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1. Effect of the State of Stress in the Fracturing and Micro-Seismic Activity of Hydraulically-Fractured Granite

   Gunarathna, G; Goncalves da Silva, B (January 2019, US Rock Mechanics/Geomechanics Symposium)

   Hydraulic fracturing can be recognized as an emerging method used in the mining of heat in Enhanced Geothermal Systems as well as in the extraction of oil and gas entrapped within shale formations. While there are several experimental studies focusing on the initiation and propagation of hydraulically-induced fractures under uniaxial and biaxial loading conditions, a very limited number of experimental studies investigate the effect of triaxial loading conditions on fracture initiation and propagation. This study describes an experimental setup, which was designed to allow one to independently apply and control three orthogonal stresses in prismatic granite specimens while simultaneously applying a hydraulic pressure inside pre-fabricated flaws. Moreover, the test setup allows one to observe and subsequently interpret the fracturing processes through visual and acoustic emission (AE) monitoring. The observations obtained in the current study using a triaxial state of stress were interpreted and compared with existing experimental studies that used other states of stress. It was observed that whitening of some grains and high-amplitude AE events occurred where visible cracks eventually developed for the triaxial state of stress investigated. Comparison with previous studies, in which only vertical loads (uniaxial) were applied, shows that the aperture of the hydraulically-induced fractures for the triaxial condition is significantly smaller than for the uniaxial loadings and that the coalescence patterns are, in general, stress-state-dependent. In terms of AE data, the total number of AE events in the specimens subject to triaxial stresses were significantly higher than in the tests using uniaxial stresses, even though most of the events (65%) had a relatively low-amplitude (<50dB) in contrast to the uniaxial tests, in which low-amplitude events were typically less than 50%. 

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2. Effects of fluid diffusivity on hydraulic fracturing processes using visual analysis

   Baptista-Pereira, C; Goncalves da Silva, B (January 2019, US Rock Mechanics/Geomechanics Symposium)

   Hydraulic fracturing arises as a method to enhance oil and gas production, and also as a way to recover geothermal energy. It is, therefore, essential to understand how injecting a fluid inside a rock reservoir will affect its surroundings. Hydraulic fracturing processes can be strongly affected by the interaction between two mechanisms: the elastic effects caused by the hydraulic pressure applied inside fractures and the poro-mechanical effects caused by the fluid infiltration inside the porous media (i.e. fluid diffusivity); this, in turn, is affected by the injection rate used. The interaction between poro-elastic mechanisms, particularly the effect of the fluid diffusivity, in the hydraulic fracturing processes is not well-understood and is investigated in this paper. This study aims to experimentally and theoretically comprehend the effects of the injection rate on crack propagation and on pore pressures, when flaws pre-fabricated in prismatic gypsum specimens are hydraulically pressurized. In order to accomplish this, laboratory experiments were performed using two injection rates (2 and 20 ml/min), applied by an apparatus consisting of a pressure enclosure with an impermeable membrane in both faces of the specimen, which allowed one to observe the growth of a fluid front from the pre-fabricated flaws to the unsaturated porous media (i.e. rock), before fracturing took place. It was observed that the fracturing pressures and patterns are injection-rate-dependent. This was interpreted to be caused by the different pore pressures that developed in the rock matrix, which resulted from the significantly distinct fluid fronts observed for the two injection rates tested. 

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3. A Numerical Study on Soot Formation and Evolution in Pressurized Turbulent Sooting Flames

   https://doi.org/10.2514/6.2021-0189  

   Zhou, Dezhi; Vaage, Anders; Boyette, Wesley R.; Guiberti, Thibault; Roberts, William L.; Yang, Suo (January 2021, AIAA Scitech 2021 Forum)

   Understandings on soot formation and evolution in pressurized flames are of significant interest due to the increasing operating pressures in different combustors and the accompanying increased soot emissions. In this study, a series of pressurized turbulent sooting flames at 1 bar, 3 bar and 5 bar, are simulated to study the pressure effect on the soot formation and evolution. The inflow conditions are chosen such that the Reynolds number at different pressures keeps constant. Via a Radiation Flamelet Progress Variable (RFPV) approach with a conditional soot sub-filter Probability Density Function (PDF) to consider the turbulence-chemistry-soot interaction, quantitatively good agreements (e.g., maximum discrepancy within one order of magnitude) are achieved for soot volume fraction predictions compared with the experimental data at different pressures. Soot volume fraction source terms are then discussed to show the pressure effect on nucleaion, condensation, surface growth and oxidation at different axial positions in these flames. 

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4. The Role of Background Stress State in Fluid‐Induced Aseismic Slip and Dynamic Rupture on a 3‐m Laboratory Fault

   https://doi.org/10.1029/2022JB024371  

   Cebry, S. B. L.; Ke, C. ‐Y.; McLaskey, G. C. (August 2022, Journal of Geophysical Research: Solid Earth)

   Abstract Fluid injection stimulates seismicity far from active tectonic regions. However, the details of how fluids modify on‐fault stresses and initiate seismic events remain poorly understood. We conducted laboratory experiments using a biaxial loading apparatus with a 3 m saw‐cut granite fault and compared events induced at different levels of background shear stress. Water was injected at 10 mL/min and normal stress was constant at 4 MPa. In all experiments, aseismic slip initiated on the fault near the location of fluid injection and dynamic rupture eventually initiated from within the aseismic slipping patch. When the fault was near critically stressed, seismic slip initiated only seconds after MPa‐level injection pressures were reached and the dynamic rupture propagated beyond the fluid pressure perturbed region. At lower stress levels, dynamic rupture initiated hundreds of seconds later and was limited to regions where aseismic slip had significantly redistributed stress from within the pressurized region to neighboring locked patches. We found that the initiation of slow slip was broadly consistent with a Coulomb failure stress, but that initiation of dynamic rupture required additional criteria to be met. Even high background stress levels required aseismic slip to modify on‐fault stress to meet initiation criteria. We also observed slow slip events prior to dynamic rupture. Overall, our experiments suggest that initial fault stress, relative to fault strength, is a critical factor in determining whether a fluid‐induced rupture will “runaway” or whether a fluid‐induced rupture will remain localized to the fluid pressurized region. 

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5. Inexpensive Verification via Electromechanical Impedance for Additively Manufactured Parts

   S. M. Strutner, C. Tenney (January 2018, SAMPE Conference)

   Currently, verifying additively manufactured (AM) parts requires time consuming and expensive nondestructive evaluation (NDE) processes such as computed tomography (CT) x-ray scanning. While such methods provide details on flaw type and location, they require significant cost and time. Often, in production environments, significant value is gained by rapidly screening part specimens for flaws at all. Cost-effective per-specimen testing for production runs of AM parts is important for their use to be economically justified. In this work, Northrop Grumman Corporation and Virginia Tech explored impedance-based testing as a means to evaluate AM titanium specimens. Specimens with and without manually-designed flaws were fabricated through a metal- based AM process and evaluated using the impedance-based technique. CT scans confirmed that the intended flaws in the experimental specimens were present. Impedance-based examination also showed the presence of unintended defects. After machining away the unintended defective regions, the flaw-containing defective specimen had a clearly different impedance ‘signature’ than non-flawed baseline specimens. Additional analysis confirmed that the impedance test method required cheaper capital equipment and required less technician time to examine test results. Taken together, this means that the impedance-based this method can reduce the total cost of utilizing AM for metal part manufacturing. 

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