Surrounding Rock Formations Research Articles

The expansion mechanism of expansive surrounding rock in Guiyang City, China

Expansive surrounding rock, such as mudstone, poses significant risks to supporting structures like tunnel linings due to its tendency to expand. To explore the expansion mechanism of expansive surrounding rock and address underground engineering challenges associated with it, this study, based on the first phase of Guiyang Metro Line S1, samples expansive surrounding rock from the section and conducts basic physical tests, X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses on the samples. The influence of water content (0%, 2.93%, 6.84%, and 8.20%) on the mechanical properties of mudstone, as well as its expansion characteristics under natural conditions, were examined. Results indicate that the microscopic surface of mudstone is rough, with cracks distributed around pores. These elements interconnect to form natural water channels, with clay minerals and non-clay minerals comprising 47.2% and 52.8% of the sample, respectively. Mudstone contains multiple pores that gradually develop as water content increases. Mudstone expansion significantly reduces its compressive strength. When lateral constraints are applied, mudstone exhibits significantly greater expansion compared to axial and radial changes. When mudstone encounters water, the expansion pressure fluctuates significantly. Upon water absorption following dehydration, cracks in the rock sample develop rapidly, accelerating collapse. These findings offer valuable insights for the construction and design of tunnels in expansive surrounding rock formations within karst areas.

Research on an Equivalent Algorithm for Predicting Gas Content in Deep Coal Seams

This document introduces a novel equivalent algorithm for forecasting gas content within deep coal seams, which is subject to constraints stemming from the advancements and precision achieved in well and roadway engineering endeavors. This algorithm meticulously acknowledges that coal seam gas content comprises three fundamental components: the inherent gas emission rate of the equivalent stratum, the residual gas content retained within the coal seam itself, and the influence imparted by the gas content within the coal seam. Furthermore, the approach thoroughly considers variations in the level of porosity development within the coal seam and its surrounding rock formations, as well as the occurrence of gas within these structures. The equivalent layer is classified into two distinct groups: the sandstone zone and the clay zone. The sandstone zone utilizes pertinent parameters pertaining to fine sandstone, whereas the clay zone distinguishes between clay rock and thick mudstone. The influencing factor considerations solely encompass natural elements, such as the coal seam’s occurrence and geological structure. The residual gas content employs either existing measured parameters or acknowledged experimental parameters specific to the coal seam. Based on this predictive approach, an intelligent auxiliary software (V1.0) for mine gas forecasting was devised. The software calculates the gas content of deep coal seams within the mine at intervals of 100 m × 100 m, subsequently fitting the contour lines of gas content across the entire area. The gas content predictions derived from this equivalent algorithm demonstrate robust adaptability to variations in gas content caused by construction activities, and the prediction results exhibit an acceptable level of error on-site. Notably, the prediction process is not constrained by the progress of tunnel engineering, ensuring that the prediction outcomes can accurately represent the distribution characteristics of deep coal seam gas content. After a year of application, the prediction results have consistently met on-site requirements, providing a scientific foundation for the implementation of effective gas prevention and control measures in the mining area. Furthermore, this approach can effectively guide the formulation of medium- and long-term gas prevention and control plans for mines.

On the Significance of Hydrate Formation/Dissociation during CO2 Injection in Depleted Gas Reservoirs

Summary The study aims to quantitatively assess the risk of hydrate formation within the porous formation and its consequences on injectivity during storage of CO2 in depleted gas reservoirs considering low temperatures caused by the Joule-Thomson (JT) effect and hydrate kinetics. Hydrates formed during CO2 storage operation can occupy porous spaces in the reservoir rock, reducing the rock’s permeability and thus becoming a hindrance to the storage project. The aim was to understand which mechanisms can mitigate or prevent the formation of hydrates. The key mechanisms we studied included water dry-out, heat exchange with surrounding rock formation, and capillary pressure. A semicompositional thermal reservoir simulator is used to model the fluid and heat flow of CO2 through a reservoir initially composed of brine and methane. The simulator can model the formation and dissociation of both methane and CO2 hydrates using kinetic reactions. This approach has the advantage of computing the amount of hydrate deposited and estimating its effects on the porosity and permeability alteration. Sensitivity analyses are also carried out to investigate the impact of different parameters and mechanisms on the deposition of hydrates and the injectivity of CO2. Simulation results for a simplified model were verified with results from the literature. The key results of this work are as follows: (1) The JT effect strongly depends on the reservoir permeability and initial pressure and could lead to the formation of hydrates within the porous media even when the injected CO2 temperature was higher than the hydrate equilibrium temperature; (2) the heat gain from underburden and overburden rock formations could prevent hydrates formed at late time; (3) permeability reduction increased the formation of hydrates due to an increased JT cooling; and (4) water dry-out near the wellbore did not prevent hydrate formation. Finally, the role of capillary pressure was quite complex, as it reduced the formation of hydrates in certain cases and increased in other cases. Simulating this process with heat flow and hydrate reactions was also shown to present severe numerical issues. It was critical to select convergence criteria and linear system tolerances to avoid large material balance and numerical errors.

Mechanisms of Crustal-Mantle Material and Energy Exchanges: Impacts on Hydrocarbon and Geothermal Resources.

The exchange of matter and energy between crust and mantle significantly influences the formation and development of oil, gas, and geothermal resources. Understanding how these exchanges impact these resources is crucial in geological science. In many oil-rich basins in China, significant accumulations of H2, CO2, geothermal energy, and other associated resources linked to deep mantle materials or geological processes have been discovered. Therefore, investigating the effects of crust-mantle material and energy exchanges on these resources is of utmost importance. This paper aims to systematically analyze the effects of mantle materials (e.g., H2, CO2, catalytic elements) and energy upwelling into basins. It synthesizes the impacts of various organic-inorganic interactions on hydrocarbon generation, evolution of organic source rocks, reservoir development, and the formation and accumulation of oil and gas. Finally, it proposes a mechanism detailing how interactions between mantle matter and energy influence specific resources. Simultaneously, this study employs numerical simulations to uncover the specific impact of magma intrusions on the geothermal field of surrounding rock formations. It demonstrates that magma chambers, continuously supplied with mantle energy, create regional thermal anomalies and facilitate the development of high-temperature geothermal resources. This underscores that mantle materials and energy not only significantly influence the generation of oil and gas but also govern the formation of geothermal resources and the evolution of thermal reservoirs. This research provides a theoretical foundation for understanding the subsequent formation of oil and gas resources under the influence of deep geological processes. Moreover, it furnishes a scientific basis for evaluating and exploring the symbiotic relationship between oil and gas resources and geothermal reservoirs.

Integrated approach of predicting rock stability in high mountain valley underground caverns

High mountain valleys are characterized by the development of intricate ground stress fields due to geological processes such as tectonic stress, river erosion, and rock weathering. These processes introduce considerable stability concerns in the surrounding rock formations for underground engineering projects in these regions, highlighting the imperative need for rigorous stability assessments during the design phase to ensure construction safety. This paper introduces an innovative approach for the pre-evaluation of the stability of surrounding rocks in underground caverns situated within high mountain valleys. The methodology comprises several pivotal steps. Initially, we conduct inverse calculations of the ground stress field in complex geological terrains, combining field monitoring and numerical simulations. Subsequently, we ascertain stress-strength ratios of the surrounding rocks using various rock strength criteria. To assess the stability characteristics of the surrounding rocks in the 1# spillway cave within our project area, we employ numerical simulations to compute stress-strength ratios based on different rock strength criteria. Furthermore, we undertake a comparative analysis, utilizing data from the 5# Underground Laboratory (Lab 5) of Jinping II Hydropower Station, aligning the chosen rock strength criterion with the damage characteristics of Lab 5′s surrounding rocks. This analysis serves as the cornerstone for evaluating other mechanical responses of the surrounding rocks, thereby validating the pre-evaluation methodology. Our pre-evaluation method takes into account the intricate geological evolution processes specific to high mountain valleys. It also considers the influence of the initial geostress field within the geological range of underground caverns. This comprehensive approach provides a robust foundation for the analysis and assessment of the stability of surrounding rocks, especially in high mountain valley areas, during the design phase of underground engineering projects. The insights derived from this analysis hold substantial practical significance for the effective guidance of such projects.

Comparative analysis of polyacrylate latex modified cement grout for water blocking and rock reinforcement with six alternative materials

Grouting plays a crucial role in mitigating water inrush disasters and reinforcing weak surrounding rock formations, and the effectiveness of grouting operations is heavily dependent on the choice of grouting materials. This study undertakes an assessment of the feasibility of deploying polyacrylate latex modified cement grouting material (PLMC) for the purposes of water obstruction and surrounding rock reinforcement, achieved through a comparative analysis against six alternative grouting materials, each of which has been applied in practical engineering. The experimental results demonstrate that the initial setting time and fluidity of PLMC fall within the range of other grouting materials, meeting the project requirements. PLMC achieves the highest filling rate at 77.42%, while the filling rates of other grouting materials range from 39.69% to 74.13%. PLMC exhibits good performance in bleeding ratio (2.77%) and resilience against dynamic scouring (retention rate is 72.95%), ranking as the third highest among the seven compared materials for both indexes. Additionally, the uniaxial compressive strengths of PLMC grout stone body and grouting-reinforced body reached 11.94 MPa and 22.74 MPa, respectively, representing the best performances. In comparison, the highest levels of these two indexes for other grouting materials are 8.78 MPa and 21.46 MPa, respectively. Compared with other grouting materials, the porosity of PLMC grout stone body is the lowest, and the interface between rock and grout in PLMC-reinforced body exhibits no significant gaps. The findings suggest that polyacrylate latex modified cement grouting material exhibits exceptional workability characteristics, positioning it as a promising candidate for practical applications.

A new method of mining pressure control for roof cutting in advance working face

After the extraction of deep coal resources, the original stress state is disrupted, leading to increased abutment stress ahead of the working face. This heightened pressure causes significant deformation in the roadway preceding the working face. To tackle this issue, a pioneering method called Roof Cutting in Advance of the Working Face(RCAWC) has been introduced to mitigate deformation by proactively alleviating pressure. This method leverages the principle of conservation of energy within the rock mass system, augmenting the fragmentation energy of the roof rock mass through controlled blasting. This process aims to release the accumulated energy within the rock mass, thereby reducing pressure. This research not only elucidates the mechanism employed by RCAWC to counteract mining pressure but also delineates the evolving pattern of bearing pressure at various RCAWC heights. The findings reveal a shift in the abutment stress curve from a single-peaked to a double-peaked pattern after roof cutting. Notably, there is a significant reduction in abutment stress as the RCAWC height increases. However, as the thick roof is cut off, the negative correlation effect diminishes. Subsequently, an engineering implementation was carried out. Following the adoption of RCAWC technology, roadway deformation decreased by 63.6 % to 75 %, and the size of the protective coal pillar reduced by approximately one-third. These findings underscore the remarkable effectiveness of RCAWC technology in impeding the propagation of mining pressure towards roadways, reducing the extent of abutment stress, and ultimately alleviating stress and deformation within the surrounding rock formation.

Load Calculation Method for Deep-Buried Layered Soft Rock Tunnel Based on Back-Analysis of Structural Deformation

After the excavation and unloading of deep-buried soft rock tunnels, support structures often experience deformation-related disasters such as concrete cracking, steel frame bending and twisting, and primary support instability under different forms of load. Accurately calculating the load borne by the primary support structure is the key to ensuring design rationality and construction safety. Especially in layered soft surrounding rock formations, the magnitude and distribution of the loads are different from those of conventional rock and soil masses, resulting in limited applicability of existing load calculation methods to similar formations. Therefore, based on the measured deformation of the tunnel structure, while considering the different geometric forms of the primary support structure during partial excavation, this paper proposes a deformation-structure (D-S) load calculation method. By comparing the calculation results of this method and a large number of sample data for typical deep-buried layered soft rock tunnels, the reliability of the D-S load calculation method is verified. In addition, the variation law of the loads during the tunnel construction period is enunciated, and the magnitude and distribution of the loads acting on the primary support are clarified. The D-S load calculation method provides a theoretical basis for load calculation in deep-buried layered soft rock tunnels.

Disasters of gas-coal spontaneous combustion in goaf of steeply inclined extra-thick coal seams

In light of the escalating global energy imperatives, mining of challenging-to-access resources, such as steeply inclined extra-thick coal seams (SIEC), has emerged as one of the future trends within the domain of energy advancement. However, there is a risk of gas and coal spontaneous combustion coupling disasters (GCC) within the goaf of SIEC due to the complex goaf structure engendered by the unique mining methodologies of SIEC. To ensure that SIEC is mined safely and efficiently, this study conducts research on the GCC within the goaf of SIEC using field observation, theoretical analysis, and numerical modeling. The results demonstrate that the dip angle, the structural dimensions in terms of width-to-length ratio, and compressive strength of the overlying rock are the key factors contributing to the goaf instability of SIEC. The gangue was asymmetrically filled, primarily accumulating within the central and lower portions of the goaf, and the filling height increased proportionally with the advancing caving height, the expansion coefficient, and the thickness of the surrounding rock formation. The GCC occurs in the goaf of SIEC, with an air-return side range of 41 m and an air-intake side range of 14 m, at the intersection area of the “<”-shaped oxygen concentration distribution (coal spontaneous combustion) and the “>”-shaped gas concentration distribution (gas explosion). The optimal nitrogen flow rate is 1000 m3/h with an injection port situated 25 m away from the working face for the highest nitrogen diffusion efficacy and lowest risk of gas explosion, coal spontaneous combustion, and GCC. It has significant engineering applications for ensuring the safe mining of SIEC threatened by the GCC.

Investigation on basic properties and microscopic mechanisms of polyacrylate latex modified cement grouting material for water blocking and reinforcement

Grouting is a highly effective engineering intervention utilized to address water inrush disasters and reinforce weak surrounding rock formations, with the grouting material playing a crucial role in determining the outcomes of grouting operations. To simultaneously meet the requirements of water plugging and reinforcement, a new polyacrylate latex modified cement grout was prepared, consisting of superfine cement, polyacrylate latex, a defoaming agent, and a coagulation accelerator. This study reports on a series of tests conducted to examine the workability, mechanical properties, and microstructures of polyacrylate latex modified cement grout with varying ratios. The experimental results indicate that the addition of polyacrylate latex significantly reduces the bleeding ratio of the grout, as well as improves its groutability and enhances its resistance to dynamic water scouring, particularly under high-flow conditions. The viscosity of the composite grout increases by adding polyacrylate, leading to a decrease in its fluidity. As the proportion of polyacrylate latex increases, the early and final strength of the grout stone body increases while the porosity decreases. The hydration degree of the grout shows a growing trend with the addition of polyacrylate latex based on the observations by low-field nuclear magnetic resonance tests. The modified grout stone body is dense, primarily due to the chemical reaction between calcium ions produced by cement hydration and carboxyl groups produced by the hydrolysis of ester groups on the polyacrylate latex, leading to cross-linking between the inorganic and organic phases and the formation of a complex network structure. Excellent working performance particularly in terms of anti-scour and load-bearing capacity makes polyacrylate modified cement grouting materials highly suitable for use in the simultaneous treatment of water inrush and reinforcement.

A hardening load transfer function for rock bolts and its calibration using distributed fiber optic sensing

Confinement of rock bolts by the surrounding rock formation has long been recognized as a positive contributor to the pull-out behavior, yet only a few experimental works and analytical models have been reported, most of which are based on the global rock bolt response evaluated in pull-out tests. This paper presents a laboratory experimental setup aiming to capture the rock formation effect, while using distributed fiber optic sensing to quantify the effect of the confinement and the reinforcement pull-out behavior on a more local level. It is shown that the behavior along the sample itself varies, with certain points exhibiting stress drops with crack formation. Some edge effects related to the kinematic freedom of the grout to dilate are also observed. Regardless, it was found that the mid-level response is quite similar to the average response along the sample. The ability to characterize the variation of the response along the sample is one of the many advantages high-resolution fiber optic sensing allows in such investigations. The paper also offers a plasticity-based hardening load transfer function, representing a "slice" of the anchor. The paper describes in detail the development of the model and the calibration/determination of its parameters. The suggested model captures well the coupled behavior in which the pull-out process leads to an increase in the confining stress due to dilative behavior.

Variability of Radionuclide Sorption Efficiency on Muscovite Cleavage Planes

AbstractIn deep geological repositories for nuclear waste, the surrounding rock formation serves as an important barrier against radionuclide migration. Multiple potential host rocks contain phyllosilicates, which have shown high efficiency in radionuclide sorption. Recent experimental studies report a heterogeneous distribution of adsorbed radionuclides on nanotopographic mineral surfaces. In this study, the energetic differences of surface sorption sites available at nanotopographic structures such as steps, pits, and terraces are investigated. Eleven important surface sites are selected and the energies of ad‐ and desorption reactions are obtained from density functional theory calculations. The adsorption energies are then used for the parameterization of a kinetic Monte Carlo model simulating the distribution of adsorbed europium on a typical nanotopographic muscovite surface. On muscovite, silicon step sites are favorable for europium sorption and lead to an increased adsorption in regions with high step concentrations. Under identical chemical conditions, sorption on typical nanotopographic surfaces is increased by a factor of three compared to atomically flat surfaces. Desorption occurs preferentially at terrace sites, leading to an overall 2.5 times increased retention at nanotopographic structures. This study provides a mechanistic explanation for heterogeneous sorption on nanotopographic mineral surfaces due to the availability of energetically favorable sorption sites.

Geographic and transport controls of temperature response in karst springs

Thermal responses of groundwater are useful signatures to reveal the recharge-discharge process and characterize aquifer structure in karst groundwater. Four karst springs with varying circulation depths ranging from ∼ 60 to 820 m in South China, were used to decipher the mechanism of heat exchange between recharge water and surrounding rock formations. An analytical solution of heat transfer in karst conduit was proposed to simulate the peak or trough temperatures of pulse flow. Hydraulic diameter, flow velocity, input temperature and circulation depth in karst aquifer, mainly depending on the recharge water volume after rainfall events, are important transport factors to control the thermal equilibrium depth and the efficiency of heat exchange that directly controls the behavior of warm peaks or cold troughs in groundwater temperature. In shallow karst aquifers wherein the circulation depth is less than its theoretical thermal equilibrium depth, the recharge water does not sufficiently exchange heat with the surrounding rocks, resulting in clear event-scale warm peaks in summer and cold troughs in winter. Otherwise, the recharge water will firstly reach a thermal equilibrium state and then mix with the warmer base flow at deeper location, which is verified in a karst spring with anomalous event-scale temperature drops after rainfall events all year around. The thermal response simulations obtained from this work provide new ways and insights to characterize geographic structure of karst aquifer and elucidate heat transfer mechanism in karst aquifer systems.

Hydraulic conductivity, microstructure and texture of compacted claystone/ bentonite mixtures saturated with different solutions

The French reference concept for the disposal of intermediate- and high-level nuclear waste in the Callovo-Oxfordian sedimentary rock formation (COX-claystone) considers the employment of crushed COX-claystone and its mixture with MX80-bentonite as potential backfill materials installed in drifts and shafts upon termination of the operational phase of the future repository. The fraction of MX80-bentonite in the mixture is limited to 30% in wet weight. Over time, the backfill will be saturated with solutions that originate in the surrounding rock formation and percolate through the concrete lining left in place. The main characteristic of the leachate will be its high pH-value. In addition to the swelling pressure, the sealing properties of the backfill are determined by the hydraulic conductivity. This laboratory experimental study aimed to understand the hydraulic conductivity evolution of potential backfill materials under realistic conditions in terms of compaction conditions, solution chemistry, temperature and hydraulic gradient, and to relate results determined at the macroscale to results of microstructural and textural analysis. Also, the impact of the solution chemistry was evaluated by these means. In the case of mixture-samples, no stabilization of hydraulic conductivity was detected, despite the duration of the experiments being about one year. There was no detectable impact of the solution chemistry in the course of experiments attributable to low reaction kinetics under imposed conditions. Subsequent microstructural analysis indicated material swelling triggering rearrangements in the pore space and lowering the fraction of hydraulic conductive voids. Interestingly, neither the saturation nor the solution chemistry had an impact on the texture of materials.

Thermomechanical behaviour of well cement in different geological formations under the coupled effects of temperature and pressure

It is crucial to ensure that the cement sheaths remain intact in all types of deep well applications subject to high temperatures and high pressures underground. It should be noted, however, that the behaviour of annulus cement sheath is not self-controlled and is governed by the response of steel casing and the surrounding rock formation in such extreme underground conditions. In this study, we investigate the thermomechanical response of wellbore materials in a typical oil well consisting of steel casing, annulus cement sheaths and surrounding rock formations subjected to continuous steam injection. A section of an oil well was simulated by using the COMSOL Multiphysics, a finite element method, using Class G Portland cement as the annulus cement. In two formation scenarios, stiff (carbonate) and compliant (sandstone), temperature and pressure were coupled to evaluate wellbore material behaviour. It was found that wellbore materials in carbonate were more susceptible to stress than those in sandstone. Despite the fact that all materials reached their highest temperatures in both formations, the retention time of the maximum temperature in the cement sheath in sandstone was shorter than in carbonate. Regardless of the thickness of the cement sheath, the highest strains were located at the casing-cement interface in carbonates. As a result of the temperature and pressure ramp, the cement sheath in sandstone failed by generating tensile radial cracks along the lowest layer thickness of the cement sheath. In compliant formations such as sandstone, the cement sheath would likely fail due to tensile cracking along its lowest thickness.

Magnetic modelling and physical properties of magnesite deposits and regional sequences of Serra das Éguas, Brumado, Brazil

AbstractThe Serra das Éguas Complex, in Brumado, state of Bahia, located in the northeastern region of Brazil, is a geological structure comprising volcanochemical, siliciclastic and carbonate units that contain layers and lenses of magnesite, important as a refractory in steel, cement, glass and copper and other applications. Considering the importance of such sort of deposit, this work aims to (i) study the physical properties of magnesite deposits via direct measurements of radiometric spectrometry and magnetic susceptibility associated with chemical analysis of the available borehole core samples; (ii) analyse the magnetic and radiometric data acquired by the combined airborne survey over the area with known magnesite deposits; (iii) estimate the magnetic response of the magnesite deposits and surrounding rock formations via 2D forward and 3D inverse magnetic modelling; (iv) evaluate and, if necessary, reconsider the stratigraphic sequences of the identified geological units of the Complex using the results of the 2D and 3D magnetic modelling. Through forward/inverse models, we noted an expressive magnetic signature likely related to the iron formations and metamafic and meta‐ultramafic rocks. The radiometric data analysis on the drill cores showed that the reddish magnesite has a well‐defined U/Th signature. Furthermore, the results show that the carbonate unit, which contains the magnesite deposits, has the lowest amplitude of magnetization values, and both signatures might be helpful for their identification. Additionally, the magnetic models show that the carbonate unit lies in the upper part of the stratigraphic sequence of the Serra das Éguas Complex. This conclusion drives us to propose a new stratigraphy sequence, where the volcanochemical unit that includes the iron formations lies at the bottom of this stratigraphic sequence.

Wellbore Integrity After a Blowout: Stress Evolution Within the Casing-Cement Sheath-Rock Formation System

A robust evaluation of the casing-cement sheath-rock formation system is foundational in ensuring wellbore integrity throughout a well's lifecycle. Analytical theory for a multi-layer, thick-walled cylinder is used to evaluate the aggregate stress distributions within the casing-cement sheath-rock formation system, honoring boundary conditions of radial stress and displacement continuity. These stress distributions are adjusted for scenarios that a well may experience in its lifetime. Each layer of the casing-cement sheath-rock formation system is evaluated separately against various possible mechanical failure mechanisms, all of which can compromise wellbore integrity.A blowout scenario after a mismanaged loss-of-well-control situation induces high stress loads in the casing-cement sheath-rock formation system, with the wellbore pressure rapidly decreasing during post-blowout discharge, followed by a rapid increase following successful well capping. Casing and cement sheath failures can expose the surrounding rock formation to the pressurized fluid inside the wellbore, risking hydrocarbons broaching the surface, or the seafloor. The aggregate stress distribution model is applied on a case study performed using parameters from the MC 252–1 “Macondo Well” blowout from April 20, 2010 in the deepwater Gulf of Mexico. The stress evolution suggests stability against mechanical failure mechanisms within the casing (collapse/burst and tensile/compressive), cement-sheath (inner or outer debonding, and shear cracking), and rock-formation layers (longitudinal or transverse tensile fracture initiation, along with shear-slippage along pre-existing faults in the near-well vicinity), throughout the blowout aftermath. Nevertheless, tendencies towards radial cracking and disking (tensile) failures were indicated for the cement-sheath layer as the system reaches radial stress and displacement continuity, after cement setting.

Neuspořádaný uhlík v tektonických zónách paleozoických sedimentů (devon moravskoslezského paleozoika)

Dark carbonaceous matter staining tectonic zones and deformed carbonate strata of the Moravo-Silesian Palaeozoic were studied by several methods. Samples were taken from tectonic structures in the quarry in Čebín and in the middle quarry in Mokrá near Brno. The grey-black-coloured rocks are clearly macroscopically and microscopically deformed, show traces of brittle ductile shear deformation and with foliation developed. The dark colour is caused by the presence of black carbon matter, which is documented by methods of optical and electron (BSE) microscopy.The mineral assemblage has the character of hydrothermal mineralization migrating along tectonic structures. Mineralization consists mainly of quartz, carbonates (calcite, dolomite), phyllosilicates (mica, chlorite, kaolinite), pyrite and it also includes black carbon. Apatite is one of the interesting and unusual components. The content of organic and elemental carbon determined by the thermo-optical method in intensively mineralized zones is around 2.5 mass%.The carbonaceous matter was more accurately identified using Raman spectra. The spectra at the two studied localities have a very similar shape and are very close to the spectra of black carbon in low-grade carbon coal matter, very disordered carbon and/or amorphous carbon (coal, kerogen). The spectra show the presence of peaks in the D, G and 2D regions and are different from the spectra of ordered and disordered graphite. The presence of a small peak G in the analysed spectra (Lorentzian function) also indicates the possible presence of a small amount of more ordered carbon in the studied black carbon matter.The components of the black mineralized zones were most likely mobilized from the surrounding rock formations during the Variscan tectono-metamorphic events. The similarity with the spectra of poorly ordered carbon matter from low metamorphic conditions shows transformation temperatures of 150–280 °C, which is in accordance with other thermometric methods in the region of the southern edge of the Moravo-Silesian Palaeozoic.

Development of a 3-D Dynamic Fracture Process Analysis code to Simulate Intermediate Loading Rate

For the stimulation solutions for oil and gas wells, the creation of multiple fractures that extend deep into surrounding rock formation is important to improve productivity or injectivity. For this purpose, the application of “not too slow but not too fast dynamic loading rate (hereafter intermediate dynamic loading rate)” has been proposed in the literature, which utilizes solid propellants. While previous researchers have experimentally demonstrated their applicability, our understanding of the detailed mechanism of rock fracturing for such an intermediate dynamic loading rate has not been mature enough. We have self-developed a state-of-the-art 3D combined finite-discrete element simulator that can model the complex dynamic fracture process of rocks. This paper applied the developed code to investigate its applicability to this class of problem characterized by the intermediate dynamic loading rate. NRC (new rock cracker) based on the water vaporized pressure due to the thermit reaction of the non-explosive ingredients was used. Based on the measured pressure as an input to the developed code, the 3D dynamic fracture process analysis (DFPA) was conducted. The involving dynamic fracture process and resultant fracture pattern were discussed. In addition, the required future tasks for more reasonable numerical simulation were also pointed out.

Hydro-mechanical path dependency of claystone/bentonite mixture samples characterized by different initial dry densities

In the context of the French Cigéo-project, a mixture composed of 70% processed Callovo-Oxfordian claystone spoil and 30% MX80-bentonite could be a potential backfill material, whose installation aims to stabilize the surrounding rock formation and to limit the propagation of the excavation damaged zone. The backfill material must sustain the overburden pressure, despite that it might be exposed to different hydraulic and mechanical paths. The reference concept considers employing conventional compaction techniques, although their employment involves spatial variations in the dry density after compaction. In general, as the initial dry density has a significant impact on the hydro-mechanical behavior of backfill materials, it is of major importance to relate the variations in the initial dry density to differences in the behavior. This experimental laboratory study aimed to analyze how variations in the initial dry density affect the swelling and compression behavior of the claystone/bentonite mixture, in particular in unsaturated state. Further, it evaluated whether those variations affected possible hydro-mechanical path dependences. The experimental program comprised suction-controlled oedometer and constant-volume swelling pressure experiments, in which samples characterized by different initial dry densities were exposed to different hydro-mechanical paths. The analysis of microstructural and water retention characteristics complemented the program. Major results indicated that the magnitude of swelling pressure at a given suction depends considerably on the initial dry density, but it is independent of the imposed hydro-mechanical path. Interestingly, the dependency of the yield behavior on the hydro-mechanical path appears to be more pronounced as the initial dry density increases.