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Molecular dynamics simulation of n-alkanes and CO2 confined by calcite nanopores Mirella Simoes Santos, Luis Franco, Marcelo Castier, and Ioannis George Economou Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02451 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018
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Molecular dynamics simulation of n-alkanes and CO2 confined by calcite nanopores Mirella S. Santos1, Luís F. M. Franco2, Marcelo Castier1, Ioannis G. Economou1,* 1
Chemical Engineering Program, Texas A&M University at Qatar, PO Box 23874, Doha, Qatar 2
School of Chemical Engineering, University of Campinas, Av. Albert Einstein 500, CEP: 13083-852, Campinas, Brazil *
corresponding author at:
[email protected] Abstract Recent advances on the recovery of oil and gas from shale and tight reservoirs have put on focus the need for a better understanding of the behavior of fluids under confinement. Confinement effects must be considered when the pore size is on the order of a few nanometers. Pores of such a small scale are abundant in shale and tight reservoirs, justifying the unique properties and characteristics observed in fluids of such reservoirs. Furthermore, the development of techniques for geological carbon reinjection and storage makes essential the understanding of how CO2 interacts with the reservoir medium and its fluids. In this work, we use molecular dynamics simulations to predict the behavior of n-alkanes and CO2 mixtures confined by calcite slit nanopores. We observe that CO2 displaces the hydrocarbons adsorbed on the calcite surface, while the number of calcium sites controls the amount of CO2 adsorbed on the pore surface. This suggests that the reinjection of CO2 in tight oil and gas reservoirs may help enhance hydrocarbon recovery. Furthermore, temperature, pore size, CO2 fraction and n-alkane length are shown to be critical factors for the selective adsorption of CO2 over n-alkanes. Keywords: molecular dynamics simulation, confinement, calcite, adsorption, tight reservoirs, shale reservoirs
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Introduction Fluids in unconventional hydrocarbon reservoirs exhibit equilibrium properties that differ largely from those in traditional reservoirs, thus having unique PVT behavior1. For example, various authors2-5 observed that the bubble point of n-octane and n-decane under confinement can be different by up to 20 K when compared with the bulk value, information that should be considered when simulating and predicting the behavior of oil and gas reservoirs. Furthermore, a combination of different experimental techniques in recent years allowed a better description of the pore size distribution of different oil and gas reservoirs, revealing a significant presence of pores smaller than 50 nm, where confinement effects play a major role6-10. Also relevant to the behavior of confined fluids is the pore geometry11,12. These facts contribute to the importance of considering nanometric pores when describing tight and shale hydrocarbon reservoirs, which leads to improving well production forecast and the prediction of recoverable oil-in-place13. Shale and tight reservoirs are favorable sites for geological carbon sequestration, which is a promising technique to reduce the carbon footprint of the oil and gas industry14,15. Besides reducing CO2 emissions, CO2 injection into geological reservoirs can increase the availability of hydrocarbons adsorbed within the reservoir prior to the injection, thus being often associated with enhanced oil recovery processes (EOR)16. A clear understanding of the interactions of CO2 with the reservoir rocks and its hydrocarbon content is relevant for the development of next generation technologies for CO2 enhanced oil recovery, which can be applied to a broader range of reservoirs around the world17. Molecular simulations constitute a valuable tool to investigate phenomena in a nanometric scale, as, for example, those observed in tight oil and gas reservoirs. Molecular Dynamics (MD) simulations have been successfully applied to model fluids within reservoir rocks such as clays18, organic matter19,20, silica21-23, quartz24, and carbonates25. Nonetheless, the selective adsorption in calcite, regardless of its relevance for different hydrocarbon fields, especially in the Middle East, remains to be studied more extensively. In this work, we apply MD simulations to describe calcite slit-shaped nanopores. The main goal of this work is to improve the understanding the effects inserting CO2 in such nanopores containing n-alkanes. The adsorption of mixtures containing CO2 and methane, n-
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butane, or n-octane is studied. Furthermore, we also investigate the effect of temperature, pore width, CO2 concentration, and density within the nanopore.
Methodology MD simulations of calcite slit-shaped nanopores containing different mixtures of CO2 with methane, n-butane or n-octane were carried out in order to analyze the effects of CO2 on the adsorption of n-alkanes. In all simulations, the confined system comprises a slit-shaped calcite nanopore with hydrocarbons and CO2. A pore containing a large amount of CO2 is an idealization of what might occur during the reinjection of CO2 into oil and gas reservoirs. Due to the anisotropic behavior of the pressure tensor in the pore, the confined fluids are characterized here in terms of the density inside the pore. The calcite plane considered was 1014 orthogonal to the z direction. All the MD simulations were performed using the GROMACS simulation package (Version 4.5.6)26,27. The × dimensions of the simulation box were set equal to 4.990 × 4.856 nm. The z coordinate of the calcite crystal was 1.212 nm while the pore width (z direction) was specified for each case studied, ranging from 3 to 8 nm. This range of width was defined considering experimentally measured pore size distributions of shale and tight hydrocarbon reservoirs and the fact that confinement effects are negligible in wider pores9,10. The initial configuration of the fluid inside the pore consisted of two layers of the n-alkane with a layer of CO2 in between them. This means that, in the initial configurations, CO2 was placed in the middle of the pore, while the n-alkanes were close to the calcite surfaces. We used the TraPPE-UA28,29 force field to describe the linear n-alkanes and CO2. For calcite, we employed the force field developed by Xiao et al.30. In the TraPPE force field, intramolecular Lennard-Jones and Coulomb interactions between atoms connected by at most three bonds are excluded and the total potential energy of the system is defined as follows: = + + + ! "
(1)
The dispersion (van der Waals) and electrostatic contributions are described by the 12-6 Lennard-Jones potential and Coulomb potential, respectively, according to the expression: 3 ACS Paragon Plus Environment
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# () (*
= $%&
' ")*
,
,
)*
)*
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+ +" -#-# − " // 0
(2)
where 1 2 is the distance between the pair of atoms i and j, 3 and 32 are the partial charges of the
atoms and 45 is the vacuum permittivity 45 = 8.85418782 × 108 9 :;< . The Lennard-
Jones parameters for the TraPPE force field can be found in Table 1 and for the calcite force field in Table 2. For the unlike interactions between fluid molecules the Lorentz-Berthelot combining rule was used, for the fluid-solid interactions the geometric combining rule was applied to both = and =? , and the solid-solid interaction parameters can be found in Table 3. For the TraPPE force field, the bonds have a fixed length of 0.154 nm. Equations 3 and 4 represent the contribution to the potential energy due to angle bending and dihedral angle torsion: =
@A >
B − B5 >
(3)
! " = C< D1 + cos HI + C> D1 − cos 2HI + C8 D1 − cos 3HI
(4)
The parameters in these equations have the following values for the TraPPE force field: KL ⁄KM = 6.250 × 10$ K rad>, B5 = 114°, C ⁄KM = −68.19 K, C8 ⁄KM = 791.32 K. For the calcite force field, the bonds between carbon and oxygen have a fixed length
of 0.118 nm, while there are no bonds between calcium and carbonate. The parameters for the angle and dihedral contributions for calcite are as follows: KL ⁄KM = 2.228 × 10U K rad>,
B5 = 120°, C ⁄KM = 3477.1 K.
Table 1 - Lennard-Jones parameters and charges for the TraPPE force field28,29 for n-alkanes and CO2. Pseudo-atom CH4 CH3 (n-alkane) CH2 C (in CO2) O (in CO2)
= DkJ nm ⁄molI 3.57 × 10U 2.52 × 10U 2.21 × 10U 2.08 × 10[ 1.70 × 10?
=? DkJ nm? ⁄molI 1.32 × 10> 9.06 × 108 5.81 × 108 4.32 × 10$ 2.11 × 108
3 0.0 0.0 0.0 +0.70 -0.35
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Table 2 - Lennard-Jones parameters and charges for the calcite force field30. =\ and ]\ refer to the oxygen and carbon in the carbonate ion =]8> . Ca Cm Om
= DK^ :; ⁄;_` I 2.52 × 10[ 1.44 × 10U 1.77 × 10?
=? DK^ :;?⁄;_` I 1.42 × 108 4.61 × 108 2.03 × 108
3 +1.668 +0.999 -0.889
Table 3 – Pairwise Lennard-Jones interaction parameters for the calcite force field30. =\ and ]\ refer to the oxygen and carbon in the carbonate ion =]8> . Ca Cm Cm Om
Om Cm Om Om
= DkJ nm ⁄molI =? DkJ nm? ⁄molI 0.0 9.49 × 10[ 4.61 × 10? 1.43 × 10> 3.08 × 10$ 9.04 × 10
