Heterogeneous Geological Material Research Articles

Heat flow characteristics and thermal resistance model for soil-rock mixtures during freezing-thawing processes: Damping properties

As a special heterogeneous geological material, soil-rock mixtures are widely distributed in nature and used in civil engineering. Heat flow is one of the most significant factors affecting the development of water balance and redistribution, ground temperature evolution, and heat transport processes in cold regions. This study explores the heat flow characteristics and generalized thermal resistance model for soil-rock mixtures during freezing-thawing processes. A series of freezing-thawing experiments for soil-rock mixtures with different rock contents (i.e., 10%, 25%, and 40%) were conducted. The effects of rock content and water–ice phase transition on heat flux during the freezing-thawing processes were analyzed, and the damping properties of heat flow for soil-rock mixtures during freeze–thaw cycles were found and illustrated. Subsequently, a generalized thermal resistance model for soil-rock mixtures was proposed and discussed. The results show that the freezing-thawing processes and rock content significantly influence the heat flux and soil temperature of the soil-rock mixtures. Regardless of rock content, the sums of heat flux in freezing and thawing stages all firstly decreased, and then gradually tends to be stable with freeze–thaw cycles. Besides, the reduction of volumetric unfrozen water content increases with the freeze–thaw cycles, and the amplitude of unfrozen water reduction for sample with high rock content is larger than that with low rock content. Additionally, owing to the effect of the latent heat released by water/ice phase transition and thermal sensitive property of rock, the variation of heat flow for soil-rock mixtures presents damping property in the stages without completely freezing, and the variation of heat flow exhibits reversed damping property in the post-freezing stages due to the denser structure and ice crystal growth. Furthermore, based on the thermal resistance in series, characteristics of water–ice phase transition, and thermal sensitive property of rocks, a new generalized thermal resistance model of soil-rock mixtures during freezing-thawing processes is proposed.

Detecting REE-rich areas in heterogeneous drill cores from Storkwitz using LIBS and a combination of k-means clustering and spatial raster analysis

This paper presents a novel approach to calculate matrix-matched intensity limits for the spatially resolved detection of rare earth element (REE) enrichments in highly heterogeneous geological material from the Storkwitz carbonatite based on scanning Laser Induced Breakdown Spectroscopy (LIBS). A drill core from the Storkwitz carbonatite was mapped in detail with a LIBS drill core scanner. For reference purposes, μ-EDXRF was applied for qualitative REE detection, and a microprobe was used for quantitative REE analysis.Microprobe analysis of thin sections verified the existence of rare earth elements and revealed an accumulation of REEs around mineral rims. Microprobe measurements also revealed that the main carrier of rare earth elements in the analysed drill core are REE-carbonates with contents as high as 24.4 wt% for Ce, 15.4 wt% for La, and 9.2 wt% for Nd. Apatite and pyrochlore are carrier for rare earth elements as well, however, with significantly lower concentrations of Ce (1.0 wt%), La (0.3 wt%), and Nd (0.3 wt%).K-means clustering was applied on the LIBS mapping to separate classes that show similarities in chemical composition. The classes represent carbonates, silicates, and rock matrix, respectively. According to the microprobe results, most REE-carbonates were found to be related to elevated porosity or microfractures in the rock matrix, forming very thin rims of idiomorphic and hypidiomorphic minute crystals and rarely dense aggregates around exposed minerals and fractured rock fragments. Therefore, a buffer zone calculation was performed on the distinct mineral classes to extract pixel belonging to mineral rims only. Rims without any enrichment in rare earth elements were then used to calculate matrix-matched intensity limits for REE enrichments. Based on the observed similarity in chemical composition for rim pixel and rock matrix pixel, this intensity limit could be transferred to all pixels belonging to the rock matrix.Based on the calculated intensity limits for La, 1.4 area % of the sample was found to be enriched in REE-carbonates. This result was validated qualitatively by comparing it to a μ-EDXRF mapping of the same sample. Optical comparison showed good agreement and pixel counting confirmed similar zones of enrichment. The results reveal that La enrichments in REE-carbonates are reliably detected with LIBS. A mass balance calculation has shown that about 88% of the REE enrichments in the investigated drill core seem to occur as pure REE-carbonates. The fast detection of drill core areas that are enriched in REEs makes LIBS a viable addition to the currently used methods in REE exploration.

Determining the momentum transfer in regolith-like targets using the TUM/LRT electro-thermal accelerator

Asteroids are small solid bodies that formed during the early stages of the solar system, even before the formation of our own planet. Asteroids have been known to strike our planet Earth ever since its formation, causing a great threat to all existing life forms. One of the means to avert a potential collision of an asteroid with Earth is to deflect the asteroid away from its trajectory towards Earth by means of an impacting spacecraft. This idea is already part of mission designs such as AIDA (Asteroid Impact and Deflection Assessment).The study of the surfaces of numerous asteroids through spacecraft, ground-based spectroscopic observations and other techniques has revealed that the surfaces of these small bodies are covered with loose, granular, heterogeneous geological material. This is called ‘regolith’. If an impactor were to collide with an asteroid, the physical phenomenon that needs to be understood is the reaction of the regolith to the impact. To understand the deflection of the asteroid, the transfer of momentum from the impactor to the asteroid needs to be characterised. The impact can produce ejecta which would also add momentum to the target. Therefore, there are two components to this momentum gained by target body: (1) direct transfer of momentum from the impactor to the asteroid, (2) momentum caused by ejecta flying in the direction opposite to that of the impactor. The momentum transfer can thus be described by the so-called momentum multiplication factor β, which is the ratio of the momentum gained by the target to the momentum of the impactor at the time of collision. In this work, small-scale impact experiments using polyvinylchloride (PVC) projectiles on regolith-like materials and simulants - sand, glass beads and the Johnson Space Center (JSC)-1A lunar regolith simulant - have been conducted. We used the electro-thermal accelerator of the Technical University of Munich (TUM)/Lehrstuhl für Raumfahrttechnik (LRT) to determine the momentum transfer from the projectiles onto these regolith targets. We derived relationships between the kinetic energy and the crater mass and characterized the ejecta cloud. We confirmed the dependency of β on the impact velocity of the projectile (impactor) - higher momentum transfer occurs at larger impact velocities. We have also seen a trend that β gets larger for a lower cohesion and higher porosity of the target material. Thus, a thorough understanding of the regolith properties of AIDA’s target asteroid will be crucial towards understanding and determining the momentum transfer.

Microstructure and micro-geomechanics evaluation of Pottsville and Marcellus shales

Shales attracted great interest as source rocks for hydrocarbons, and as Caprocks that provide seals for conventional reservoirs. Recently, their integrity has been investigated in the containment of injected CO2 and as unconventional reservoirs for oil and gas. Hydraulic fracturing has been utilized to produce hydrocarbons from shales for more than a decade, but the fundamental mechanism to initiate and propagate these fractures remains unclear. The shales are complex and heterogeneous geological material highly impacted by depositional environment, diagenesis and earth stresses. Macro and Microstructure of shales are dominated by distinct laminated layering, which causes the anisotropy in petrophysical and mechanical properties. Due to this heterogeneity, when subjected to a load, both plastic and elastic deformations manifest simultaneously at different degrees in different directions, leading to a difference in failure/fracture response of the bulk rock. The objective of this paper is to gain a fundamental understanding of how and where the fractures initiate when the rock is under stress, as in the case of pressure buildup or hydraulic fracturing.Indentation tests were conducted on two shale formations, Pottsville, AL, and Marcellus, PA, at both micro and nanometer scales on retrieved drilled rock core samples to get the mechanical properties of the bulk and individual mineralogical phases present. Optical microscopy was used in the selection of indentation maps, surface roughness and large view of observations. High-resolution electron microscopy offered an insight into the internal arrangement of different minerals within the rock matrix. Energy Dispersive X-Ray Spectroscopy (EDS) analysis was combined to provide a spatial link between geochemistry and geomechanics at micro and nano scale.Results from indentation showed Pottsville shale sample had overall better sealing properties than Marcellus shale, because of 1) higher bulk mechanical properties: Pottsville have average Young's Modulus of 9.55 and 11.03 Gpa from different directions, the values for Marcellus was 7.48 and 8.96 Gpa, Pottsville also have average hardness of 0.5 and 0.53 Gpa from different directions, the values for Marcellus was 0.24 and 0.31 Gpa; 2) more uniform grain size: from <1 μm to 50 μm for Pottsville, while <1 μm–200 μm for Marcellus; 3) higher rigid grain content: Pottsville has rigid grain content of 10.37% comparing to 4.01% of Marcellus.; and 4) the potential of fracture/deformation healing and re-sealing within few months: deformation on Pottsville almost recovered to its original state (<6% difference), while on Marcellus, the number is 43% of its maximum deformation after four months. The value of this approach is a reduction of time and cost in geomechanical evaluation for shales, the measured results can be used as input or for validation of predictive rock models to contribute to the decoding the complex mechanical responses of shales.

Monitoring Local Changes in Granite Rock Under Biaxial Test: A Spatiotemporal Imaging Application With Diffuse Waves

AbstractDiffuse acoustic or seismic waves are highly sensitive to detect changes of mechanical properties in heterogeneous geological materials. In particular, thanks to acoustoelasticity, we can quantify stress changes by tracking acoustic or seismic relative velocity changes in the material at test. In this paper, we report on a small‐scale laboratory application of an innovative time‐lapse tomography technique named Locadiff to image spatiotemporal mechanical changes on a granite sample under biaxial loading, using diffuse waves at ultrasonic frequencies (300 kHz to 900 kHz). We demonstrate the ability of the method to image reversible stress evolution and deformation process, together with the development of reversible and irreversible localized microdamage in the specimen at an early stage. Using full‐field infrared thermography, we visualize stress‐induced temperature changes and validate stress images obtained from diffuse ultrasound. We demonstrate that the inversion with a good resolution can be achieved with only a limited number of receivers distributed around a single source, all located at the free surface of the specimen. This small‐scale experiment is a proof of concept for frictional earthquake‐like failure (e.g., stick‐slip) research at laboratory scale as well as large‐scale seismic applications, potentially including active fault monitoring.

A stochastic computational method based on goal-oriented error estimation for heterogeneous geological materials

Computational modeling of geological materials is challenging. Firstly, they are heterogeneous with numerous uncertainties in the input parameters and secondly, the computational cost of modeling geological structures is time consuming due to the large and different length scales involved. In this article, we propose an efficient computational method for heterogeneous geological materials based on goal oriented error estimation and adaptive mesh refinement. Instead of estimating the error in a specific norm, the proposed novel error estimation approach which is called dual-weighted residual error estimation, approximates the error with respect to the quantity of interest. The dual-weighted residual error estimation is a dual-based scheme which requires an adjoint problem. The adjoint or dual problem is described by defining the quantity of interest in a functional form. Then by solving the primal and dual problems, errors in terms of the specified quantities are calculated. In many applications in engineering geology, the material is heterogeneous. In such cases, the material properties can be regarded as random fields. This variety of material properties leads to non-uniform distributions of the solution gradients, e.g. stresses. Therefore, it is vital to apply a reliable error estimation approach to be able to do efficiently the mesh-adaptivity procedure with regard to varying material parameters with pre-defined correlation lengths. Hence, the proposed error estimator is extended by accounting for a random field model to describe the material properties. Local estimated errors are exploited in order to accomplish the mesh adaptivity procedure. The goal-oriented mesh adaptivity controls the local errors in terms of the prescribed quantities. Both refinement and coarsening processes are applied to raise the efficiency. The performance of the proposed computational approach is demonstrated for several examples.

Data-fusion of high resolution X-ray CT, SEM and EDS for 3D and pseudo-3D chemical and structural characterization of sandstone

When dealing with the characterization of the structure and composition of natural stones, problems of representativeness and choice of analysis technique almost always occur. Since feature-sizes are typically spread over the nanometer to centimeter range, there is never one single technique that allows a rapid and complete characterization. Over the last few decades, high resolution X-ray CT (μ-CT) has become an invaluable tool for the 3D characterization of many materials, including natural stones. This technique has many important advantages, but there are also some limitations, including a tradeoff between resolution and sample size and a lack of chemical information. For geologists, this chemical information is of importance for the determination of minerals inside samples. We suggest a workflow for the complete chemical and structural characterization of a representative volume of a heterogeneous geological material. This workflow consists of combining information derived from CT scans at different spatial resolutions with information from scanning electron microscopy and energy-dispersive X-ray spectroscopy.

Numerical Modeling of Pore Pressure Influence on Fracture Evolution in Brittle Heterogeneous Rocks

Rock is a heterogeneous geological material. When rock is subjected to internal hydraulic pressure and external mechanical loading, the fluid flow properties will be altered by closing, opening, or other interaction of pre-existing weaknesses or by induced new fractures. Meanwhile, the pore pressure can influence the fracture behavior on both a local and global scale. A finite element model that can consider the coupled effects of seepage, damage and stress field in heterogeneous rock is described. First, two series of numerical tests in relatively homogeneous and heterogeneous rocks were performed to investigate the influence of pore pressure magnitude and gradient on initiation and propagation of tensile fractures. Second, to examine the initiation of hydraulic fractures and their subsequent propagation, a series of numerical simulations of the behavior of two injection holes inside a saturated rock mass are carried out. The rock is subjected to different initial in situ stress ratios and to an internal injection (pore) pressure at the two injection holes. Numerically, simulated results indicate that tensile fracture is strongly influenced by both pore pressure magnitude and pore pressure gradient. In addition, the heterogeneity of rock, the initial in situ stress ratio (K), the distance between two injection holes, and the difference of the pore pressure in the two injection holes all play important roles in the initiation and propagation of hydraulic fractures. At relatively close spacing and when the two principal stresses are of similar magnitude, the proximity of adjacent injection holes can cause fracturing to occur in a direction perpendicular to the maximum principal stress.

Numerical Simulation of 3D Hydraulic Fracturing Based on an Improved Flow-Stress-Damage Model and a Parallel FEM Technique

The failure mechanism of hydraulic fractures in heterogeneous geological materials is an important topic in mining and petroleum engineering. A three-dimensional (3D) finite element model that considers the coupled effects of seepage, damage, and the stress field is introduced. This model is based on a previously developed two-dimensional (2D) version of the model (RFPA2D-Rock Failure Process Analysis). The RFPA3D-Parallel model is developed using a parallel finite element method with a message-passing interface library. The constitutive law of this model considers strength and stiffness degradation, stress-dependent permeability for the pre-peak stage, and deformation-dependent permeability for the post-peak stage. Using this model, 3D modelling of progressive failure and associated fluid flow in rock are conducted and used to investigate the hydro-mechanical response of rock samples at laboratory scale. The responses investigated are the axial stress–axial strain together with permeability evolution and fracture patterns at various stages of loading. Then, the hydraulic fracturing process inside a rock specimen is numerically simulated. Three coupled processes are considered: (1) mechanical deformation of the solid medium induced by the fluid pressure acting on the fracture surfaces and the rock skeleton, (2) fluid flow within the fracture, and (3) propagation of the fracture. The numerically simulated results show that the fractures from a vertical wellbore propagate in the maximum principal stress direction without branching, turning, and twisting in the case of a large difference in the magnitude of the far-field stresses. Otherwise, the fracture initiates in a non-preferred direction and plane then turns and twists during propagation to become aligned with the preferred direction and plane. This pattern of fracturing is common when the rock formation contains multiple layers with different material properties. In addition, local heterogeneity of the rock matrix and macro-scale stress fluctuations due to the variability of material properties can cause the branching, turning, and twisting of fractures.

Sorption properties of152Eu and241 Am in geological materials: Eu as an analogue for monitoring the Am behaviour in heterogeneous geological environments

In order to confirm the similar behavior of Eu and Am in heterogeneous geological materials, we carried out the batch experiments for determining the sorption property of radionuclides,152Eu and241Am. We used four different types of core rocks including biotite banded gneiss, biotite gneiss, metabasite and andestic tuff, and selected two samples per each lithology, one of which is fracture-bearing and another is fracture-free. Except for metabasites, rock samples of each type are similar in their compositions. We calculated sorption ratios of two radionuclides from the experimental results. Biotite gneiss and tuff had similar sorption trends for152Eu and241Am regardless of the existence of fractures, whereas two metabasite samples showed very different sorption properties. Such difference in the sorption trends revealed a close relationship with chemical compositions of the host rocks. Nevertheless,152Eu and241Am showed similar adsorption trends for all the samples with variable contact times regardless of petrography and pH variations, and particularly, the sorption trends of152Eu and241Am in the metabasites were similar. This observation suggests that Eu and Am have similar sorption properties on geological materials. Therefore, Eu can be used as a useful analogue of Am in all kinds of geological environments regardless of variations in lithology and pH of groundwater. In addition, sorption ratios of152Eu and241 Am are correlated with the contents of P2O5 and TiO2, suggesting that the chemical components such as P2O5 and TiO2 might be important for deciphering the interaction between the radionuclide and groundwater.

Coupled analysis of flow, stress and damage (FSD) in rock failure

Abstract Rock is a heterogeneous geological material that contains natural weakness of various scales. When rock is subjected to mechanical loading, these pre-existing weaknesses can close, open, grow or induce new fractures, which can in turn change the structure of the rock and alter its fluid flow properties. Experimental results provide strong evidence that rock permeability is not a constant, but a function of stresses and stress-induced damage. A flow-stress-damage (FSD) coupling model for heterogeneous rocks that takes into account the growth of existing fractures and the formation of new fractures is proposed herein. Implemented with the Rock Failure Process Analysis code (F-RFPA 2D ), this FSD model is used to investigate the behaviour of fluid flow and damage evolution, and their coupling action, in samples that are subjected to both hydraulic and biaxial compressive loadings. The modeling results suggest that the nature of fluid flow in rocks varies from material to material, and strongly depends upon the heterogeneity of the rocks.