Finite element modelling of the effective elastic properties of partially saturated rocks

Simulation of effective physical properties from microtomographic 3D images of porous structures allows one to relate properties of rocks directly to their microstructure. A static FEM code has been previously used to estimate effective elastic properties of fully saturated monomineralic (quartz) rock under wet and dry conditions. We use the code to calculate elastic properties under partially saturated conditions. The numerical predictions are compared to the Gassmann theory combined with Wood's formula (GW) for a mixture of pore fluids, which is exact for a monomineralic macroscopically homogeneous porous medium. Results of the numerical simulations performed for two Boolean sphere pack distributions show significant deviation from the GW limit and depend on the spatial distribution of fluids. This is shown to be a numerical artefact caused by incomplete equilibration of fluid pressure, which is primarily due to insufficient spatial resolution. To investigate the effect of pore-size and pore geometry, we perform FEM simulations for a model with regular pore geometry, where all pore channels have the same size and shape. Accuracy of these simulations increases with the total cross-section area of the channels and the size of individual channels. For the case where the total cross-section of the channels is large enough (on the same order as total porosity), there is a minimum of 4 voxels per channel diameter required for adequate fluid pressure equilibration throughout the pore space. Increasing the spatial resolution of the digital models reduces the discrepancy between the simulations and theory, but unfortunately increases the memory and CPU requirements of the simulations

Seismic Effects of Viscoelastic Pore Fill on Double-Porosity Rocks

© ASCE. We present a new model for elastic wave dispersion and attenuation in porous rocks saturated with viscous fluids and viscoelastic substances. The main idea is that the dispersion is mainly caused by frequency dependence of the normal and tangential stiffness of compliant pores that are hydraulically connected to stiff pores. First, we assume that the compliant pore (microcrack or grain-to-grain contact) is filled with an elastic solid. This allows us to obtain the stiffness of the compliant pore using a known solution of the elastic problem for a circular interlayer sandwiched between two parallel plates. The solution is then extended to viscoelastic pore fill using elastic/viscoelastic correspondence principle. The model predicts both, squirt-flow and viscous shear relaxation, and substantial increase in the rock rigidity with increasing frequency (or decreasing temperature) for rocks saturated with viscoelastic substances. The results are illustrated by an example of a typical rock saturated with water and with heavy oil

Seismic attenuation due to wave-induced fluid flow in a porous rock with spherical heterogeneities

Most natural porous rocks have heterogeneities at nearly all scales. Heterogeneities of mesoscopic scale that is, much larger than the pore size but much smaller than wavelength can cause significant attenuation and dispersion of elastic waves due to wave induced flow between more compliant and less compliant areas. Analysis of this phenomenon for a saturated porous medium with a small volume concentration of randomly distributed spherical inclusions is performed using Waterman-Truell multiple scattering theorem, which relates attenuation and dispersion to the amplitude of the wavefield scattered by a single inclusion. This scattering amplitude is computed using recently published asymptotic analytical expressions and numerical results for elastic wave scattering by a single mesoscopic poroelastic sphere in a porous medium.This analysis reveals that attenuation and dispersion exhibit a typical relaxation-type behavior with the maximum attenuation and dispersion corresponding to a frequency where fluid diffusion length (or Biot's slow wave length) is of the order of the inclusion diameter. In the limit of low volume concentration of inclusions the effective velocity is asymptotically consistent with the Gassmann theory in the low-frequency limit, and with the solution for an elastic medium with equivalent elastic inclusions (no-flow solution) in the low-frequency limit. Attenuation (expressed through inverse quality factor ) scales with frequency in the low frequency limit and with in the high frequency limit. These asymptotes are consistent with recent results on attenuation in a medium with a periodic distribution of poroelastic inclusions, and in continuous random porous media

Cyano-bridged perovskite [(CH3)3NOH]2[KM(CN)6],[M: Fe(III), and Co(III)] for high-temperature multi-axialferroelectric applications with enhanced thermaland nonlinear optical performance

Highly stable ferroelectrics with reversible high-temperature phase transitions and switchable nonlinear optical behaviour are much coveted targets for emerging optoelectronic applications. Here, we demonstrate a cyano-bridged perovskite [(CH3)3NOH]2[KCo(CN)6] (TMAO-Co), a new analogue of the multi-axial ferroelectric [(CH3)3NOH]2[KFe(CN)6] (TMAO-Fe) with improved thermal stability and enhanced second-order nonlinear optical response. Indeed, for TMAO-Co the Curie temperature (Tc) is shifted to a higher value of ca. 416 K (improvement by ca. 10 K versus TMAO-Fe); the separation between Tc and the decomposition threshold is 46 K. TMAO-Co is a biaxial ferroelectric as revealed by P(E) hysteresis loop measurements along the a and c crystallographic directions with spontaneous polarization values of 0.9 and 0.63 μC cm−2 at 293 K, respectively. The SHG response of TMAO-Co is two times higher than that of TMAO-Fe. The improved stability of TMAO-Co to thermal and optical loads allowed for demonstration of bistable switching of nonlinear optical response between SHG-on and SHG-off states by temperature sweeping. Structurally, TMAO-Co reproduces the unusual characteristics of TMAO-Fe, i.e. the first-order phase transition between (polar) monoclinic to (nonpolar) cubic phases involving bond switching and is assisted by the pronounced increase of disorder of the TMAO cations above Tc. Combined temperature-resolved Raman and infrared spectroscopic measurements were employed to track the symmetry increase above Tc, which is primarily associated with changes in hydrogen-bonding. Consistent with the bond-switching character of the phase transitions, a pronounced shift to higher wavenumbers is observed for the O–H stretching modes. The DFT calculations demonstrate that the system's polarization along the a-axis mostly comes from the rotation of the [(CH3)3NOH]2 cluster, while the atomic displacement of the framework contributes largely to that along the c axis

Obesity-induced nucleosome release predicts poor cardio-metabolic health

Objective: While circulating nucleosome levels are high in obese mouse models, it is unknown where these nucleosomes originate from and whether they are a marker of cardio-metabolic health in humans. Here, we aimed to determine whether an association exists between circulating nucleosomes and the risk of developing obesity, metabolic syndrome (MetS) and/or a dysfunctional cardiovascular performance. / Methods: We randomly selected 120 participants of the Kardiovize Brno 2030 study across three BMI strata: BMI 18–25, 25–30, and > 30. We assessed the association between circulating nucleosome levels and the risk of obesity, MetS, and poor cardiovascular health. We then cultured human neutrophils, adipocytes, and hepatoma cells to study nucleosome origins in a fat-rich environment. Results: Circulating nucleosome levels positively correlated with BMI (R = 0.602, p < 0.05), fatty liver index (R = 0.622, p < 0.05), left ventricular mass (R = 0.457, p < 0.05), and associated with MetS (p < 0.001) and poor cardiovascular health (p < 0.001). Incubating neutrophils with 1–10 μM free fatty acids triggered nucleosome production without concomitant cell death. Nucleosomes were not produced during pre-adipocyte differentiation or upon incubation of hepatic cells with palmitic acid. / Conclusions: Neutrophils are a bona fide source of circulating nucleosomes in an obesogenic environment and in overweight/obese patients. High nucleosome levels are associated with MetS and cardiovascular performance, and might represent novel candidate biomarkers for cardio-metabolic health