Teddy Fen-Chong

Adsorption-Induced Deformation of Microporous Materials: Coal Swelling Induced by CO2–CH4 Competitive Adsorption

Carbon dioxide injection in coal seams is known to improve the methane production of the coal seam, while ensuring a safe and long-term carbon sequestration. This improvement is due to the preferential adsorption of CO(2) in coal with respect to CH(4): an injection of CO(2) thus results in a desorption of CH(4). However, this preferential adsorption is also known to cause a differential swelling of coal, which results in a significant decrease in the reservoir permeability during the injection process. Recent studies have shown that adsorption in coal micropores (few angströms in size) is the main cause of the swelling. In this work, we focus on the competitive adsorption behavior of CO(2) and CH(4) in micropores. We perform molecular simulations of adsorption with a realistic atomistic model for coal. The competitive adsorption is studied at various temperatures and pressures representative of those in geological reservoirs. With the help of a poromechanical model, we then quantify the subsequent differential swelling induced by the computed adsorption behaviors. The differential swelling is almost insensitive to the geological temperatures and pressures considered here and is proportional to the CO(2) mole fraction in the coal.

Poroelastic Analysis of Partial Freezing in Cohesive Porous Materials

We revisit the poromechanics set up by Olivier Coussy for better understanding of the mechanical effect of partial freezing in cohesive porous materials. This approach proves to be able to quantitatively predict swelling even if the in-pore liquid does not expand when solidifying. In this case, dilation results from the so-called cryosuction process that dominates thermal shrinkage under cooling, as shown in our analysis conducted on the historical experiment run by Beaudoin and MacInnis (1974, “The Mechanism of Frost Damage in Hardened Cement Paste,” Cem. Concr. Res., 4, pp. 139–147) on benzene saturated 24-h old cement paste. Both mechanisms are also at work when freezing water saturated early age cement paste with air voids. In this case, the cryosuction process results in shrinkage and should be added to the thermal shrinkage, their respective amplitudes being temperature dependent but, a priori, of the same order of magnitude.

Water Removal by Freeze-Drying of Hardened Cement Paste

This paper investigates the water removal extent of freeze-drying (F-drying) technique on hardened cement pastes. Two cement pastes with w/c = 0.5, 0.3 were prepared, and the authors measured the hydration degree, specific surface area, porosity, and solid skeleton density of the material samples at curing ages from 7 days to 180 days. From these results, the C-S-H gel volumes of samples are predicted from Jennings model and also evaluated from drying data. The good agreement between the results of two approaches confirms that F-drying removes only partially the water in gel pores of LD C-S-H structure. At the age of 180 days, the residual porosity saturated by water after F-drying is evaluated as 28% (w/c = 0.5) and 55% (w/c = 0.3) with dried C-S-H estimated as 28% (w/c = 0.5) and 13% (w/c = 0.3).

Cardinal series to sort out defective samples in magnetic resonance data sets

NMR signals are unavoidably impaired with noise stemming from the electronic circuits of the spectrometer. This noise is most often white and Gaussian and can be greatly reduced by applying low pass analogue and digital filters. Nevertheless, extra noise with other statistics than Gaussian may interfere with the signal, e.g. when auxiliary electrical devices are placed near the magnet of the NMR spectrometer. This paper reports on how one can make use of this difference in statistics to remove the noise caused by electrical devices before any further data processing. The algorithm is based on the use of a new linear low pass filter, which consists in fitting NMR data in the time domain with a cardinal series and whose spectral width can be controlled. Over other filtering methods such filter has the advantage of not distorting the signal neither at the beginning nor the end of the acquisition period. The performance of the method is demonstrated by applying it to a data set collected in a flow PGSE experiment and impaired with noise emanated from a brushed DC electric motor.

Cardinal series to filter oversampled truncated magnetic resonance signals

Digital low pass filters are routinely used to improve the signal-to-noise ratio of NMR signals, e.g. FID or echoes, when pass band widths of the available analogue filters do not correspond to the spectral width of the signals. Applying digital filters will always necessitate an oversampling of the signal to filter. The digital filters with which the commercial spectrometers are nowadays equipped and most of those known to date from literature were designed to be applied to signals in the time domain. Nevertheless, most of them are aimed at optimising the filtering of signals in the frequency domain and tend to distort them in the time domain, especially when applied to truncated signals. Herein we propose a low pass filter that preserves all the features of the signal in both domains. The method consists in fitting raw NMR data with a finite sum of truncated cardinal sine functions and requires nothing but the signal being a band-limited function. We devised sensible and, in practice, hardly restrictive rules for setting parameters of the filter and applied it to various computer-simulated and experimentally measured truncated data sets to demonstrate its success in filtering both FID and echo signals.

A Micromechanics Model for Solute Diffusion Coefficient in Unsaturated Granular Materials

A new micromechanics analysis of solute diffusion in unsaturated granular materials is put forward. This permits predicting the percolation effect. To do so, the pore water is divided up into four phases that account for the different spatial distributions and diffusion properties: interconnected capillary water, isolated capillary water, wetting layer and water film. A parameter denoted as connectivity ratio is introduced to account for the connectivity of the capillary pore water. Our model agrees well with experimental results on unsaturated sands and glass beads from literature. It also permits interpreting the physical meaning of the saturation exponent of Archie’s law: The latter is found to be correlated with the connectivity ratio of the unsaturated granular materials.

Effect of supercooling on the instantaneous freezing dilation of cement-based porous materials

This study seeks to assess the instantaneous deformation of saturated cement-based porous materials induced by supercooling of pore fluid. Thermodynamic equilibria account for the physical behaviors of the confined crystallization of ice for partially frozen porous systems. Pore pressure is built up when ice forms in pores, and this can be intimately related to the pore structure of the porous materials. To overcome the metastability of supercooled water that surpasses a normal poromechanical description, a special program for cooling has been designed. The hydraulic pressure resulting from rapid ice formation and the thermal shock caused by heat release account for an observed dilation peak. The lower the supercooled temperature, the larger the dilation peak becomes. Rapid pore pressure relaxation accounts for significant contraction following the dilation peak. The pore structure also has a significant impact on the freezing deformation of the cement-based porous materials. Results obtained in these examples are comparable with previously published data, which confirms the importance of the detrimental effects of supercooling in porous materials.

Micromechanical Modeling of Transport Properties of Cement-Based Composites: Role of Interfacial Transition Zone and Air Voids

The transport properties of cement-based composites, including solute diffusivity, electrical conductivity and water permeability, are regarded as durability indicators of cement-based composites. These transport properties are closely related to the microstructure, or rather to the pore structure of materials. Among all the microstructural aspects, the interfacial transition zone (ITZ) between the cement paste matrix and aggregates, and the air voids are believed to play an essential role in the transport properties. However, their impacts on the transport properties are difficult to be quantified. This paper develops a closed-form four-phase micromechanical model accounting for the local properties of ITZ and the saturation states of air voids. The effects of ITZ and air voids on the transport properties of cement-based composites are addressed quantitatively in the model. The Katz–Thompson equation is reinterpreted by the model in particular. It is shown that the local properties of ITZ and volume fraction of aggregates act mutually on the overall transport properties, the influence of air voids depends significantly on the water saturation, and a critical saturation degree is found to be 1/3.

A study of the behaviors of cement-based materials subject to freezing

The freezing behavior of cement-based materials is studied in this paper. The freezing-related pore structure is investigated, the fundamental relationship between liquid water saturation degree and freezing temperature is established and the material cryo-deformation is given through poroelastic approach. The effective pressure from freezing is divided into contributions of hydraulic pressure by viscous water flow and pore crystallization pressure by phase change as well as the interface factor. The cryo-deformations of air-entrained mortars with different spacing factors are evaluated through the poromechanical model. The results indicate that both thermal contraction and negative liquid pressure dominate the freezing shrinkage of materials, and the hydraulic pressure contributes most to the freezing expansion as spacing factor increasing.

Poroelastic Contribution to Freezing in Cement Paste

An effective pore pressure model is used to predict the mechanical effect induced by in-pore partial freezing of a liquid phase initially saturating a cohesive porous solid. This allows discussing the influence of the thermodynamic and poroelastic properties of the different constituents of the unsaturated porous material on its strain under drained or undrained conditions.