Geological repositories subject to the injection of large amounts of anthropogenic carbon dioxide will undergo chemical and mechanical instabilities for which there are currently little experimental ...data. This study reports on experiments where low and high pCO2 (8~MPa) aqueous fluids where injected into natural rock samples. The experiments were performed in flow-through triaxial cells, where the vertical and confining stresses, temperature, and pressure and composition of the fluid were separately controlled and monitored. The axial vertical strains of two limestones and one sandstone were continuously measured during separate experiments for several months, with a strain rate resolution of 10^-11 s-1. Fluids exiting the triaxial cells where continuously collected and their compositions analysed. The high pCO2 fluids induced an increase in strain rates of the limestones by up to a factor of 5, compared to the low pCO2 fluids. Injection of high pCO2 fluids into the sandstone resulted in deformation rates one order of magnitude smaller than the limestones. The creep accelerating effect of high pCO2 fluids with respect to the limestones was mainly due to the acidification of the injected fluids, resulting in a significant increase in solubility and reaction kinetics of calcite. Compared to the limestones, the much weaker response of the sandstone was due to the much lower solubility and reactivity of quartz in high pCO2 fluids. In general, all samples showed a positive correlation between fluid flow rate and strain rate. X-ray tomography results revealed significant increases in porosity at the inlet portion of each core; the porosity increases were dependent on the original lithological structure and composition. The overall deformation of the samples is interpreted in terms of simultaneous dissolution reactions in pore spaces and intergranular pressure solution creep.
The World Health Organization refers to stroke, the second most frequent cause of death in the world, in terms of pandemic. Present treatments are only effective within precise time windows. Only 10% ...of thrombolysis patients are eligible. Late assessment of the patient resulting from admission and lack of knowledge of the symptoms is the main explanation of lack of eligibility.
The aim is the measurement of the time of access to treatment facilities for stroke victims, using ambulances (firemen ambulances or EMS ambulances) and private car. The method proposed analyses the potential geographic accessibility of stroke care infrastructure in different scenarios. The study allows better considering of the issues inherent to an area: difficult weather conditions, traffic congestion and failure to respect the distance limits of emergency transport.
Depending on the scenario, access times vary considerably within the same commune. For example, between the first and the second scenario for cities in the north of Rhône county, there is a 10 min difference to the nearest Primary Stroke Center (PSC). For the first scenario, 90% of the population is 20 min away of the PSC and 96% for the second scenario. Likewise, depending on the modal vector (fire brigade or emergency medical service), overall accessibility from the emergency call to admission to a Comprehensive Stroke Center (CSC) can vary by as much as 15 min.
The setting up of the various scenarios and modal comparison based on the calculation of overall accessibility makes this a new method for calculating potential access to care facilities. It is important to take into account the specific pathological features and the availability of care facilities for modelling. This method is innovative and recommendable for measuring accessibility in the field of health care. This study makes possible to highlight the patients' extension of care delays. Thus, this can impact the improvement of patient care and rethink the healthcare organization. Stroke is addressed here but it is applicable to other pathologies.
In order to better understand the interaction between pore‐fluid overpressure and failure patterns in rocks we consider a porous elasto‐plastic medium in which a laterally localized overpressure line ...source is imposed at depth below the free surface. We solve numerically the fluid filtration equation coupled to the gravitational force balance and poro‐elasto‐plastic rheology equations. Systematic numerical simulations, varying initial stress, intrinsic material properties and geometry, show the existence of five distinct failure patterns caused by either shear banding or tensile fracturing. The value of the critical pore‐fluid overpressure pc at the onset of failure is derived from an analytical solution that is in excellent agreement with numerical simulations. Finally, we construct a phase‐diagram that predicts the domains of the different failure patterns and pc at the onset of failure.
Removal of oxyanions (selenite, selenate, arsenate, phosphate and nitrate) during calcite formation was experimentally studied using aqueous carbonation of calcium hydroxide under moderate pressure ...(P
CO2
≅
20
bar) and temperature (30
°C). The effects of Ca(OH)
2 dose (10 and 20
g), Ca(OH)
2 source (commercial pure material or alkaline paper mill waste) and oxyanion initial concentration (from 0 to 70
mg
atom/L) were investigated for this anisobaric gas–liquid–solid system.
The Ca(OH)
2 carbonation reaction allowed successfully the removal of selenite (>90%), arsenate (>78%) and phosphate (≅100%) from synthetic solutions. Conversely, nitrate and selenate had not any physicochemical affinity/effect during calcite formation.
The rate of CO
2 transfer during calcite formation in presence of oxyanions was equal or slower than for an oxyanion-free system, allowing to define a retarding kinetic factor
RF that can vary between 0 (no retarding effect) to 1 (total inhibition). For selenite and phosphate
RF was quite high, close to 0.3. A small retarding effect was detected for arsenate (
RF
≈
0.05) and no retarding effect was detected for selenate and nitrate (
RF
≈
0). In general,
RF depends on the oxyanion initial concentration, oxyanion nature and Ca(OH)
2 dose.
The presence of oxyanions could also influence the crystal morphology and aggregation/agglomeration process. For example, a
c-axis elongation of calcite crystals was clearly observed at the equilibrium, for calcite formation in presence of selenite and phosphate.
The oxyanions removal process proposed herein was inspired on the common physicochemical treatment of wastewater using calcium hydroxide (Ca(OH)
2). The particularity, for this novel method is the simultaneous calcium hydroxide carbonation with compressed carbon dioxide in order to stabilise the solid matter. This economical and ecological method could allow the removal of various oxyanions as well as the ex situ mineral sequestration of CO
2; particularly, when the Ca(OH)
2 source comes from alkaline solid waste.
•We performed a global-to-specific white matter (WM) analysis in patients with temporal lobe epilepsy (TLE)•Track-weighted fractional anisotropy was related to patients' clinical and cognitive ...status•We used TractLearn, a manifold learning pipeline, to automatically detect patient-specific WM lesions•We found WM abnormalities in several fascicles, clustered on certain tract segments•Our analyses highlighted interindividual variability and the need for precision medicine
An accurate description of brain white matter anatomy in vivo remains a challenge. However, technical progress allows us to analyze structural variations in an increasingly sophisticated way. Current methods of processing diffusion MRI data now make it possible to correct some limiting biases. In addition, the development of statistical learning algorithms offers the opportunity to analyze the data from a new perspective. We applied newly developed tractography models to extract quantitative white matter parameters in a group of patients with chronic temporal lobe epilepsy. Furthermore, we implemented a statistical learning workflow optimized for the MRI diffusion data – the TractLearn pipeline – to model inter-individual variability and predict structural changes in patients. Finally, we interpreted white matter abnormalities in the context of several other parameters reflecting clinical status, as well as neuronal and cognitive functioning for these patients. Overall, we show the relevance of such a diffusion data processing pipeline for the evaluation of clinical populations. The “global to fine scale” funnel statistical approach proposed in this study also contributes to the understanding of neuroplasticity mechanisms involved in refractory epilepsy, thus enriching previous findings.
Understanding the grain-scale processes leading to reservoir compaction during hydrocarbons production is crucial for enabling physics-based predictions of induced surface subsidence and seismicity ...hazards. However, typical laboratory experiments only allow for pre- and post-experimental microstructural investigation of deformation mechanisms. Using high-resolution time-lapse X-ray micro-tomography imaging (4D µCT) during triaxial deformation, the controlling grain-scale processes can be visualized through time and space at realistic subsurface conditions. We deformed a sample of Slochteren sandstone, the reservoir rock from the seismogenic Groningen gas field in the Netherlands. The sample was deformed beyond its yield point (axial strain > 15%) in triaxial compression at reservoir P–T-stress conditions (100 °C, 10 MPa pore pressure, 40 MPa effective confining pressure). A total of 50 three-dimensional µCT scans were obtained during deformation, at a spatial resolution of 6.5 µm. Time lapse imaging plus digital volume correlation (DVC) enabled identification of the grain-scale deformation mechanisms operating throughout the experiment, for the first time, both at small, reservoir-relevant strains (< 1%), and in the approach to brittle failure at strains > 10%. During small-strain deformation, the sample showed compaction through grain rearrangement accommodated by inter-granular slip and normal displacements across grain boundaries, in particular, by closure of open grain boundaries or compaction of inter-granular clay films. At intermediate and large strains (> 4%), grain fracturing and pore collapse were observed, leading to sample-scale brittle failure. These observations provide key input for developing microphysical models describing compaction of the Groningen and other producing (gas) reservoirs.
Highlights
Time-lapse synchrotron micro-CT imaging reveals grain-scale deformation processes for compaction of Slochteren sandstone, from the Groningen gas field
At small axial strains, digital volume correlation shows local strains by small displacements along grain boundaries, without intra-granular cracks
After yielding, the sample deformed through pervasive grain failure and pore collapse
The formation of solid calcium carbonate (CaCO3) from aqueous solutions or slurries containing calcium and carbon dioxide (CO2) is a complex process of considerable importance in the ecological, ...geochemical and biological areas. Moreover, the demand for powdered CaCO3 has recently increased considerably in various fields of industry. The aim of this study was therefore to synthesize fine particles of calcite with controlled morphology by hydrothermal carbonation of calcium hydroxide at high CO2 pressure and at moderate and high temperatures (30 and 90 deg C). The morphology of precipitated particles was identified by transmission electron microscopy (TEM/EDS) and scanning electron microscopy (SEM/EDS). In addition, an X-ray diffraction analysis was performed to investigate the carbonation efficiency and purity of the solid product. Carbonation of dispersed calcium hydroxide (Ca(OH)2(s)+CO2(aq)- > CaCO3(s)+H2O) in the presence of supercritical (PT=90bar, T=90 deg C) or gaseous (PT=55bar, T=30 deg C) CO2 led to the precipitation of sub-micrometric isolated particles ( < 1mum) and micrometric agglomerates ( < 5mum) of calcite. For this study, the carbonation efficiency (Ca(OH)2-CaCO3 conversion) was not significantly affected by pressure-temperature (PT) conditions after 24h of reaction. In contrast, the initial rate of calcium carbonate precipitation increased from 4.3mol/h in the '90bar-90 deg C'system to 15.9mol/h in the '55bar-30 deg C'system. The use of high CO2 pressure may therefore be desirable for increasing the production rate of CaCO3, carbonation efficiency and purity, to approximately 48kg/m3h, 95% and 96.3%, respectively, in this study. The dissipated heat for this exothermic reaction was estimated by calorimetry to be -32kJ/mol in the '90bar-90 deg C'system and -42kJ/mol in the '55bar-30 deg C'system.
Gas–solid carbonation experiments were carried out by using an infrared microscope coupled to a reaction cell. The hydroxide ions (OH) consumption and the production of molecular water (H
2O) and ...carbonate (CO
3
2−) vibration bands were directly monitored as a function of time. Herein, we demonstrated that the gas–solid carbonation of calcium hydroxide (or portlandite) was exclusively activated by initial adsorbed water-molecules (water activity
≈
0.6 in the lab room) at low temperature (30
°C) and low CO
2 pressure (0.5–1.5
bar). We assume that carbonation reaction was then rapidly autocatalysed by the water production and followed by a passivation step due to the formation of a dense layer of carbonate around the reacting particles of portlandite. The fast carbonation and passivation steps were satisfactory fitted by using a kinetic pseudo-second-order model. Moreover, the infrared measurements provided complementary insights with relevance to the reaction mechanism of gas–solid carbonation of calcium hydroxide. Herein, the formation of metastable aragonite was identified and a hydrated calcium carbonate was suspected during carbonation process.
On the other hand, when initial adsorbed water onto reacting particles was removed by in situ vacuum drying (
P
<
10
−5 mbar,
T
=
110
°C) prior to injection of CO
2 in the reaction cell (water activity
≈
0), the carbonation of calcium hydroxide particles was no more detected by infrared spectroscopy at low temperature (30
°C). However, there was evidence for a very limited carbonation reaction at higher temperature (300
°C) and low CO
2 pressure (<1
bar). Another carbonation mechanism is required to explain this observation, for example the migration of oxygen atoms from the solid towards adsorbed CO
2.
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