20 research outputs found
Monte Carlo algorithm based on internal bridging moves for the atomistic simulation of thiophene oligomers and polymers
We introduce a powerful Monte Carlo (MC) algorithm for the atomistic
simulation of bulk models of oligo- and poly-thiophenes by redesigning MC moves
originally developed for considerably simpler polymer structures and
architectures, such as linear and branched polyethylene, to account for the
ring structure of the thiophene monomer. Elementary MC moves implemented
include bias reptation of an end thiophene ring, flip of an internal thiophene
ring, rotation of an end thiophene ring, concerted rotation of three thiophene
rings, rigid translation of an entire molecule, rotation of an entire molecule
and volume fluctuation. In the implementation of all moves we assume that
thiophene ring atoms remain rigid and strictly co-planar; on the other hand,
inter-ring torsion and bond bending angles remain fully flexible subject to
suitable potential energy functions. Test simulations with the new algorithm of
an important thiophene oligomer, {\alpha}-sexithiophene ({\alpha}-6T), at a
high enough temperature (above its isotropic-to-nematic phase transition) using
a new united atom model specifically developed for the purpose of this work
provide predictions for the volumetric, conformational and structural
properties that are remarkably close to those obtained from detailed atomistic
Molecular Dynamics (MD) simulations using an all-atom model. The new algorithm
is particularly promising for exploring the rich (and largely unexplored) phase
behavior and nanoscale ordering of very long (also more complex)
thiophene-based polymers which cannot be addressed by conventional MD methods
due to the extremely long relaxation times characterizing chain dynamics in
these systems
Transport Properties of Shale Gas in Relation to Kerogen Porosity
Kerogen is a micro-porous amorphous solid, which consist the major component of the organic
matter scattered in the potentially lucrative shale formations hosting shale gas. Deeper
understanding of the way kerogen porosity characteristics affect the transport properties of
hosted gas is important for the optimal design of the extraction process. In this work, we employ
molecular simulation techniques in order to investigate the role of porosity on the adsorption
and transport behavior of shale gas in overmature type II kerogen found at many currently
productive shales. To account for the wide range of porosity characteristics present in the real
system, a large set of 60 kerogen structures that exhibit a diverse set of void space attributes
was used. Grand Canonical Monte Carlo (GCMC) simulations were performed for the study
of the adsorption of CH4, C2H6, n-C4H10 and CO2 at 298.15 K and 398.15 K and a variety of
2
pressures. The amount adsorbed is found to correlate linearly with the porosity of the kerogen.
Furthermore, the adsorption of a quaternary mixture of CH4, C2H6, CO2 and N2 was
investigated in the same conditions, indicating that the composition resembling that of the shale
gas is achieved under higher temperature and pressure values, i.e. conditions closer to these
prevailing in the hosting shale field. The diffusion of CH4, C2H6 and CO2, both as pure
components and as components of the quaternary mixture, was investigated using equilibrium
Molecular Dynamics (MD) simulations at temperatures of 298.15 and 398.15 K and pressures
of 1 and 250 atm. In addition to the effect of temperature and pressure, the importance of
limiting pore diameter (LPD), maximum pore diameter (MPD), accessible volume (Vacc) and
accessible surface (Sacc) on the observed adsorbed amount and diffusion coefficient was
revealed by qualitative relationships. The diffusion across the models was found to be
anisotropic and the maximum component of the diffusion coefficient to correlate linearly with
LPD, indicating that the controlling step of the transport process is the crossing of the limiting
pore region. Finally, the transport behavior of the pure compounds was compared with their
transport properties when in mixture and it was found that the diffusion coefficient of each
compound in the mixture is similar to the corresponding one in pure. This observation agrees
with earlier studies in different kerogen models comprising wider pores that have revealed
negligible cross-correlation Onsager coefficients
Transport Properties of Shale Gas in Relation to Kerogen Porosity
Kerogen is a micro-porous amorphous solid, which consist the major component of the organic
matter scattered in the potentially lucrative shale formations hosting shale gas. Deeper
understanding of the way kerogen porosity characteristics affect the transport properties of
hosted gas is important for the optimal design of the extraction process. In this work, we employ
molecular simulation techniques in order to investigate the role of porosity on the adsorption
and transport behavior of shale gas in overmature type II kerogen found at many currently
productive shales. To account for the wide range of porosity characteristics present in the real
system, a large set of 60 kerogen structures that exhibit a diverse set of void space attributes
was used. Grand Canonical Monte Carlo (GCMC) simulations were performed for the study
of the adsorption of CH4, C2H6, n-C4H10 and CO2 at 298.15 K and 398.15 K and a variety of
2
pressures. The amount adsorbed is found to correlate linearly with the porosity of the kerogen.
Furthermore, the adsorption of a quaternary mixture of CH4, C2H6, CO2 and N2 was
investigated in the same conditions, indicating that the composition resembling that of the shale
gas is achieved under higher temperature and pressure values, i.e. conditions closer to these
prevailing in the hosting shale field. The diffusion of CH4, C2H6 and CO2, both as pure
components and as components of the quaternary mixture, was investigated using equilibrium
Molecular Dynamics (MD) simulations at temperatures of 298.15 and 398.15 K and pressures
of 1 and 250 atm. In addition to the effect of temperature and pressure, the importance of
limiting pore diameter (LPD), maximum pore diameter (MPD), accessible volume (Vacc) and
accessible surface (Sacc) on the observed adsorbed amount and diffusion coefficient was
revealed by qualitative relationships. The diffusion across the models was found to be
anisotropic and the maximum component of the diffusion coefficient to correlate linearly with
LPD, indicating that the controlling step of the transport process is the crossing of the limiting
pore region. Finally, the transport behavior of the pure compounds was compared with their
transport properties when in mixture and it was found that the diffusion coefficient of each
compound in the mixture is similar to the corresponding one in pure. This observation agrees
with earlier studies in different kerogen models comprising wider pores that have revealed
negligible cross-correlation Onsager coefficients
Modeling of bulk kerogen porosity: Methods for control and characterization
Shale gas is an unconventional source of energy, which has attracted a lot of attention during the last years.
Kerogen is a prime constituent of shale formations and plays a crucial role in shale gas technology. Significant experimental effort
in the study of shales and kerogen has produced a broad diversity of experimentally determined structural and thermodynamic
properties even for samples of the same well. Moreover, proposed methods reported in the literature for constructing realistic
bulk kerogen configurations have not been thoroughly investigated. One of the most important characteristics of kerogens is their
porosity, due to its direct connection with their transport properties and its potential as discriminating and classifying metric
between samples. In this study, molecular dynamics (MD) simulations are used to study the porosity of model kerogens. The
porosity is controlled effectively with systematic variations of the number and the size of dummy LJ particles that are used during
the construction of system’s configuration. The porosity of each sample is characterized with a newly proposed algorithm for
analyzing the free space of amorphous materials. It is found that, with moderately sized configurations, it is possible to construct
percolated pores of interest in the shale gas industry
Special Issue on "Monte Carlo Simulation of Soft Matter Systems" EDITORIAL
ISSN:2296-424