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1. Viscoplastic modeling of elastic and creep deformation of fractured Berea sandstone.

   Talukdar, M.; Sone, H.; Griffith, W. A. (June 2021, 55th US Rock Mechanics/Geomechanics Symposium)

   null (Ed.)

   We observed and modeled the elastic, inelastic and time-dependent viscous properties of damaged Berea Sandstone samples to investigate the impact of damage on the rheological properties of rocks. Cylindrical Berea Sandstone plugs were prepared both parallel and perpendicular to bedding. We impacted the samples with Split Hopkinson Pressure Bar to pervasively fracture the specimens at different strain rates. Longitudinal mode-I fractures are dominant in specimens impacted at relatively low strain rates (about 130 /s), whereas shear fractures also form in specimens deformed at high strain rates (up to 250 /s). The damaged rocks were subjected to multiple steps of differential stress loading and hold stages under 15 MPa confining pressure. A key observation is that higher damaged specimens showed greater axial and volumetric creep strain deformation during loading and hold stages. Poisson ratio also increase with increasing damage. We modeled the volumetric strain of the sandstone specimens using a Perzyna viscoplasticity law that employs the Modified Cam Clay model as the yield criterion (Haghighat et al. 2020). We deduced that fractured rocks undergo substantial bulk time-dependent deformation due to volumetric compaction and fracture closure. Damage increase results in decrease of the effective viscosity of the material. 

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2. Induced seismicity in the Dallas-Fort Worth Basin: Enhanced seismic catalogue and evaluation of fault slip potential

   https://doi.org/10.1190/segam2020-3428222.1  

   Li, Bing Q.; Avouac, Jean-Philippe; Ross, Zachary E.; Du, Jing; Rebel, Estelle (September 2020, SEG Technical Program Expanded Abstracts 2020)

   null (Ed.)

   We present an updated catalogue of seismicity in the Dallas-Fort Worth basin from 2008 to the end of 2019 using state-of-the-art phase picking and association methods based on machine learning. We then calculate the pore pressure and poroelastic stress changes on a monthly basis between 2000 and 2020 for the whole basin, incorporating fluid injection/extraction histories at 104 saltwater injection and 20576 production wells. These pore pressure and poroelastic stress changes are calculated using coupled analytical solutions for a point source injection in a 3D homogeneous isotropic medium, and are superposed for all wells. We suggest that the poroelastic effects of produced gas and water contribute significantly to fault instability. 

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3. 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)

   ABSTRACT: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 of reacted shale. 1. INTRODUCTIONDetermination 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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4. Relating strain localization and Kaiser effect to yield surface evolution in brittle rocks

   https://doi.org/10.1093/gji/ggaa130  

   Gajst, Hannah; Shalev, Eyal; Weinberger, Ram; Marco, Shmuel; Zh, Wenlu; Lyakhovsky, Vladimir (June 2020, Geophysical Journal International)

   null (Ed.)

   SUMMARY The yield surfaces of rocks keep evolving beyond the initial yield stress owing to the damage accumulation and porosity change during brittle deformation. Using a poroelastic damage rheology model, we demonstrate that the measure of coupling between the yield surface change and accumulated damage is correlated with strain localization and the Kaiser effect. Constant or minor yield surface change is associated with strong strain localization, as seen in low-porosity crystalline rocks. In contrast, strong coupling between damage growth and the yield surface leads to distributed deformation, as seen in high-porosity rocks. Assuming that during brittle deformation damage occurs primarily in the form of microcracks, we propose that the measured acoustic emission (AE) in rock samples correlates with the damage accumulation. This allows quantifying the Kaiser effect under cyclic loading by matching between the onset of AE and the onset of damage growth. The ratio of the stress at the onset of AE to the peak stress of the previous loading cycle, or Felicity Ratio (FR), is calculated for different model parameters. The results of the simulation show that FR gradually decreases in the case of weak coupling between yield surface and damage growth. For a strong damage-related coupling promoting significant yield surface change, the FR remains close to one and decreases only towards the failure. The model predicts that a steep decrease in FR is associated with a transition between distributed and localized modes of failure. By linking the evolving yield surface to strain localization patterns and the Kaiser effect, the poroelastic damage rheology model provides a new quantitative tool to study failure modes of brittle rocks. 

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5. The Deep Structure and Rheology of a Plate Boundary-Scale Shear Zone: Constraints from an Exhumed Caledonian Shear Zone, NW Scotland

   https://doi.org/10.2113/2020/8824736  

   Lusk, Alexander D. J.; Platt, John P.; Roeske, ed., Sarah M. (September 2020, Lithosphere)

   Abstract Below the seismogenic zone, faults are expressed as zones of distributed ductile strain in which minerals deform chiefly by crystal plastic and diffusional processes. We present a case study from the Caledonian frontal thrust system in northwest Scotland to better constrain the geometry, internal structure, and rheology of a major zone of reverse-sense shear below the brittle-to-ductile transition (BDT). Rocks now exposed at the surface preserve a range of shear zone conditions reflecting progressive exhumation of the shear zone during deformation. Field-based measurements of structural distance normal to the Moine Thrust Zone, which marks the approximate base of the shear zone, together with microstructural observations of active slip systems and the mechanisms of deformation and recrystallization in quartz, are paired with quantitative estimates of differential stress, deformation temperature, and pressure. These are used to reconstruct the internal structure and geometry of the Scandian shear zone from ~10 to 20 km depth. We document a shear zone that localizes upwards from a thickness of >2.5 km to <200 m with temperature ranging from ~450–350°C and differential stress from 15–225 MPa. We use estimates of deformation conditions in conjunction with independently calculated strain rates to compare between experimentally derived constitutive relationships and conditions observed in naturally-deformed rocks. Lastly, pressure and converted shear stress are used to construct a crustal strength profile through this contractional orogen. We calculate a peak shear stress of ~130 MPa in the shallowest rocks which were deformed at the BDT, decreasing to <10 MPa at depths of ~20 km. Our results are broadly consistent with previous studies which find that the BDT is the strongest region of the crust. 

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