Clay-rich Rocks Research Articles

Empirically informed convolutional neural network model for logging curve calibration

Environmental calibration of logging curves is critical for petrophysical interpretation and sweet spot characterization. Wellbore failure frequently occurs in clay-rich shale rocks during drilling, leading to biased logging interpretation and uncertainty. To reduce the biased correction or erroneous decision making in the interpreter-dominated logging curve calibration process, we develop an empirically informed convolutional neural network (EiCNN) logging curve correction strategy to calibrate the borehole failure-induced logging curve abnormity more accurately. The EiCNN method, together with high-quality logging curves as labeled samples, provides a nonlinear mapping between input logging curves and calibrations for the distorted curves. The EiCNN method completely alleviates biased correction or decision making by the interpreter-dominated method. It has a strong generalization ability, using many empirically interpreted high-quality data as input samples. The field validation wells demonstrate that the EiCNN model can precisely correct the distorted logging curves of mudstone segments with a correlation coefficient of >0.95. Moreover, the validation and test wells illustrate that the EiCNN method is capable of precisely correcting logging curves of interlayer mudstone, implying that the EiCNN method, to a certain degree, can also accurately perform environmental correction of logging curves from thin mudstone layers.

Water Effects on Elastic S-Wave Propagation and Attenuation Across Single Clay-Rich Rock Fractures: Insights from Ultrasonic Measurements

Though previous studies have shown that the presence of water strongly influences wave responses of rock fractures, water effects on wave behaviours across clay-rich rock fractures have been ambiguous until now. In the present study, we conducted considerable ultrasonic measurements on single rock fractures filled with kaolinite-dominant gouges at varying water saturation degrees to investigate the water effects on elastic S-wave propagation and attenuation across clay-rich rock fractures. The experimental results reveal that the S-wave velocity across single clay-rich rock fractures slightly increases and decreases with the progressively increasing water saturation degree. An increase in water saturation leads to a concave trend of the spectral amplitudes, while it moderately affects the central frequency of transmitted S-waves. In addition, the seismic quality factor across single clay-rich rock fractures follows an exponential growth trend with the water saturation, suggesting the exponentially negative relation between S-wave attenuation and the water saturation. We attribute the water saturation-dependent S-wave attributes across single clay-rich rock fractures to the combined effects of the local flow and the degradation of grain contacts. Compared to P-wave, S-wave exhibits less sensitivity to varying water saturation of clay-rich rock fracture. Upon increasing water saturation, S-waves display similar velocity and central frequency trends with P-waves. The tendencies of spectral amplitude and seismic quality factor for S-waves are approximately opposite to those for P-waves as the water saturation degree increases. We interpret these discrepancies by the fact that S-wave attributes across single water-saturated clay-rich rock fractures mainly depend on the properties of the skeletal frame, while the characteristics of the particles, pore fluid, and skeleton frame dominate P-wave behaviours. The outcomes of the current work facilitate our understanding of the fluid effects on the interaction of waves with clay-rich rock discontinuities.

Effect of water saturation on Cerchar abrasivity index (CAI) of clay-rich rocks at different scratch lengths

Predicting the cutter consumption rate of mechanical excavators using the Cerchar abrasivity index (CAI) results is possible. However, changing operational parameters in the field may cause differences between the estimated cutter consumption rate based on CAI test results and the real cutter consumption rate observed in the field. One of the essential parameters in the field that affects the cutter consumption rate is the increase in the water content of the excavated rocks. In theory, an increase in the water content of rocks is expected to reduce their abrasivity. In addition, the prediction of cutter consumption rate is based solely on CAI test results conducted on dry surfaces. However, especially during the excavation of low-strength clay-rich rocks, the excavated rock material can adhere to the cutters and block them, leading to a higher cutter consumption rate than initially predicted. This study aims to analyze the rock-pin interaction in CAI tests conducted on wet surfaces of clay-rich rocks and investigate the effect of water saturation on CAI at different scratch lengths. CAI tests were conducted on dry and wet surfaces of 20 clay-rich rocks with low strength at 10, 30, 50, and 70 mm scratch lengths, respectively. The relationships between the CAI test results obtained on dry and saturated rock surfaces were statistically analyzed for each scratch length. A positive linear correlation was found between CAI values obtained on dry (CAIDry) and saturated surfaces (CAISat) at each scratch length. The percentage variations in CAI values at each scratch length were analyzed in detail. It was found that 90 % of samples have higher CAIDry than CAISat at 10 mm scratch length. However, it was observed that as the scratch length increased, the relationship between the CAI values obtained in dry and saturated conditions changed, and the proportion of samples with CAIDry higher than CAISat decreased to 65 %, 55 %, and 25 % at scratch lengths of 30 mm, 50 mm, and 70 mm, respectively. In CAI tests conducted on saturated surfaces, it was observed that the excavated rock material adhered to the test pins, and as the scratch length increased, the increase in CAISat values was considered to be due to this situation. This study revealed the importance of considering the adhesion potential of rocks to cutters at water-saturated conditions for the accurate prediction of cutter consumption in the excavation of clay-rich rocks.

Diffusion of HTO, 36Cl and 22Na in the Mesozoic rocks of Northern Switzerland: III. Cross-lab comparison of diffusion measurements on argillaceous twin samples

In the past decades, there has been a high interest in evaluations of argillaceous rock formations to host repositories for spent fuel or radioactive waste material. One of the main reasons for this interest is that argillaceous rocks are dominated by clay minerals, resulting in a fine-grained sediment matrix with micropores, low pore connectivity and thus low permeability, and good self-sealing properties. This entails that diffusion is the prime mechanism for transport of gas, fission products and radionuclides in clay-rich rocks. The determination of diffusion parameters is therefore key to evaluate the quality of the rock from long-term safety perspectives. Cross-lab comparison of diffusion data is however often challenging as various methods, concepts and models are utilised in the different laboratories across the globe. Here, a direct cross-lab comparison study of through-diffusion experiments was performed to compare and assess the effect of experimental method (through-diffusion) and modelling uncertainties of the parameters by comparing results obtained by two independent laboratories. The R&D group ‘Disposal’ at SCK CEN (Belgium) and the PSI-LES group (Switzerland) both performed through-diffusion experiments on (nearly) the same sample material using their in-house experimental and modelling methodologies for through-diffusion experiments. Adjacent (twin) samples at five depths between 870 and 940 m in the Trüllikon1-1 borehole (Switzerland) were selected and each lab subjected these five clay rock samples to diffusion in synthetic pore water with three different tracers, HTO, 36Cl− and 22Na+, representative for neutral, anionic and cationic transport behaviour. The two labs used a similar design of diffusion cell and worked with similar experimental conditions, but there were small differences in the experimental set-up/conditions and in the modelling approach. The independently determined diffusion parameters from SCK CEN and PSI-LES for all three tracers confirmed previously observed uncertainties. For all three radionuclides, the variability of the effective diffusion coefficients estimated independently by both institutes was less than a factor 2 and in general much lower (deviations ranging between 0 and 73%). Besides, the parameter estimations of the capacity factor (or accessible porosity in case of HTO and 36Cl−) agreed well. Moreover, the experimental datasets of HTO and 36Cl− were also cross-fitted. The evaluation revealed that the minor deviations can be attributed predominantly to variations in temperature (experimental conditions) and to a lesser extent to minor distinctions in the modelling approach. It is important to acknowledge that local heterogeneity might also contribute to these differences.

Laboratory Testing and Classification of Mudrocks: A Review

Mudrocks are fine-grained clay-rich rocks that comprise different lithotypes forming more than 60% of all sedimentary rocks, and thus, they occur frequently in engineering projects either as natural ground or as made ground. These rocks may display a range of engineering behaviours controlled mostly by their composition and structural features. Due to rapid breakdown and susceptibility to volume changes, they may cause problems both during and after construction. Research into the susceptibility of mudrocks to breakdown aims to predict problematic behaviour and provide guidance for avoiding or mitigating these effects. Low-durability materials that disintegrate during sampling and testing can be especially difficult to assess. The paper reviews laboratory techniques for mudrock characterization as well as describes geological and engineering geological classification schemes generally used to describe and classify these materials. The value of some of the tests and determinations in the evaluation of a series of mudrock data taken from the literature is presented.

Influence of Rock Firing on the Wettability of Clay-Rich Sandstones

Rock firing is a conventional technique of core preparation to prevent clay swelling and fines migration in clay-rich rock; however, the influence of rock firing on the wetting behaviors and alterations of mineralogical compositions of clay-rich sandstones is not yet explicit. Thus, we assessed and compared the impacts of rock burning at two different temperatures (700 and 1100 °C) on the changes in wetting states of Bentheimer (BN) and clay-rich Bandera Gray (BG) sandstones through contact angle measurement, Amott wettability index determination, and thermogravimetric analysis (TGA). Results showed that rock firing makes the rocks more water-wet, but the clay-rich BG sandstone becomes significantly more water-wet than the quartz-dominated BN rock with increasing temperature. There was no change in rock porosity due to firing, but some clay remained, whereas some clays were transformed into other minerals after rock firing. Particularly, there were thermal transformations of clay to quartz and the disappearance of dolomite, chlorite, and clinochlore. Such transformation of these minerals into alkali metal oxide silicates dispersed on the rock matrix makes the surface more polar and enhances the interactions with water molecules, resulting in more water wettability for the BG sandstone. The study suggests that rock firing could change the clay-rich rock-wetting state into a more water-wet condition than the clay-poor or zero-clay sandstone samples; thus, the influence of rock burning on rock-wetting behavior should be considered to ensure an accurate prediction of the rock-wetting state due to rock burning. Moreover, burning at 1100 °C is recommended to achieve effective clay transformation and removal in clay-rich rock to prevent swelling, fines migration, and clay–fluid interactions.

Extensive experimental investigation on the effect of thermal treatment and lateral pressure on the shear behavior of intact mudstone

The coupling environment of temperature (T) and lateral pressure at great depths promotes intact rocks to shear failure, posing a serious threat to underground engineering. Temperature effect on shear behaviour is of particular importance due to the possible mineralogical alterations in mineral composition, especially in clay-rich rocks such as mudstone that has a great affinity for water. Accordingly, the effect of thermal treatment on the shear behaviour of intact mudstone was investigated, in this study, using the Short Core in Compression (SSC) method. Three temperatures of RT, 250 and 500 °C, and four lateral pressures of 0.0, 0.5, 2.0, and 4.0 MPa were adopted. Numerical and experimental observations showed that the resulting fractures in SCC samples are shear and by increasing the lateral pressure, shear failure is promoted. Compared with other rock types such as granite and sandstone, shear properties in mudstone have only one positive trend with temperature increase up to 500 °C, by increasing T from RT to 500 °C, mode II fracture toughness, peak friction angle, and the cohesion increased by about 15 to 47%, 4.9%, and 47.7%, respectively. The bilinear Mohr–Coulomb failure criterion can be used to model the peak shear strength behaviour of intact mudstone before and after thermal treatment.

Effect of CO2-H2O-Smectite Interactions on Permeability of Clay-Rich Rocks Under CO2 Storage Conditions

Effect of CO2-H2O-Smectite Interactions on Permeability of Clay-Rich Rocks Under CO2 Storage Conditions

Adsorption of methane and carbon dioxide by water-saturated clay minerals and clay rocks

Understanding the effects of water on gas adsorption in geological media is of high importance in order to efficiently control numerous subsurface engineering process operating at gas/rock interfaces. Due to preferential interaction with clay surfaces, water fills their porous body, greatly reducing CH4 and CO2 adsorption capacity. In order to quantitatively describe CH4 and CO2 adsorption by hydrated clay minerals, this work proposes to rely on the mechanism of gas uptake by dissolution in pre-adsorbed pore water. This approach was employed to characterise water-saturated porous media of increasing complexity: mesoporous silica SBA-15 and silica gel with different pore sizes and geometry, isolated illite and montmorillonite and natural clay-rich rock (the Callovo-Oxfordian formation – COx, France) in powdered and crushed states. It was found that the solubility in water can reliably explain the CO2 uptakes by hydrated pore systems regardless of their nature as well as the CH4 uptakes, but only for solids with large mesopores and montmorillonite mineral. A so called “adsorption enhanced gas uptake in pore water” for CH4, exceeding its solubility in bulk water by a factor of 5–8, was observed for the systems with narrow pore sizes, highlighting the impact of surface energy on gas uptake and the occurrence of the interaction of weakly-soluble methane with the surface, promoting its uptake in comparison to pure dissolution.

Shallow Composition and Structure of the Upper Part of the Exhumed San Gabriel Fault, California: Implications for Fault Processes

Quantifying shallow fault zone structure and characteristics is critical for accurately modeling the complex mechanical behavior of earthquakes as energy moves within faults from depth. We examine macro- to microstructures, mineralogy, and properties from drill core analyses of fault-related rocks in the steeply plunging ALT-B2 geotechnical borehole (total depth of 493 m) across the San Gabriel Fault zone, California. We use macroscopic drill core and outcrop-sample analyses, core-based damage estimates, optical microscopy, and X-ray diffraction mineralogic analyses to determine the fault zone structure, deformation mechanisms, and alteration patterns of exhumed deformed rocks formed in a section of the fault that slipped 5-12 million years ago, with evidence for some Quaternary slip. The fault consists of two principal slip zones composed of cohesive cataclasite, ultracataclasite, and intact clay-rich, highly foliated gouge within upper and lower damage zones 60 m and 50 m thick. The upper 6.5 m thick principal slip zone separates Mendenhall Gneiss and Josephine Granodiorite, and a lower 11 m thick principal slip is enclosed within the Josephine Granodiorite. Microstructures record overprinted brittle fractures, cohesive cataclasites, veins, sheared clay-rich rocks, and folded foliated and carbonate-rich horizons in the damage zones. Carbonate veins are common in the lower fault zone, and alteration and mineralization assemblages consist of clays, epidote, calcite, zeolites, and chloritic minerals. These data show that shallow portions of the fault experienced fluid-rock interactions that led to alteration, mineralization, and brittle and semi-brittle deformation that led to the formation of damage zones and narrow principal slip zones that are continuous down-dip and along strike.

Comprehensive investigation of rock fracture behaviour in clay-rich rocks under the effect of temperature: Experimental study under three loading modes (I, I/II, II)

Rock fracture behaviour is a crucial aspect in many geological and engineering projects and can be affected by different environmental factors such as temperature, especially in projects where the rocks are exposed to a change in temperature such as geothermal energy, oil and gas exploitation, nuclear waste areas, etc. Therefore, this research aims to investigate the effect of temperature on rock fracture behaviour under four loading modes; pure mode I, pure mode II, and two mixed-mode loadings I/II in SCB specimens of clay-rich rocks (mudstone). Thirty-six SCB specimens were prepared and tested under three temperatures (Ambient temperature (AT), 250 and 500 °C), and some important parameters such as fracture toughness, fracture envelope, crack propagation speed, and crack acceleration were measured and discussed as a function of temperature. The main results concluded that due to the gradual loss of water from the structure of high clay content in the mudstone during thermal treatment, the behavior of mudstone gradually changed from ductile to brittle behaviour, and as a result, fracture toughness and crack propagation speed increased significantly. In addition, our results showed that the area under the fracture envelope which represents fracture resistance to growth under mixed-mode loading increased by 1.86 and 2.68 orders in magnitude after thermal treatment at temperatures of 250 and 500 °C, respectively. Under the four loading mode mixities, fracture grows through three main stages, high acceleration (0–5 mm), high deceleration (5–10 mm), and low deceleration (10–15 mm) stages. Crack propagation speed and crack acceleration depend strongly on the mode mixity ratio (KII/KI) and it increased by changing the loading mode from pure mode I to mixed mode loading I/II and pure mode II.

Influence of salt (NaCl) on hydric and hygric dilatation of clay-rich rocks

• Hydric (liquid water) dilatation decreases with increasing salinity of pore water. • Salt contamination reverses hygric (water vapour) dilatation phases of clay-rich rocks. • Crystallisation of NaCl develops higher strain than hydration of clay interlayers. • Changes in volume occur more frequently once clay-rich rocks are exposed to salts. • Salt weathering of clay-rich rocks produces granular disintegration and scaling. The mechanisms through which crystallisation of salts and hydration of swelling clays cause deterioration of rocks are generally understood. However, how these two decay mechanisms operate synergistically is less clear. In this study, we investigated the impact of a salt (NaCl) on the deterioration of clay-rich basaltic scoria from which the churches forming the UNESCO World Heritage Site of Lalibela in Ethiopia are hewn. The basaltic scoria is rich in smectite (swelling clay) and zeolitic minerals which have micropores and an affinity for efficient water absorption. Hydric and hygric swelling experiments were carried out on 50 mm cube quarry samples and continuous measurements were taken with Linear Variable Displacement Transformers (LVDT). Hydric (liquid water) tests involved partially submerging the samples in deionised and saline solutions. Hygric (water vapour) tests were done in an environmental cabinet where the RH fluctuated between 60% to 90% RH with salt-free and salt-contaminated samples. The results showed that contamination with salts alters both hydric and hygric behaviour. Hydric swelling was reduced in most samples when contaminated with salt, while the expansion and contraction phases in hygric tests were reversed after contamination with NaCl. Continuous measurement of dilatation allowed us to quantify the strain developed during the phase transition of NaCl. Due to the crystallisation pressure, hygric dilatation increased significantly in salt-contaminated samples. Salt-contaminated samples also showed frequent episodic changes in volume that corresponded both with the hydration/dehydration of clay interlayers and crystallisation/dissolution of NaCl. These volume changes correspond with the damage profile on the surface of the samples, i.e., contour scaling and granular disintegration. The results indicate that the mechanism through which salt contamination accelerates the decay of clay-rich rocks is via an increase in the frequency and degree of hygric dilatation.

Implications of slow fault slip for hydraulic-fracturing-induced seismicity

Since the discovery of slow-slip phenomena, scientific understanding of the behavior of active fault systems has been transformed significantly. It is now recognized that tectonic fault systems are characterized by a spectrum of slip behavior, from “regular” (stick-slip) earthquakes that radiate elastic wave energy and occur on a timescale of seconds, to slow-slip events with durations ranging from minutes to years. More recently, slow-slip phenomena have been observed and modeled in association with injection-induced seismicity. This includes evidence for predominantly slow fault slip during injection that triggered dynamic rupture elsewhere on a fault. In the case of hydraulic fracturing, slow-slip behavior is consistent with the frictional characteristics of faults in clay-rich rocks. A change in pore pressure or slip rate can cause a fault to transition from slow to unstable slip. Through real-time monitoring of slip-slip processes and, potentially, the development of operational adjustments to reduce the hazard of damaging ground motions, a better understanding of slow-slip processes could contribute to improved risk mitigation for induced earthquakes. Effective tools for direct observation of slow-slip processes include tiltmeters, strainmeters, global navigation satellite systems, interferometric synthetic aperture radar, and distributed fiber-optic sensing.

Experimental study on the effect of thermal treatment on the mechanical properties of clay-rich rocks (mudstone)

Temperature is one of the most important environmental factors that affect rock engineering properties. The clay-rich rocks such as mudstone are abundant in the earth's crust and have a high affinity to water. So the behavior of these rocks differs from others after thermal treatment, which might force the water out of the clay. Therefore, we investigated the effect of thermal treatment on the mechanical properties of clay-rich rock, mudstone, under three temperatures; ambient temperature, 250, and 500 °C. Our results showed that thermal cracks begin to appear in the mudstone at a temperature between 500 and 600 °C. Several mechanical properties were measured in the lab; uniaxial compressive strength (UCS), elastic modulus (E), Poisson's ratio (ν), Brazilian tensile strength (BTS), fracture toughness of tensile (KIC), and shear (KIIC) cracks. Due to the drying of clay minerals through the dehydration and dihydroxylation processes, the following mechanical properties of mudstone significantly increased after treating the rock with a temperature of 500 °C; UCS, E, BTS, KIC, and KIIC have improved by 91%, 39%, 16%, 41%, and 94%, respectively, while ν decreased by 80%. This reduction of ν, due to increasing the thermal treatment temperature, caused the behavior of mudstone to transform gradually from ductile to brittle material which means that the failure of such rocks, at this temperature, might be a sudden one. This research presents new insights into the effect of temperatures below 500 °C on clay-rich rocks for use in different engineering fields.

Hydro-mechanical response of Opalinus Clay in the CO2 long-term periodic injection experiment (CO2LPIE) at the Mont Terri rock laboratory

Guaranteeing the sealing capacity of caprocks becomes paramount as CO2 storage scales up to the gigaton scale. A significant number of laboratory experiments have been performed with samples of intact rock, showing that low-permeability and high-entry pressure caprocks have excellent sealing capacities to contain CO2 deep underground. However, discontinuities, such as bedding planes, fractures and faults, affect the rock properties at the field scale, being at the same time challenging to monitor in industrial-scale applications. To bridge these two spatial scales, Underground Research Laboratories (URLs) provide a perfect setting to investigate the field-scale sealing capacity of caprocks under a well-monitored environment. In particular, the CO2 Long-term Periodic Injection Experiment (CO2LPIE) at the Mont Terri rock laboratory, Switzerland, aims at quantifying the advance of CO2 in Opalinus Clay, an anisotropic clay-rich rock with bedding planes dipping 45° at the experiment location. To assist in the design of CO2LPIE and have an initial estimate of the system response, we perform plane-strain coupled Hydro-Mechanical simulations using a linear transversely isotropic poroelastic model of periodic CO2 injection for 20 years. Simulation results show that pore pressure changes and the resulting stress variations are controlled by the anisotropic behavior of the material, producing a preferential advance along the bedding planes. CO2 cannot penetrate into Opalinus Clay due to the strong capillary effects in the nanoscale pores, but advances dissolved into the resident brine. We find that the pore pressure oscillations imposed at the injection well are attenuated within tens of cm, requiring a close location of the monitoring boreholes with respect to the injection interval to observe the periodic signal.Article highlightsUnderground rock laboratory experiments permit examining the caprock sealing capacity at a representative scale for CO2 storage;We perform coupled transverse isotropic hydro-mechanical simulations to gain insight on the response of shaly rock to CO2 periodic injection;Simulation results assist in the design of the injection amplitude and period and monitoring of the long-term periodic CO2 injection experiment.

TOC estimation of shale oil reservoir by combining nuclear magnetic resonance logging and nuclear physics logging

Abstract The evaluation of source rock properties has become a vital step in logging interpretation. Total organic carbon (TOC) content is the key to estimating the quality and hydrocarbon generation potential of source rocks. In the shale oilfield of the Junggar Basin, the conventional method of calculating the TOC of hydrocarbon source rocks cannot satisfy logging evaluation requirements. This paper predominantly deals with a method for the quantitative estimation of TOC in source rocks via nuclear physics and nuclear magnetic resonance (NMR) logs. According to this method, the total hydrogen index of the source rock is the sum of the response of kerogen, clay minerals and fluid, expressed by corrected neutron porosity. The hydrogen index of fluid and clay minerals is indicated by the effective porosity of NMR and the estimated clay content, respectively. To eliminate the hydrogen index of fluid, the effective NMR porosity is subtracted from the corrected neutron porosity. On this basis, a new and overlapping method suitable for clay-rich rocks and oil reservoirs is proposed. This method was developed by overlaying the scaled clay content curve on the hydrogen index curve. In non-source rocks, the two curves regularly overlap. However, in organic-rich rocks the two curves will separate. The separation distance between the two curves was used to estimate TOC continuously. Possessing sound application and benefiting from the measured results of sweet spots, this method provides new insights for TOC quantitative prediction in shale oil reservoirs.

Experimental observations on the effect of thermal treatment on the crack speed and mode I and II fracture toughness in brittle and ductile rocks

The mechanical properties of rocks are greatly affected by thermal treatment, as this treatment can either improve or worsen these properties depending on the mineralogical composition. Therefore, the effect of thermal treatment on the (1) crack speed properties such as crack propagation speed, crack acceleration, and deceleration, and (2) fracture resistance to grow under two loading modes (pure mode I and pure mode II) was investigated for both brittle and ductile rocks. The ductile type was mudstone, which is a clay-rich rock with a high affinity for water. While the brittle type was granite, which does not contain water-active minerals such as clay or lime. Eighteen semi-circular specimens of mudstone and granite were prepared and tested at room temperature (RT), 250 and 500 °C. The results showed one behavioral trend for the effect of temperature on the mudstone. Due to the drying of clay minerals by increasing the temperature, the crack speed and fracture toughness improved significantly under pure mode I and II after thermal treatment. On the other hand, the thermal treatment of granite caused two behavioral trends. The first trend shows that crack speed and fracture toughness increased under pure mode I and II by increasing the temperature from RT to 250 °C due to crack arrest by randomly distributed thermal microcracks. In the second trend, they decreased below their normal levels under treatment from room temperature to T = 500 °C due to the extension and coalescence of thermal microcracks. The granite behavior changes depending on the density and extension of thermal microcracks. Under the three temperatures of RT, 250, and 500 °C, and for the two rock types, crack propagation speed, crack acceleration, and deceleration under pure mode II were greater than that under pure mode I. Where the fracture goes through three main stages growth, high acceleration, high deceleration, and low deceleration stages. In addition, the effect of temperature on the fracture toughness ratio (KIIC/KIC) was evaluated and the results showed that increasing the temperature increased the fracture toughness ratio. This means that after thermal treatment, the improvement rate of fracture toughness under pure mode II is better than that under pure mode I, and the crack tends to grow under pure mode I rather than mode II.

Characterization of Pennsylvanian Paleosols in Indiana with a Special Reference to Rare Earths (REE) and Lithium

This study characterizes Pennsylvanian paleosols from coal-bearing strata in Indiana, with the main emphasis on the abundance and distribution on rare earths (REE) and lithium (Li). Eleven locations in southwest Indiana were selected for this study, targeting paleosols of both the Carbondale Group and the Raccoon Creek Group. Most paleosols were directly underlying coal beds. Lithologically, paleosols range from grey mudstones to greenish-gray claystones and occasional siltstones. In the Carbondale Group, the thickest paleosol horizons occur under the Danville and the Houchin Creek Coal Members, with their thickness reaching close to 10 feet. In the Raccoon Creek Group, the paleosol under the Wise Ridge Coal appears to be thickest (4.6 to 11.4 feet) and best developed. Total REE content in paleosol samples ranges from 138.8 ppm to 728.55 ppm, with an average range of 208.3 ppm (Colchester paleosol) to 342.9 ppm in the paleosol under the Upper Block Coal. Light REE dominate over heavy REE, and the variation in the light REE is responsible for the largest variations of the total REE. The largest amount of the heavy REE (61 ppm) occurs in the paleosol under the Lower Block Coal, whereas the paleosol under the Upper Block Coal has the largest variation in the heavy REE content (49 to 148 ppm). REE patterns for paleosol samples show very weak to no fractionation among light, medium, and heavy REE, with no major anomalies present. The majority of paleosols have REE values that are roughly representative of the upper continental crust. Only some paleosols are slightly enriched and these are paleosol samples under the Danville Coal and the Upper Block Coal. Lithium content in the paleosol samples shows a variation from 21 ppm to 530 ppm. On average, the Wise Ridge Coal is richest in Li (249.3 ppm), followed by Viking B Coal (190.8 ppm). The coals of the Brazil and Staunton Formations have higher Li content than the younger coals. Although this study has not uncovered large concentrations of REE or lithium, the wide range of concentrations and potential ease of their extraction from these clay-rich rocks imply that the paleosols deserve further examination, and the current study can be used as a screening guide for more focused investigation.

A constitutive model for anisotropic clay-rich rocks considering micro-structural composition

A new constitutive model is developed in this paper for clay-rich rocks. This model describes elastic, instantaneous and time-dependent plastic deformations , and induced damage. The dependency of mechanical behavior of clayey rocks on water saturation degree is taken into account. Further, the structural anisotropy commonly exhibited by sedimentary rocks is also incorporated in the proposed model. Moreover, differently with most full phenomenological models , the impact of micro-structural composition on the macroscopic mechanical behavior is explicitly described by the presented model. More precisely, the macroscopic elastic properties and plastic criterion are determined by using analytical homogenization techniques. These properties explicitly depend on mineralogical compositions and porosity. The damage process is physically related to the interface debonding between clay matrix and mineral particles. The proposed model is applied to the Callovo-Oxfordian claystone investigated in the context of geological disposal of radioactive waste in France. Its efficiency is assessed through comparisons between numerical predictions and experimental data from laboratory tests under different loading paths and water saturation degrees.

Physical model simulation of rock-support interaction for the tunnel in squeezing ground

The existence of squeezing ground conditions can lead to significant challenges in designing an adequate support system for tunnels. Numerous empirical, observational and analytical methods have been suggested over the years to design support systems in squeezing ground conditions, but all of them have some limitations. In this study, a novel experimental setup having physical model for simulating the tunnel boring machine (TBM) excavation and support installation process in squeezing clay-rich rocks is developed. The observations are made to understand better the interaction between the support and the squeezing ground. The physical model included a large true-triaxial cell, a miniature TBM, laboratory-prepared synthetic test specimen with properties similar to natural mudstone, and an instrumented cylindrical aluminum support system. Experiments were conducted at realistic in situ stress levels to study the time-dependent three-dimensional tunnel support convergence. The tunnel was excavated using the miniature TBM in the cubical rock specimen loaded in the true-triaxial cell, after which the support was installed. The confining stress was then increased in stages to values greater than the rock's unconfined compressive strength. A model for the time-dependent longitudinal displacement profile (LDP) for the supported tunnel was proposed using the tunnel convergence measurements at different times and stress levels. The LDP formulation was then compared with the unsupported model to calculate the squeezing amount carried by the support. The increase in thrust in the support was back-calculated from an analytical solution with the assumption of linear elastic support. Based on the test results and case studies, a recommendation to optimize the support requirement for tunnels in squeezing ground is proposed.