of the sampling intervals which saw large increases in flow, only two locations did not have preexisting fractures mapped at the site. Given the lack of noble gas release and a clear increase in permeability, we infer that most of the strain accommodation of the fractured media occurred along previously existing fractures, rather than the creation of new fractures, even for a high strain rate event. These results have significant implications for how we conceptualize the deformation of rocks with fracture networks above themore »percolation threshold, with application to a variety of geologic and geological engineering problems.« less