Differential Stress Research Articles

Combined ERT and borehole logs for mapping the soil-rock interface in a granitic environment

This study used the efficiency of electrical resistivity tomography (ERT) and borehole logs to map the soil-rock interface beneath four traverses (RS1, RS2, RS3, and RS4) in the granitic terrain of Perak, Peninsular Malaysia. The study aimed to evaluate the impacts of the soil-rock characteristic features and interfaces on groundwater and infrastructure development to meet the needs of the increasing inhabitants yearly. The borehole- and ERT-derived lithologic units are strongly correlated. The delineated lithologic units include the topsoil, weathered granitic units (medium stiff to hard silty clay or clayey silt with <800 Ωm), thin to wide-sized weathered/fractured units, and fresh granitic bedrock. These soil-rock profiles and weathered/fractured apertures support sustainable groundwater developments with drill depths above 45 m. In contrast, the delineated clay/silt alternating with stiffer soils, low load-bearing deep-weathered/fractured zones, and bedrock boulders in most places, except beneath traverse RS3, have high affinities for water retention and differential stresses. These features can adversely impact poorly reinforced foundations. Hence, structural elements of the foundations, such as footings or piles, should be placed on stable bedrock, particularly in the central to western parts of the study area. This study has reduced the paucity of information on using ERT and borehole logs for soil-rock interface studies in the study area.

Reef habitats structure symbiotic microalgal assemblages in corals and contribute to differential heat stress responses

Reef habitats structure symbiotic microalgal assemblages in corals and contribute to differential heat stress responses

Surface faulting earthquake clustering controlled by fault and shear-zone interactions

Surface faulting earthquakes are known to cluster in time from historical and palaeoseismic studies, but the mechanism(s) responsible for clustering, such as fault interaction, strain-storage, and evolving dynamic topography, are poorly quantified, and hence not well understood. We present a quantified replication of observed earthquake clustering in central Italy. Six active normal faults are studied using 36Cl cosmogenic dating, revealing out-of-phase periods of high or low surface slip-rate on neighboring structures that we interpret as earthquake clusters and anticlusters. Our calculations link stress transfer caused by slip averaged over clusters and anti-clusters on coupled fault/shear-zone structures to viscous flow laws. We show that (1) differential stress fluctuates during fault/shear-zone interactions, and (2) these fluctuations are of sufficient magnitude to produce changes in strain-rate on viscous shear zones that explain slip-rate changes on their overlying brittle faults. These results suggest that fault/shear-zone interactions are a plausible explanation for clustering, opening the path towards process-led seismic hazard assessments.

Abstract B012: Differential stress responses drive asparagine exchange in pancreatic cancer

Abstract Pancreatic ductal adenocarcinoma (PDAC) is the third leading cause of cancer-related death in the United States, contrasting a relatively low incidence rate. A principal barrier to the treatment of pancreatic cancer is the densely fibrotic tumor microenvironment and the high interstitial pressure which acts to collapse blood vessels, impairing the delivery of chemotherapy. This lack of functional vasculature limits nutrient availability within the tumor, causing cancer cells to develop numerous metabolic adaptations to allow for proliferation in hypoxic, austere conditions. Recent studies have demonstrated several clonal behaviors of cancer cells within individual pancreatic tumors, although it is unclear if this is true of metabolism. Exploring this, we found the metabolic profiles and sensitivities of a series of clonal cell lines derived from a single pancreatic tumor clustered into two distinct groups. These groups demonstrated differential sensitivities to inhibitors of mitochondrial metabolism; whereby the growth of the sensitive clones during impaired respiration can be rescued through co-culture with the insensitive cells. Profiling metabolites exchanged by the clonal populations, we identified this is mediated by asparagine released by insensitive cells. Mechanistically, the production of asparagine is driven by differential stress response pathway activation between clones. We further determined that exogenous asparagine supports the growth of both cancer and non-cancerous cells during limited respiration. Conversely, we found that the use of asparaginase to degrade asparagine sensitizes PDA tumors to mitochondrial inhibition. Collectively these data demonstrate distinct metabolic classes of cancer cells within single tumors and identify asparagine as a potential target to sensitize tumors to mitochondrial inhibition. Citation Format: Cecily Anaraki, Galloway Thurston, Nina G. Steele, Samuel A. Kerk, Hanna Hong, Peter Sajjakulnukit, Anthony Andren, Catherine Felton, Alica Beutel, Rima Singh, Heather R. Christofk, Andrea Viale, Howard Crawford, Marina Pasca di Magliano, Claus Jorgensen, Costas A. Lyssiotis, Christopher J. Halbrook. Differential stress responses drive asparagine exchange in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr B012.

Periosteal Skeletal Stem Cells Can Migrate into the Bone Marrow and Support Hematopoietic Regeneration

Hematopoietic stem cells (HSC) reside within the bone marrow microenvironment, coined the HSC niche, and are supported by various cell types, including bone marrow mesenchymal stem cells (BM-MSCs). BM-MSCs are multipotent stromal cells capable of self-renewal and trilineage differentiation into chondrocytes, osteoblasts, and adipocytes. BM-MSCs secrete factors important for hematopoiesis, such as C-X-C motif ligand 12 (CXCL12) and stem cell factor (SCF). Functional stromal cells are important for bone marrow regeneration after chemotherapy or radiation-induced injury to avoid myelosuppression, a common side effect of these treatments. However, it is not known what governs bone marrow regeneration and how the cells within the bone marrow are repopulated post-injury. Here we describe a novel model of whole bone transplantation to study hematopoietic regeneration and identify periosteal skeletal stem cells (P-SSCs) as key contributors to stromal recovery following bone marrow injury. Skeletal stem cells are skeletal lineage cells that can be found in multiple locations within the body, including the periosteum. The periosteum is a thin, two-layered membrane that covers the surface of bones and has been well studied in the context of bone fracture. P-SSCs can be found in the inner layer of the periosteum and contribute to bone development, regeneration and homeostasis. However, despite the close relationship between bone and bone marrow physiology, the potential role of P-SSCs in hematopoiesis and within the bone marrow has yet to be explored. In this study, we have developed a bone transplantation model that involves isolating a femur from a donor mouse and subcutaneously transplanting it into a recipient mouse (Figure 1A). This model mimics the initial bone marrow cellular necrosis and fatty infiltration seen after bone marrow injury and the subsequent regeneration over time. We discovered that over 99% of the BM-MSCs (CD45-Ter119-CD31-CD51+CD200+) within the engrafted femur were derived from the graft, while over 98% of the hematopoietic cells (CD45+Ter119+) within the engrafted femur were derived from the hosts. Notably, however, we observed that P-SSCs were migrating from the periosteum and into the bone marrow, aiding in stromal regeneration. To assess the contribution of P-SSCs to the recovering bone marrow stroma, we crossed Periostin (Postn)-CreER mice with a tdTomato (Tmt) line. After 5 months, at which point there is no significant difference between host femur and graft femur cellularity, we observed that 83.9% (± 7.18%) of the BM-MSCs within the engrafted femur were Tmt+, indicating periosteal origin (Figure 1B). Once in the bone marrow, these P-SSCs are reprogrammed into BM-MSC-like cells and facilitate hematopoietic recovery, as shown by expression levels of HSC maintenance factors. Compared to steady state P-SSCs, Tmt+ graft BM-MSCs increased CXCL12 and SCF expression by 6.92-fold (p<0.0001) and 5.74-fold (p<0.0001), respectively. Additionally, unlike BM-MSCs, P-SSCs are resistant to transplantation stress, and actively proliferate in the early days following bone transplantation. Using RNA sequencing, qPCR, and flow cytometric analyses, we were able to show that at steady state, P-SSCs appear to be even more quiescent than BM-MSCs, potentially contributing to their stress resistance. Our bone transplantation model has enabled us to study the bone marrow regeneration process using genetic mouse models and led us to uncover a role for P-SSCs in supporting hematopoiesis. Additionally, we have assessed the differential stress response of P-SSCs and BM-MSCs after transplantation. This study highlights a novel bone transplantation model to investigate bone marrow injury and regeneration. It also reveals a critical role of P-SSCs in stromal regeneration and hematopoietic recovery. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Influences of Schistosity Structure and Differential Stress on Failure and Strength Behaviors of an Anisotropic Foliated Rock Under True Triaxial Compression

Influences of Schistosity Structure and Differential Stress on Failure and Strength Behaviors of an Anisotropic Foliated Rock Under True Triaxial Compression

Differential integrated stress response and asparagine production drive symbiosis and therapy resistance of pancreatic adenocarcinoma cells

The pancreatic tumor microenvironment drives deregulated nutrient availability. Accordingly, pancreatic cancer cells require metabolic adaptations to survive and proliferate. Pancreatic cancer subtypes have been characterized by transcriptional and functional differences, with subtypes reported to exist within the same tumor. However, it remains unclear if this diversity extends to metabolic programming. Here, using metabolomic profiling and functional interrogation of metabolic dependencies, we identify two distinct metabolic subclasses among neoplastic populations within individual human and mouse tumors. Furthermore, these populations are poised for metabolic cross-talk, and in examining this, we find an unexpected role for asparagine supporting proliferation during limited respiration. Constitutive GCN2 activation permits ATF4 signaling in one subtype, driving excess asparagine production. Asparagine release provides resistance during impaired respiration, enabling symbiosis. Functionally, availability of exogenous asparagine during limited respiration indirectly supports maintenance of aspartate pools, a rate-limiting biosynthetic precursor. Conversely, depletion of extracellular asparagine with PEG–asparaginase sensitizes tumors to mitochondrial targeting with phenformin.

Observational and theoretical evidence for frictional-viscous flow at shallow crustal levels

Along the Hikurangi Subduction Margin, accretionary prism uplift has exposed the Hungaroa fault zone, an inactive thrust developed within the Middle to Late Eocene Wanstead Formation. Within the ~33 m-wide fault core, deformation of the smectitic, calcareous mudstone matrix produced a penetrative foliation that locally wraps around clasts. Deformation occurred at temperatures constrained by syntectonic calcite vein clumped isotope thermometry, which yielded a narrow range of Δ 47 values between 0.445 ± 0.024‰ and 0.482 ± 0.013‰, corresponding to a mean calcite precipitation temperature of 82 −12 +13 °C. Optical and scanning electron microscopy analyses reveal that calcite underwent: dissolution along stylolites and clast, vein, and microlithon margins; precipitation in foliation-parallel and foliation-perpendicular extension veins; and precipitation in hybrid veins and strain fringes. Maximum differential stress estimates obtained from calcite twin densities (44.1 ± 13.9 to 96.6 ± 20.8 MPa) are consistent with those sustainable by a cohesionless fault at ~3 km depth with a friction coefficient in the range measured for two calcareous mudstones (μ = 0.38 to 0.50) and a micrite clast (μ = 0.61 and 0.64). Marlstone clasts within the foliated calcareous mudstone matrix contain mutually cross-cutting shear fractures and extension veins with crack-seal textures, providing evidence for temporal fluctuations in shear strength resulting from pore fluid overpressure transients. At strain rates imposed during laboratory experiments, frictional sliding involves granular flow processes. Yet, calcite microstructures indicate that diffusive mass transfer played an important role in accommodating deformation. We model the fault zone rheology assuming diffusion-controlled frictional-viscous flow, with deformation at strain rates γ ˙ ≤ 10 −9 s −1 able to have taken place at very low shear stresses ( τ < 10 MPa) given sufficiently short diffusion distances ( d < 0.1 mm), even in the absence of pore fluid overpressures. However, if grain-scale and fracture-scale processes change the diffusion distance, fault zones deforming via frictional-viscous flow can exhibit temporally variable strain rates. Thus, our results suggest that the shallow (up-dip) limit of the seismogenic zone is not a simple function of temperature in fault zones governed by a frictional-viscous flow rheology. • Displacement occurred via distributed deformation across a 33 m-wide fault core. • Fault core rocks experienced spatiotemporal variations in stress and strength. • Frictional-viscous flow can explain the weakness of shallow crustal thrust faults. • Frictional-viscous flow can occur at temporally variable strain rates.

Deriving Three‐Dimensional Properties of Fracture Networks From Two‐Dimensional Observations in Rocks Approaching Failure Under Triaxial Compression: Implications for Fluid Flow

AbstractApproximating the three‐dimensional structure of a fault network at depth in the subsurface is key for robust estimates of fluid flow. However, only observations of two‐dimensional outcrops are often available. To shed light on the relationship between two‐ and three‐dimensional measurements of fracture networks, we examine data from a unique set of 11 X‐ray synchrotron triaxial compression experiments that reveal the evolving three‐dimensional fracture network throughout loading. Using machine learning, we derive relationships between the two‐ and three‐dimensional measurements of three properties that control fluid flow: the porosity, and volume and tortuosity of the largest fracture at a particular differential stress step. The models predict the porosity and volume of the largest fracture with R2 scores of &gt;0.99, but predict the tortuosity with maximum R2 scores of 0.68. To test the assumption that different rock types may require different equations between the two‐ and three‐dimensional properties, we develop models for both individual rock types (granite, monzonite, marble, sandstone) and all of the experiments. Models developed using all of the experiments perform better than models developed for individual rock types, suggesting fundamental similarities between fracture networks in rocks often analyzed separately. Models developed with several parallel two‐dimensional observations perform similarly to models developed with several perpendicular two‐dimensional observations. When the models are developed with statistics of the two‐dimensional observations, the models primarily depend on the mean and median when they predict the porosity, and minimum when they predict the volume and tortuosity.

An experimental study of the transition from tensile failure to shear failure in Carrara marble and Solnhofen limestone: Does “hybrid failure” exist?

Failure of brittle rocks under axisymmetric extensional loading, when the minimum principal stress is tensile, results in the formation of one or more opening-mode cracks that are oriented normal to the extension axis. When all the principal stresses are compressive, failure occurs through the formation of numerous grain-scale tensile cracks, which coalesce into a shear-mode fault that is inclined at >20° to the maximum principal stress. There have been few attempts to study the transition between these failure modes, particularly those that incorporate microstructural analyses. A transitional mode of failure, termed “hybrid failure”, is often proposed to describe the orientation of faults between these end-member loading conditions, but this concept has traditionally been based on a parabolic, “Griffith-type” failure envelope that describes the growth of the single most critically-oriented crack. By integrating axisymmetric rock deformation experiments with microstructural analyses, it can be shown that failure under these transitional conditions involves the formation of several transgranular tensile cracks, which coalesce into a shear-mode fault that is inclined at <10° to the maximum principal stress. The stress intensity at the tip of each crack interacts with adjacent cracks to produce a stair-step geometry along the failure surface. These are what may be appropriately considered “hybrid faults”, the angle of which systematically increase as the maximum principal stress, and thus the differential stress increases. There is, however, no evidence of a distinct “hybrid failure” mode, which involves the in-plane propagation of a single crack that is subject to a combined shear stress and tensile normal stress. The results of this experimental study provide novel insights into the mechanical behaviour of carbonate rocks and into the interpretation of meso-scale natural examples of failure under mixed stress-states.

The evolving representative elementary volume size in crystalline and granular rocks under triaxial compression approaching macroscopic failure

SUMMARY Determining the size of the representative elementary volume (REV) for properties of fracture networks, such as porosity and permeability, is critical to robust upscaling of properties measured in the laboratory to crustal systems. Although fractured and damaged rock may have higher porosity and permeability than more intact rock, and thus exert a dominant influence on fluid flow, mechanical stability and seismic properties, many of the analyses that have constrained the REV size in geological materials have used intact rock. The REV size is expected to evolve as fracture networks propagate and coalesce, particularly when fracture development becomes correlated and the growth of one fracture influences the growth of another fracture. As fractures propagate and open with increasing differential stress, the REV size may increase to accommodate the larger fractures. The REV size may also increase as a consequence of the increasing heterogeneity of the fracture network, as many smaller fractures coalesce into fewer and longer fractures, and some smaller fractures stop growing. To quantify the evolving heterogeneity of fracture networks, we track the REV size of the porosity throughout eleven triaxial compression experiments under confining stresses of 5–35 MPa. Acquiring X-ray tomography scans after each increase of differential stress provides the evolving 3-D fracture network in four rock types: Carrara marble, Westerly granite, quartz monzonite and Fontainebleau sandstone. In contrast to expectations, the REV size does not systematically increase toward macroscopic failure in all of the experiments. Only one experiment on sandstone experiences a systematic increase in REV size because this rock contains significant porosity preceding loading, and it subsequently develops a localized fracture network that spans the core. The REV size may not systematically increase in most of the experiments because the highly heterogeneous porosity distributions cause the REV to become larger than the core. Consistent with this idea, when the rock does not have a REV, the fractures tend to be longer, thicker, more volumetric, and closer together than when the rock hosts a REV. Our estimates of the REV for the porosity of the sandstone are similar to previous work: about two to four times the mean grain diameter, or 0.5–1 mm. This agreement with previous work and the &amp;lt;15 per cent change in the REV size in two of the sandstone experiments suggests that when a system composed of sandstone does not host a localized, system-spanning fracture network, estimates of the REV derived from intact sandstone may be similar to estimates derived from damaged sandstone. Using the existing REV estimates derived from intact sandstone to simulations with more damaged crust, such as the damage zone adjacent to large crustal faults, will allow numerical models to robustly simulate increasingly complex crustal systems.

Determination of mining-induced stresses using diametral rock core deformations

Knowledge of ground stresses is crucial for ground control activities such as the design of underground openings, selection of support systems, and analysis for stability. However, it is a known fact that far field stresses experience changes in orientation and magnitude due to the presence of geological structures and due to the excavations created by mining activities. As a result, in-situ stresses around drifts, ramps, and stopes in underground mines are quite different from far field or pre-mining stresses. The purpose of this research is to develop a simple and practical methodology for determining in-situ stresses. Stress relief occurs once the rock core is drilled off. Such relief is a function of the surrounding stress field. This study uses exploration rock cores that are drilled off for the purpose of orebody definition in the underground mine. The method measures and analyzes the diametral core deformations in laboratory. Two case studies from operating underground mines are presented for demonstration. In these case studies, rock core deformations are measured with a customized test apparatus and rock samples were prepared and tested for Young’s modulus and Poisson’s ratio. The differential stress, namely the difference between the local principal stresses in the plane perpendicular to the core rock axis is calculated. It is shown that this methodology is useful for determining the brittle shear ratio in the rock mass, which is of primary interest to ground control studies.

High-magnitude stresses induced by mineral-hydration reactions

AbstractFluid-rock interactions play a critical role in Earth’s lithosphere and environmental subsurface systems. In the absence of chemical mass transport, mineral-hydration reactions would be accompanied by a solid-volume increase that may induce differential stresses and associated reaction-induced deformation processes, such as dilatant fracturing to increase fluid permeability. However, the magnitudes of stresses that manifest in natural systems remain poorly constrained. We used optical and electron microscopy to show that one of the simplest hydration reactions in nature [MgO + H2O = Mg(OH)2] can induce stresses of several hundred megapascals, with local stresses of as much as ∼1.5 GPa. We demonstrate that these stresses not only cause fracturing but also induce plastic deformation with dislocation densities (1015 m−2) exceeding those typical of tectonically deformed rocks. If these reaction-induced stresses can be transmitted across larger length scales, they may influence the bulk stress state of reacting regions. Moreover, the structural damage induced may be the first step toward catastrophic rock failure, triggering crustal seismicity.

Experimental Study on the Variation Mechanism of Permeability and Seepage Characteristics of High-Rank Coal with Different Bedding.

In order to understand the variation mechanism of permeability and seepage characteristics of high-rank coal with different bedding, we prepared cylindrical raw coal samples according to the bedding angles of 0, 30, 45, 60, and 90° and conducted permeability tests under two stress paths (stress path 1, unloading confining pressure under constant axial pressure; stress path 2, simultaneous loading axial pressure and unloading confining pressure). The results show that the relationship between the permeability and effective stress of high-rank coal with different bedding in the two stress paths conforms to an exponential function, and the permeability increases gradually with an increase in differential stress. Under the two stress paths, the initial permeability of different bedding under the loading axial pressure and confining pressure shows a pattern of a maximum for parallel bedding coal samples, followed by oblique bedding coal samples, and a minimum for vertical bedding coal samples. Under path 1, the increase in the permeability of the oblique bedding is 21.4 times that of the vertical bedding and 14.94 times that of the parallel bedding, and under path 2, the increase in the permeability of the oblique bedding is 26.45 times that of the vertical bedding and 142.11 times that of the parallel bedding; the coal samples of the oblique bedding suffer the greatest damage. The increase in the permeability of parallel bedding coal samples, oblique bedding coal samples, and vertical bedding coal samples under path 2 is 1.47 times, 13.96 times, and 11.3 times the increase in the permeability of the corresponding coal samples under path 1, respectively, and the damage produced by coal samples under path 2 is greater than that under path 1.

Modeling the Impact of the Viscoelastic Layer Thickness and the Frictional Strength to the Lithosphere Deformation in a Strike-Slip Fault: Insight to the Seismicity Pattern along the Great Sumatran Fault

As an earthquake is capable of causing significant losses, a strain buildup and release model following an earthquake is of importance for mitigation purposes. In this study, we aim to model strain buildup and release on a strike-slip fault which consists of elastic–brittle (upper crust) and elastic–viscous (lower crust and upper mantle) layers using a finite element model. The fault strength during strain buildup is controlled by the friction coefficient and cohesion, in addition to the viscoelastic parameter, as shown in the deformation model using Maxwell’s material. In the strain buildup model, we found that the differential stress on the elastic layer is larger than that on the viscoelastic layer and that the differential stress increases with the thickness of the elastic layer. When the viscoelastic layer is thinner, the deformation observed on the surface is larger. However, the differential of stress in the strain release model on the elastic layer is smaller than that on the viscoelastic layer, which shows the transfer stress from the lower crust and upper mantle to the upper crust. Using the knowledge gained by varying the thickness and frictional strength of the lithosphere, we discuss the seismicity pattern observed along the Great Sumatran Fault.

Earthquake Focal Mechanisms as a Stress Meter of Active Volcanoes

AbstractThe orientation of faulting associated with volcano‐tectonic earthquakes follows the stress field there, as with tectonic earthquakes. Therefore, stress changes associated with volcanic activity change fault orientations or focal mechanisms. Zhan et al. (2022; https://doi.org/10.1029/2022GL097958) observed temporal changes of focal mechanisms associated with volcanic unrest. They decomposed the stress field into the ambient differential stress, volcano loading, and the stress change by the dike intrusion; they then evaluated their relative contributions to constrain the magnitude of the ambient differential stress that is consistent with the observation. This study indicates that focal mechanisms can be used to monitor the stress state of an active volcano. Combining focal mechanisms with other geophysical observables, such as seismic anisotropy and geodetic measurements, will give us more precise assessments of the stress state, leading to better forecasts of volcanic activity.

A TEM Study on a Polycrystalline Olivine Sample Deformed in a D-DIA under Mantle Conditions

We carried out an electron microscopy study on a polycrystalline olivine sample that was deformed with multiple deformation cycles under controlled differential stresses and strain rates at high pressures and high temperatures. Low-angle backscattered electron images thereof showed randomly oriented grains. Most of the grains were about 10–20 μm wide. The grains were irregular with wavy grain boundaries, indicating high grain boundary mobility during deformation. Transmission electron microscopy (TEM) images showed complex dislocation microstructure characteristics of high temperature, high pressure, and high strain. Free dislocations were predominantly either short and straight screw dislocations or curved dislocations with mixed screw and edge characters. Many of them split into partial dislocations. The differential stress estimated with the free dislocations was ~780 MPa, which was close to the value of differential stress attained in the final deformation cycle. We also observed dense dislocation tangles, which formed dislocation cell substructures under high strain. The existence of dislocation loops and jogs indicated significant climbing activity, providing evidence for high-temperature creep as the dominant deformation mechanism. All of the dislocations observed in this study were exclusively with a [001] Burgers vector. Dislocations with a [100] Burgers vector were absent, suggesting that the activity of the a-slip (i.e., (010)[100] and (001)[100] slip systems) was completely suppressed. These observations support a conclusion that was reported based on an X-ray texture analysis, which considered that a high pressure promotes the activities of the c-slip (i.e., (010)[001] and (100)[001] slip systems). It appears that the transition from the a-slip to the c-slip was complete with multiple deformation cycles at a relatively lower pressure of 5.1 GPa than previously thought, corresponding to a depth of 165 km in the mantle.

Elastic impedance inversion for stress indicator in weakly orthorhombic media

Hydraulic fracturing is a crucial technology for improving permeability and production of shale reservoirs. The precise estimation of the stress distribution has a significant guidance for optimizing the placement of hydraulic fracturing. Assuming that the shale gas reservoir is a weakly anisotropic medium with orthorhombic symmetry, a new stress indicator parameterized by rock mechanical parameters and fracture parameters is firstly presented to predict the differential horizontal stress ratio in shale gas reservoirs. Then, we derive a novel simplified P-to-P reflection coefficient and a logarithmic normalized elastic impedance (EI) as a function of Young's modulus, Poisson's ratio, Thomsen's WA parameter δb and normal excess compliance ZN. Next, we adopt azimuthal EI inversion in a Bayesian framework to estimate rock mechanics parameters and fracture parameters directly on a fractured shale gas field seismic data. Finally, the stress indicator is determined by utilizing four inverted parameters. Synthetic examples demonstrate that the proposed approach produces stable parameter estimates even with moderate noise, verifying the feasibility and effectiveness of the method. Test on a field data set illustrates that the inversion results can be reasonably estimated, and the stress indicator derived by the inversion accords with the geomechanics result. Compared with the previous method, the new stress indicator has a higher capability to describe stress characteristics in the shale reservoir. We conclude that this stress evaluation procedure can provide reliable guidance for well location deployment and hydraulic fracturing reformation.

Deformation Mechanisms of Blueschist Facies Continental Metasediments May Offer Insights Into Deep Episodic Tremor and Slow Slip Events

AbstractExhumed fossil subduction zones are archives of the deformation conditions and mechanisms from depths not directly accessible. Microstructural analysis of samples exhumed therefrom offers insights into the micromechanics and deformation processes associated with subduction such as earthquakes, slow earthquakes, and aseismic creep. Subducted and exhumed continental metasediments of the Tuscan Metamorphic Units of the Italian Northern Apennines contain a mylonitic foliation and quartz and carpholite dilational hydroshear veins with crack‐and‐seal textures, both developed at blueschist facies conditions (350–400°C, ∼1 GPa). As shown by overprinting relationships and mineral assemblages, these two structure types formed broadly coeval within the stability field of carpholite. Metaconglomerates and metaquartzarenites deformed mainly by dissolution‐precipitation creep and secondary by dislocation creep. Microstructural and electron backscatter diffraction analyses of the veins suggest only limited recrystallization of quartz fibers by subgrain rotation recrystallization, with adjacent metapelite bands acting as decollement horizons, likely by slip on the basal plane of phyllosilicates. We estimated differential stresses of 43–55 MPa and strain rates between 10−13 and 10−14 s−1 from the vein recrystallized quartz fibers. We propose these microstructures and deformation mechanisms to represent a geological evidence of deep episodic tremor and slow slip events in subducted continental metasediments. Pore pressure cyclically reached supralithostatic values triggering tremors causing fracturing of all involved lithotypes. Likely, slow slip was accommodated preferentially by slip on phyllosilicate bands. Aseismic creep occurred mainly by dislocation creep with subgrain rotation recrystallization in vein quartz, slip on the basal plane of phyllosilicates, and dissolution and precipitation creep in the host rock.

Effects of supercritical CO2 on viscoelastic properties of shales

Laboratory uniaxial compression creep tests, with differential stress of 30 MPa hold for 3 h, were performed on Chang-7, Longmaxi (LMX) and Barnett shales to study the influence of SC-CO 2 on short-term viscoelastic properties. To this end, the wet shale samples were treated with SC-CO 2 with a pressure of 30 MPa and a temperature of 110 °C for 14 days. We analyzed the creep data using the fractional Maxwell model. To investigate microscopic structural alterations, the surface morphology of the same location, before and after SC-CO 2 -water exposure, was examined by SEM images. Compared with dry shales, dynamic and static elastic moduli decreased by up to 25.02% and 55.83%, respectively, but the creep deformation increased by 200% for LMX and Chang-7 shales, and 500% for the Barnett shale treated by SC-CO 2 . Compared to dry sample, there is an increase in calculated fractional orders of 0.02, 0.07, 0.22 for SC-CO 2 treated samples, indicating that SC-CO 2 treatment is likely to enhance shale creep. SEM investigation confirmed physicochemical mechanisms responsible for the observed elastic damage and creep enhancement, including mineral dissolution and swelling caused by SC-CO 2 . This work would further improve our current understanding of the time-dependent deformation of shale under chemical-mechanical coupling effects during CO 2 capture utilization and storage.