Porous Rocks Research Articles

Diffusion in Porous Rock Is Anomalous.

Molecular diffusion of chemical species in subsurface environments─rock formations, soil sediments, marine, river, and lake sediments─plays a critical role in a variety of dynamic processes, many of which affect water chemistry. We investigate and demonstrate the occurrence of anomalous (non-Fickian) diffusion behavior, distinct from classically assumed Fickian diffusion. We measured molecular diffusion through a series of five chalk and dolomite rock samples over a period of about two months. We demonstrate that in all cases, diffusion behavior is significantly different than Fickian. We then analyze the results using a continuous time random walk framework that can describe anomalous diffusion in heterogeneous porous materials such as rock. This methodology shows extreme long-time tailing of tracer advance as compared to conventional Fickian diffusion processes. The finding that distinct anomalous diffusion occurs ubiquitously implies that diffusion-driven processes in subsurface zones should be analyzed using tools that account for non-Fickian diffusion.

New insights into how temperature affects the electrical conductivity of clay-free porous rocks

Summary Geothermal energy is increasingly important for the global environment and for the sustainable development of our society. Electrical surveys are widely employed for the exploration of geothermal energy, because the electrical geophysical properties provide useful information about the fluids at depth. However, although quantitative interpretation of electrical survey data relies on the knowledge about the effects of temperature on the electrical properties of fluid-bearing rocks, it remains poorly understood about how temperature affects the electrical conductivity of clay-free porous rocks. We bridge this knowledge gap by measuring the electrical conductivity and porosity of five brine saturated clean Berea sandstones with temperature ranging between 25° C and 140° C, and analysing all the factors that impact the rock conductivity. We showed that the effects of surface conductivity on the temperature dependent electrical conductivity can be negligible, whereas the temperature induced variation in the porosity and pore structure quantitatively characterized in terms of cementation exponent can be more significant. We also found that temperature affects the electrical conductivity of brine saturated Berea sandstones by impacting the brine conductivity, and the pore structure and porosity of the samples, with their importance in a descending order. The results have provided new insights into how temperature affects the electrical conductivity of clay-free porous rocks, and will help to improve the quantitative interpretation of electrical survey data for the exploration of geothermal energy.

Cushion gas effects on hydrogen storage in porous rocks: Insights from reservoir simulation and deep learning

Underground hydrogen (H2) storage (UHS) is crucial for the H2 economy, offering scalable and long-term solutions vital for reliable and sustainable H2-based energy systems. Cushion gas is a common component in UHS designs, but its impact on H2 storage performance is not yet fully understood. This study bridges this research gap by systematically investigating the effects of various cushion gas scenarios on UHS performance in porous rocks, specifically saline aquifers and depleted gas reservoirs, using reservoir simulations and deep learning. Firstly, we conducted 8,000 multiphase compositional reservoir simulations for UHS operations in two types of storage formations, considering four cushion gas scenarios: no cushion gas, methane (CH4), nitrogen (N2), and carbon dioxide (CO2). These simulations cover a wide range of geological and operational parameters relevant to practical UHS projects. From these simulations, we derived key insights into the physical mechanisms governing UHS processes and proposed critical storage performance metrics, including H2 withdrawal efficiency, produced H2 purity, produced gas–water ratio, and well injectivity. Then, we developed a unified reduced-order model (ROM) using a deep neural network (DNN) based on the comprehensive simulation results. The DNN architecture, with an input layer, five hidden layers, and an output layer, takes 12 geological and operational parameters as inputs, and forecasts the four performance metrics. The ROM accurately predicts the cyclic evolution of performance metrics and is over 5,000,000 times faster than traditional physics-based simulations, allowing for thorough uncertainty quantification of UHS performance prediction under various geological and operational conditions. Key findings of this study include: (a) UHS in porous rocks is technically promising, with improving storage performance over cycles; (b) UHS in saline aquifers has higher withdrawal efficiency and purity but lower gas–water ratio and injectivity compared to depleted gas reservoirs; (c) In saline aquifers, cushion gas reduces withdrawal efficiency and purity but significantly enhances gas–water ratio and injectivity, making it particularly beneficial in scenarios with high water production risks; (d) In depleted gas reservoirs, cushion gas is less important under the current operational conditions, as it barely affects withdrawal efficiency and purity, and these reservoirs inherently exhibit high gas–water ratio and injectivity due to initial/leftover native gas (mostly CH4); (e) The cushion gas type slightly affects storage performance, with N2 slightly outperforming CH4 and CO2 in withdrawal efficiency and purity, CO2 being better for gas–water ratio, and CH4 for injectivity. The findings of this study support cushion gas designs in future UHS projects in both saline aquifers and depleted gas reservoirs. Moreover, the developed ROM offers a highly efficient tool for conducting extensive sensitivity analysis and uncertainty quantification, optimizing operational conditions, and facilitating the screening of potential field sites for future UHS projects.

Evolution of pore structure and analysis of freeze damage in granite during cyclic freeze-thaw using NMR technique

Understanding the changes in a rock's pore structure and unfrozen water content during the freeze-thaw (F-T) process is crucial for unraveling the F-T damage mechanism of rock masses. This study employs nuclear magnetic resonance (NMR) tests to resolve the pore structure characteristics of granite specimens subjected to varying numbers of F-T cycles (0, 50, 100, and 200 cycles). Throughout the F-T process, unfrozen water (comprising bound water and free water) is quantified through T2 distribution curves, and the freeze-induced strain at different subzero temperatures is investigated. The results reveal that F-T cycles lead to the expansion of pore sizes from small to large, reducing the complexity of the pore structure. Moreover, an increased proportion of meso- and macro-pores accelerates the rate of water-ice phase transition in granite specimens, resulting in intensified freeze damage and greater maximum freeze-induced strain. Additionally, when previously damaged granite is refrozen, the presence of free water induces higher levels of freeze-induced strain. This research comprehensively analyzes the F-T damage process in granite, laying a crucial theoretical foundation for understanding freeze damage mechanisms in rock engineering.

Microscopic Insights of Phase-Transition-Induced Vapor Transport Enhancement in Porous Media

Vapor transport in porous media, often associated with liquid-vapor phase change, is an fundamental process in many emerging underground energy storage and extraction processes (i.e., seasonal solar thermal aquifer storage, geothermal extraction, extraterrestrial in-situ water extraction). By jointly using experimental imaging and numerical modeling at the micro-scale, we conduct mechanistic pore scale investigation of capillarity-dominated phase change dynamics and its influence on vapor transport in partially saturated porous rock micromodel. Strongly linked to surface roughness and wettability condition, the capillarity hinders water vaporization from rock surface micro/nano-structures as observed under the environmental scanning electron microscope. By varying the contact angle of 0°, 60°, and 120°, the lattice Boltzmann simulation shows the wettability-dependent vaporization process of capillary-hold water, where pores with hydrophilic surfaces contains significantly more liquid water than that of the hydrophobic ones under the same temperature. When saturated vapor flows through rock porous patterns, capillarity further induces water condensation on the strong water-wet surfaces. Water condensation, yet forming water bridges/islands and causing the blockage of vapor diffusion, enhances the vapor diffusion ability counterintuitively. The reduction of diffusion path is revealed as the main reason by characterizing the local vapor pressure distribution before and after the pore filling by condensate. The findings support the debatable enhancement mechanisms postulated by Philip and de Vries. This work offers the insightful interfacial hydrodynamics of vapor transport in porous media and potentially provides operational guidance for geothermal applications and beyond.

Compaction localization in geomaterials: A mechanically consistent failure criterion

Compaction bands play a key role in the deformation processes of porous rocks and explain different aspects of physical processes in geological formations. The state-of-the-art description of the localized strains that lead to compaction banding has limitations from the mechanical point of view. Thus, we describe the phenomenon using a consistent axiomatic formulation. We build a viscoplastic model using minimal assumptions; we base our model on six principles to study compaction band strain localization triggered by viscous effects. We analyze different stress states to determine the conditions that trigger compaction bands. Laboratory experiments show that a material undergoes different strain localizations depending on the confinement pressure; thus, we perform a series of numerical experiments that reproduce these phenomena under varying triaxial compression conditions. These simulations use a simple viscoplastic constitutive model for creep based on Perzyna’s viscoplasticity and show how confinement changes the strain localization type for different triaxial tests.

Effect of pore structure characteristics on gas-water seepage behaviour in deep carbonate gas reservoirs

The study of gas-water two-phase flow behavior has always been a focus and a difficult problem in the field of oil and gas seepage, and it is related to the formulation of reasonable development plans for oil and gas reservoirs. Deep carbonate gas reservoirs (depth>4500m) develop cross-scale porous media, which have extremely strong heterogeneity and very complex seepage characteristics. To the best of our knowledge, there has been no systematic study on the two-phase flow patterns of gas and water in deep carbonate gas reservoirs. This paper uses cast thin sections, scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) to study the influence of carbonate rock pore structure characteristics on gas storage and seepage capabilities. The results show that the existence of cavities enhances the gas storage capacity of the reservoir and improves the seepage capacity of pore-throat space near the cavity. Fractures can effectively communicate with isolated cavities, enhance reservoir connectivity, and thereby significantly improve the overall seepage capacity of the reservoir. The centrifugal technique was combined with NMR to study the movable fluid characteristics of carbonate reservoirs. The critical flow throat radius and movable fluid saturation of the reservoir reveal that the degree of development of cavities and fractures determines the mobility of the fluid in the reservoir. The fluid mobility in the fracture-cavity-type reservoir is the strongest, while the fluid mobility in the pore-type reservoir is the weakest. Analysis of the normalized curve of gas-water relative permeability under high temperature and pressure reveals that formation water preferentially enters connected pores with low seepage resistance, and the strong storage capacity of cavities can effectively delay the non-uniform intrusion of formation water. The results of water invasion simulation experiments show that the water invasion phenomenon can improve the gas production rate and recovery rate of pore-type reservoirs. However, the formation water flow and channeling phenomena caused by reservoir heterogeneity will cause a large amount of gas to be sealed in the pore space of cavity-type and fracture-cavity-type reservoirs, reducing the recovery rate and economic benefits of these reservoirs. These findings further provide solid fundamentals for efficient and effective exploitation of deep carbonate gas reservoirs.

Experimental and theoretical analysis of finely bedded porous rock models with internal cracks

The behavior of ultrasonic waves in synthetic rocks consisting of alternating layers of sand-based (porous and permeable) and acrylic (unporous) materials is investigated. Two synthetic layered media are created, one without cracks and the other with parallel cracks in one of the layers, to increase anisotropy and simulate a heterogeneous transversely isotropic medium with a vertical axis of symmetry. Microcomputed tomography is used to obtain images of the samples, which indicate that there are no bubbles present and that the layers are well connected without any gaps or breaks. The experiment uses ultrasonic measurements to determine the propagation of P and S waves, with transducers at frequencies of 100 kHz, 500 kHz, and 1 MHz for traveltime and waveform measurements. These three sources are chosen to verify the effective behavior of the samples, which depends on the relationship between the dominant wavelength of investigation and the layer thickness ([Formula: see text]). The measured velocities are compared with theoretical models based on Backus, Kennett (traditional models), and Backus (modified for cracked media by Schoenberg and Muir’s theory). The main finding is that the cracked medium exhibits at least twice as large a difference in behavior for P- and S-wave vertical velocities as a function of the number of layers compared with the uncracked medium.

Simulating squirt flow in realistic rock models using Graphical Processing Units (GPUs)

Summary Understanding the underlying mechanisms of seismic attenuation and moduli dispersion in fluid-saturated cracked porous rocks is of great importance for the development of non-invasive methods to characterize the subsurface. Wave-induced fluid flow at the pore scale, so-called squirt flow, is responsible for seismic attenuation and moduli dispersion at sonic and ultra-sonic frequencies and may be relevant at seismic frequencies. The squirt flow associated attenuation is usually quantified using analytical models. However, numerical experiments suggest that the squirt flow related dissipation is sensitive to fine details of the pore geometry, which can only be modeled numerically. Most of the existing numerical studies explore this phenomenon using simplified models, and there is a lack of numerical studies that model the physics in realistic pore geometries with sufficient numerical resolution. As a result, the impact of wave-induced fluid flow on the effective static and time-dependent mechanical characteristics in realistic settings is still poorly understood. I address these issues by developing a numerical method to model the effective mechanical properties of a hydro-mechanically coupled system at the pore scale suitable for graphical processing units (GPUs). A numerical evaluation of attenuation and modulus dispersion due to squirt flow in models based on 3D micro-tomography images of cracked Carrara marble is presented. It is shown that the local hydraulic conductivity can be quantitatively estimated from the numerically evaluated effective properties. The accuracy of the numerical results is carefully analyzed. This study improves the understanding of the underlying mechanisms of attenuation and moduli dispersion in fluid-saturated cracked rocks. The new method can be applied to model squirt flow for entire laboratory samples in the centimeter scale which was not possible a decade ago.

Quantifying the physical and mechanical heterogeneity of porous volcanic rocks from the Chaîne des Puys (Massif Central, France)

Quantifying the physical and mechanical heterogeneity of porous volcanic rocks from the Chaîne des Puys (Massif Central, France)

Seismic wave modeling of fluid-saturated fractured porous rock: including fluid pressure diffusion effects of discretely distributed large-scale fractures

Abstract. The scattered seismic waves of fractured porous rock are strongly affected by the wave-induced fluid pressure diffusion effects between the compliant fractures and the stiffer embedding background. To include these poroelastic effects in seismic modeling, we develop a numerical scheme for discretely distributed large-scale fractures embedded in fluid-saturated porous rock. Using Coates and Schoenberg's local-effective-medium theory and Barbosa's dynamic linear slip model characterized by complex-valued and frequency-dependent fracture compliances, we derive the effective viscoelastic compliances in each spatial discretized cell by superimposing the compliances of the background and the fractures. The effective governing equations for fractured porous rocks are viscoelastic anisotropic and numerically solved by the mixed-grid-stencil frequency-domain finite-difference method. The main advantage of our proposed modeling scheme over poroelastic modeling schemes is that the fractured domain can be modeled using a viscoelastic solid, while the rest of the domain can be modeled using an elastic solid. We have tested the modeling scheme in a single fracture model, a fractured model, and a modified Marmousi model. The good consistency between the scattered waves off a single horizontal fracture calculated using our proposed scheme and the poroelastic modeling validates that our modeling scheme can properly capture the fluid pressure diffusion (FPD) effects. In the case of a set of aligned fractures, the scattered waves from the top and bottom of the fractured reservoir are strongly influenced by the FPD effects, and the reflected waves from the underlying formation can retain the relevant attenuation and dispersion information. The proposed numerical modeling scheme can also be used to improve migration quality and the estimation of fracture mechanical characteristics in inversion.

An assessment of the durability of untreated and water repellent-treated cultural heritage stone (Döğer tuff-Turkey) by salt mist and salt crystallization tests

Salt mist and salt crystallization tests were conducted to determine the resistance of the untreated and water-repellent-treated Döğer tuff samples to salt damage. The Döğer tuffs are of volcanic origin and consist of porous pyroclastic rocks. A series of characterization studies were performed to determine the physicochemical and chemical properties (XRF), mineralogical-petrographic characteristics (polarizing optical microscopy, XRD, and SEM), and pore-size distribution (MIP) of the Döğer tuff prior to the aging tests. Following the aging experiments, the changes in weight, ultrasound pulse velocity, uniaxial compressive strength, and variations in pore size distribution were examined in the tested tuffs. In addition, SEM studies were conducted to analyze the microstructures. The untreated tuff samples were more severely affected by salt crystallization. The increases in dry weight for untreated and water-repellent-treated Döğer tuff samples were 0.27% and 0.13% after 60 cycles of salt mist exposure and 7.2% and 3.76% after salt crystallization cycles using a sodium chloride solution, respectively. Both conditions resulted from the accumulation of NaCl crystals on the tuff surface and within the pores. Salt crystal formation in a sodium sulfate solution led to dry weight change of 2.02% and 0.29% in the untreated and water-repellent-treated samples, respectively. The uniaxial compressive strength of the untreated tuff samples decreased by 36.71% in the presence of sodium chloride solution and by 72.58% in the presence of sodium sulfate solution. The ultrasound pulse velocity values decreased in all tuff samples after the salt crystallization test. However, the Döğer tuff samples were still distinguishable after both tests and their physical integrity remained intact.

An extended continuum-mechanics standard linear solid rheology for fluid-saturated porous rock

SUMMARY In a large body of rock-physics research, seismic wave velocity dispersion and attenuation in fluid-saturated porous rock are studied by constructing analytical or numerical models for time- or frequency-dependent dynamic (effective, or viscoelastic) moduli. A key and broadly used model of such kind is the Zener's, or the standard linear solid (SLS). This model is qualitatively successful in explaining many field and laboratory observations and serves as the key element of many generalizations such as the Burgers model for plastic deformations or the generalized SLS explaining band-limited or near-constant seismic attenuation. However, as a physical model of fluid-saturated porous rock, the SLS has several major limitations: disregard of inertial effects, absence of secondary wave modes and lack of key physical parameters such as porosity and Skempton coefficients. Grainy and porous rock is an unconsolidated material in which the effective density is frequency-dependent, and its effects on wave velocities may exceed those of the dynamic modulus. To overcome these limitations of the empirical SLS, we propose a rigorous rheologic model based on classical continuum mechanics and called the extended SLS, or eSLS. This rheology explains the available attenuation and dispersion observations equally well, but it is also close to Biot's model, honours all poroelastic relations, includes inertial effects, and reveals several new physical properties of the material. Detailed comparison of the eSLS and Biot's models gives a physical-mechanism-based classification of wave-induced fluid flow (WIFF) phenomena. In this classification, the so-called ‘global-scale’ flows occur in Biot's type structures within the material, whereas the ‘local-scale’ WIFF occurs in eSLS-type structures. Combining Biot's and eSLS models gives a broad class of rheologies for linear anelastic phenomena within rock with a single type of porosity. The model can be readily generalized to multiple porosities and different types of internal variables, such as describing squirt flows, wetting or thermoelastic effects. Modelling is conducted with relatively little effort, using a single matrix equation similar to a mechanical form of the standard SLS. By combining the eSLS and Biot's models, observations of dynamic-modulus dispersion and attenuation can be inverted for macroscopic mechanical properties of porous materials.

Water weakening and the compressive brittle strength of carbonates: Influence of fracture toughness and static friction

Water is ubiquitous within the pore space of rocks and has been shown to affect their physical and mechanical behaviour. Indeed, water can act on the rock strength via mechanical (i.e., reducing the effective stresses) or chemical effects (e.g., mineral dissolution, mineral alteration, subcritical crack growth, etc.). As rock macroscopic strength is controlled by both fracture toughness and friction at the grain-scale, these parameters should also be affected in presence of water. While some recent studies have measured the effect of water on both fracture toughness and frictional parameters to constrain the water weakening of porous rock compressive strength, the physical parameters, or rock characteristics, that influence this weakening are as of yet unclear. Here, we report a series of laboratory experiments in order to determine the influence of a water-saturated, as opposed to dry, environment on five limestones’ strengths. The uniaxial compressive strength, the mode-I fracture toughness and the static friction parameters are of interest. The experiments show that, for the tested limestones, water-saturated conditions provoke a reduction of the uniaxial compressive strength by up to 53 %. This reduction is accompanied by a reduction of the mode-I fracture toughness by up to 34 % and of the static friction by up to 16 %. Even though the water weakening of the uniaxial compressive strength is not influenced by the sample porosity, the mode-I fracture toughness reduction in the presence of water is accentuated for high-porosity limestones. Additionally, low porosity limestones appear to promote higher static friction reductions in water-saturated environments.

Magnetic Resonance Relaxation in Heterogeneous Materials is Analogous to First-Order Chemical Reaction

AbstractBiological tissue, pharmaceutical tablets, wood, porous rocks, catalytic reactors, concrete, and foams are examples of heterogeneous systems that may contain one or several fluid phases. Fluids in such systems carry chemical species that may participate in chemical reactions in the bulk of a fluid, as homogeneous reactions, or at the fluid/fluid or fluid/solid interfaces, as heterogeneous reactions. Magnetic resonance relaxation measures the return of 1H nuclear magnetization in chemical species of these fluids to an equilibrium state in a static magnetic field. Despite the perceived difference between reaction–diffusion and relaxation–diffusion in heterogeneous systems, similarities between the two are remarkable. This work draws a close parallel between magnetic resonance relaxation–diffusion and chemical reaction–diffusion for elementary unitary reaction $${\text{A}}\to {\text{B}}$$ A → B in a dilute solution—both in heterogeneous systems. A striking similarity between the dimensionless numbers that characterize their relevant behavior is observed: the Damköhler number of the second kind $${{\text{Da}}}^{{\text{II}}}$$ Da II for reaction and the Brownstein–Tarr number $${{\text{BT}}}_{i}$$ BT i for relaxation. The new vision of analogy between reaction- and magnetic resonance relaxation–diffusion in heterogeneous systems encourages the exploitation of similarities between reaction and relaxation processes to noninvasively investigate the dynamics of chemical species and reactions. One such example of importance in chemical engineering is provided for solid–fluid reaction in packed beds.

Deterioration of volcanic tuffs from rock dwellings in Brhlovce (Slovakia) induced by freeze-thaw cycling studied by non-destructive tests and µCT visualization

The aim of this paper is to evaluate the effect of freeze-thaw-induced weathering of volcanic tuffs from the rock dwellings in the village of Brhlovce (Slovakia) based on a non-destructive and experimental study of tuff pore space properties. Traditional techniques for characterization of the porosity and pore size distribution and new experimental techniques like spontaneous imbibition (pore interconnection) or indicative rock pore structure method (pore size distribution) were compared with pore network models acquired by X-ray computed microtomography before and after freeze-thaw cycling in a custom-built thermodilatometer VLAP04. The results offer insight into how frost weathering altered the pore network of volcanic tuffs. Brhlovce tuffs are highly susceptible to frost damage due to their high porosity, pore interconnectivity, and bimodal pore size distribution pattern, where nanometric-sized micropores predominate over larger micrometric-sized capillary pores. Poromechanical principles were applied to confirm that ice crystallization initiates in well-connected micropores with a corresponding crystallization pressure below 10 MPa. After undergoing 100 freeze-thaw cycles with temperature oscillations ranging from − 10 to 10 °C, there was a significant increase in total porosity, pore interconnectivity, and rock permeability. Additionally, the pore size distribution was altered. These parameters influence the transport of fluids and the moisture regime in rock masses, which can serve as a precursor to physical and chemical weathering processes

The effects of pore structure on wave dispersion and attenuation due to squirt flow: A dynamic stress-strain simulation on a simple digital pore-crack model

Squirt flow, a phenomenon typically observed in porous cracked rocks, occurs due to the contrasting compressibility between the pores and cracks, leading to the pore pressure diffusion and dissipation of wave energy. Understanding the influence of pore structure on wave dispersion and attenuation signatures due to squirt flow is essential for interpreting seismic and sonic logging data in various fields of earth and energy sciences, such as hydrocarbon exploration, geothermal energy exploitation, and CO2 sequestration. In this study, we construct a simple digital pore-crack model with varying pore structures and use a dynamic stress-strain simulation approach to characterize wave dispersion and attenuation signatures due to squirt flow. Numerical simulation suggests that, in addition to the commonly considered parameters such as porosity, crack density, and crack aspect ratio, additional pore structure parameters, such as pore size, pore aspect ratio, crack orientation, crack length, and crack width, significantly affect the wave dispersion and attenuation signatures. Increasing the pore size leads to the pronounced enhancement of attenuation magnitude and a decrease in characteristic frequency. We demonstrate that variations in crack length have a pronounced impact on attenuation magnitude, whereas crack width is decisive in controlling the characteristic frequency. Furthermore, it is found that the saturation paths (the gas filling the pore first or gas filling the crack first) considerably affect the velocity-saturation and attenuation-saturation relationship, suggesting that the coupling effects of pore structure and fluid distribution complicate the fluid pressure diffusion and wave attenuation behaviors. The presented results offer insights for deciphering pore structure parameters using attenuation- or dissipation-related seismic attributes.

Metal intrusion enhanced deep learning-based high temperature deterioration analysis of rock materials

The deep learning-based method demonstrates superior capability in high-temperature deterioration analysis of rock materials through scanning electron microscopy (SEM) characterization. However, the blurred boundaries between rock particles and pore structure presented in SEM images always affect the training efficiency. Hence, in this study, the metal intrusion technology and backscattered electron (BSE) observations are applied to assist a deep learning-based model to analyze the deterioration of the rock materials exposed to different levels of high temperatures. >2000 micro images for each level were inputted and trained to distinguish the features of rock after 5 temperature levels deterioration. The results reveal that the classification accuracy on the heat deterioration of the rock materials achieved 94.3%. The classification accuracy under the optimized observation area (from 240 pixels × 240 pixels to 280 pixels × 280 pixels) is stable over 90%, balancing the high precision and training efficiency. Moreover, the interpretable high-temperature degradation characteristics of the micro-damage on rock particles and pore regions can be extracted, enabling further analysis of the degradation process of high-temperature treated rocks. We expect this work will inspire a high-precision, wide-application and cost-efficient method for the deterioration analysis of rock materials in underground projects.

Role of Crack Interaction on Shear Localization in Porous Granular Rocks Deformed in the Brittle and Ductile Fields

AbstractCrack interactions leading to shear localization were quantified using microstructural analysis for brittle faults and high‐temperature ductile faults formed during experiments on quartz sandstone. In both faulting regimes, the nucleation of macroscopic faults results from the interactions of microfractures at two length scales in ensemble. Brittle faults nucleate when the longest mesoscale shear fractures and transgranular tensile cracks critically interact. In contrast, ductile faults nucleate when the longest mesoscale shear fractures and multi‐grain scale intergranular shear cracks critically interact. For both faulting regimes, we conclude the interaction and coalescence of the longest mesoscale shear fractures is the fundamental process responsible for fault nucleation. Hence, mesoscale shear fractures, which accommodate the majority of axial strain prior to shear localization in both faulting regimes, also serve as the nucleus of macroscopic faults. Locally, the growth of the mesoscale shear fractures is promoted by the interaction and coalescence of the multi‐grain scale cracks in both faulting regimes. We hypothesize that attainment of a critical microstructure for shear localization (i.e., local clustering of the longest microfractures) requires a characteristic amount of plastic axial strain, which depends on deformation conditions. In brittle faulting, distributed microfracturing is confined within limited regions of the rock volume, which expedites crack clustering and fault nucleation at low characteristic strains. In ductile faulting, distributed microfracturing occurs more uniformly throughout the rock volume, delaying shear localization to high characteristic strains. Accurate prediction of shear localization requires models that describe crack interactions of the largest flaws that account for crack clustering.

The 15 January 2022 Hunga (Tonga) eruption: A gas-driven climactic explosion

An extraordinarily powerful, explosive eruption occurred from Hunga volcano in the Tonga island arc on 15 January 2022 and generated an eruption column 58 km high. The explosive eruption also generated atmospheric gravity waves, extreme runup tsunamis and quite unusual and destructive meteotsunamis. Together these place this VEI 6 eruption as, globally, one of the largest of the past 300 yearsBased on the oceanic context of Hunga volcano, it has previously been assumed that the eruption was phreatomagmatic through a fuel-coolant Surtseyan-type interaction, but this is not supported by satellite imagery. Similarly, it has been suggested that a caldera-collapse was the eruption trigger, but this is not supported by bathymetric data or the seismicity recorded during the eruption. Here we develop a new model based on the observed energetics and time sequence of the eruption integrated with understanding of the internal structure of active volcanoes and their characteristic high flux discharges of volcanic gas.It has been shown elsewhere that magma-derived reactive gases (H2O, CO2, SO2, HCl, etc) aggressively alter the volcanic rocks in the core of a volcano leading to selfsealing of gas flow to the surface and consequent changes to deviatoric stress in the structure. Common minerals developed by these reactions include anhydrite (CaSO4), sulphides and silica (quartz). all of which have been recorded in volcanic ejecta including at Hunga.We here develop a first order numerical model that quantifies how the free discharge of such gas to the surface may progressively become choked by these sealing reactions leading to increased internal gas pressure. Hydraulic fracture of the seal occurs when the transmitted pressure of the compressed magmatic gas beneath the seal increases to a value greater than the lithostatic pressure plus the tensile strength of the sealed rock. This initiates the explosive release of compressed gas whose high-power discharge progressively develops and enlarges a crater. At the same time the explosion feeds upon itself by generating larger pressure gradients in the pressurized gas within the fractured porous rock mass of the core of the volcano. Excavation of the crater may intersect high level intrusions and produce the pumice rafts that were observed after the eruption. The eruption itself diminished in intensity as the gas pressure in the reservoir declined.At Hunga the eruption excavated an 850 m deep, 2-3 km diameter steep-walled crater. This volume may be assumed to approximate the volume of fractured porous rock (the control volume of the eruption) whose trapped gas was mined by the eruption until surrounding gas pressure was depleted. Our numerical model shows that the calculated potential energy of the trapped compressed gas matches the independent observations of the scale of the eruption. Sensor data have since shown that gas bubble flares continued for at least 6 months after the eruption indicating continued depletion of the gas reservoir of rocks surrounding the new crater. The systems-based, gas-driven model for the Hunga climactic eruption developed here also applies to Plinean-type eruptions on subaerial arc volcanoes such as at Pinatubo (Philippines) 1991.