Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology The Impact of Short-Range Force Field Parameters and Temperature Effect On Selective Adsorption of Water and CO2 On Calcite PHAN VAN CUONG, BJØRN KVAMME, TATIANA KUZNETSOVA*, BJØRNAR JENSEN Institute for Physics and Technology University of Bergen Postboks 7803 NORWAY [email protected] http://www.ift.uib.no Abstract: - Carbon dioxide can be captured and stored in geological formations. This promising technique of carbon sequestration can contribute both to greenhouse effect reduction and enhanced oil recovery. However, all processes occurring during injection, post-injection, and storage occur in porous rock, making interactions and reactions between CO2, water, and minerals to be of utmost importance. In this work, molecular dynamics (MD) simulations were used to study several aqueous interfacial systems involving CO2 focusing on the impact of force field, calcite and temperature variations. Our investigation showed that CO2 transport and interface stability were heavily affected by temperature, calcite, and force field utilized. As temperature increased, the number of CO2 molecule crossing water layer and adsorbing on calcite surface increased while adsorption stability deteriorated. When we applied Buckingham potential between water and calcite, all other interactions were Lennard-Jones (L-J), electrostatic contribution proved to be the deciding factor with the coordination of CO2 oxygen towards the calcium ions in calcite being the most important factor that ensures the stability of calcium-CO2 pairs. When Buckingham potential is applied for both water-calcite and CO2-calcite interactions, with the rest being L-J in form, the coordination of CO2 carbon towards the carbonate oxygen becomes the decisive factor. Key-Words: - Calcite; Carbon Dioxide; Water; Adsorption; Molecular Simulation 1 Introduction In this work, we used molecular dynamics Carbon dioxide can be captured, transported, and simulations to study several aqueous interfacial permanently stored in geological formations systems involving CO2 and calcite. Our main focus including spent petroleum reservoirs [1]. This was on the impact of calcite and temperature promising technique of carbon sequestration can variations on transport, adsorption, and stability of contribute both to greenhouse effect reduction and CO2 molecules and water as affected by the enhanced oil recovery (EOR). However, all presence of )4110( calcite surface. A special processes occurring during injection, post-injection, attention was paid to role of short-range and storage occur in either porous rock or inside contributions to the intermolecular potentials. rust-covered pipes, making interactions and reactions between CO2, water, and minerals to be of utmost importance. 2 System setup and molecular Calcite is one of the most abundant minerals in simulation details the Earth’s crust, with )4110( plane being the The composite system was built from a 1620-atom most stable [2] and by far the dominant observed slice of calcite crystal [7, 8], two water slabs, a morphology of calcite in situ [3]. Atomistic-scale hydrate crystal, and a carbon dioxide phase with the interactions between )4110( calcite surface and density corresponding to 200 atm and 277 K. The various substances like pure water, aqueous calcite slice was positioned in the middle of the 40 solutions, peptides, etc. have been the subject of Å-thick liquid water block and parallel to the initial several numerical studies already [4-6]; it continue water-CO2 interface. A carbon dioxide hydrate slab to draw interest because of the decisive role they composed of 4x4x2.5 structure I unit cells was play in determining both macroscopic properties and added to probe the potential competition for carbon kinetics of processes. dioxide between hydrate and calcite in reservoirs under conditions where hydrate formation is ISBN: 978-1-61804-065-7 192 Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology possible. The second water phase consisted only of Our molecular dynamics used the rigid body 500 water molecules meant to cushion the hydrate treatment for all molecules. All calcite crystal atoms crystal from the carbon dioxide. The resulting were fixed in place, as our previous studies found primary simulation cell ranged 48 x 48 x 108 Å in the rearrangement of hydrate calcite surface to be size and is shown in Fig. 1. rather small. Water molecules in hydrate were locked in space but free to rotate around their centers, while the CO2 guests were completely free. We used Linux-based Message-Passing Interface (MPI) to run the MD simulations in parallel on 88 processors of Cray XT4 supercomputing facility at the University of Bergen, Norway. 2.2.1 Force fields and the impact of short-range interactions The force field used the conventional approach Fig 1. Side view snapshot of the initial system. describing potential energy as a sum of individual non-bonded energy terms with two contributions, 2.1 Fractional charges in calcite the electrostatics and the van der Waals. Maestro/Jaguar quantum chemistry package [9, 10] PS Conventional Lorentz-Berthelot mixing rules were utilizing B3LYP with LACVP basis set and with used to calculate the cross interactions. The short- force convergence flag set was used to estimate the range potential energy between CO2 and water was partial charges in vacuum for a 210-atom calcite represented by the Lennard-Jones potential of [13] slab cleaved along the dominant )4110( plane. and modified F3C [4, 14] models, respectively. Except for the edges of the crystal, vacuum charges The inclusion of carbon dioxide in the system proved to be quite uniform. These values are listed resulted in the additional challenge concerning the in Table 1; they agree quite well with those of Fisler description of interaction between calcite and CO2. et al. [11] where the calcium ion charge were kept In the absence of available experimental results fixed at +2, and the focus was on the carbonate characterizing the behavior of carbon dioxide close group, allowing carbon and oxygen charges to vary to calcite or similar minerals, we found it necessary but constraining their sum to -2. In our approach to test a series of short-range potentials that ranged that used Maestro/Jaguar, all atomic charges in from pure Lennard-Jones, a combination of calcite were free to vary while their sum was Lennard-Jones and Buckingham potentials, and pure constrained to zero. The charges were then mapped Buckingham interactions between calcite and CO2, onto a 1620-atom calcite slab cleaved out of a larger with the Buckingham CO2 model fitted to reproduce calcite crystal along the )4110( plane. bulk properties [15] (see Table 2). Table 1. Fractional atomic charge in calcite Table 2. Lennard-Jones and Buckingham force field parameters for carbon dioxide ([13] and [15]) and Atom Fractional charge (e) water ([2] and [14]) This work Fisler et al. [13] O in H in C O in CO2 Calcium 1.881 2.000 H2O H2O Charge Carbon 1.482 1.344 [e] 0.6512 -0.3256 -0.8476 0.4238 Oxygen -1.118 -1.115 σ [Å] 2.7570 3.0330 3.1666 0.8021 ε 0.2339 0.6657 0.7732 0.04184 2.2 MD details [kJ/mol] Molecular dynamics MDynaMix package [12] was A [kJ 1491.6 1629.9 2889.7 106.2 employed, with temperature kept constant at three Å6/mol] different temperatures (277, 388, and 500 K). The B 909.23 1483300.0 293206.3 11537.16 time step was 0.5 fs, with periodic boundary [kJ/mol] conditions applied in all three directions. The cut-off C [1/Å] 2.27 4.4 3.659 3.875 radius for the Lennard-Jones potential was set to 10 Å. ISBN: 978-1-61804-065-7 193 Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology The Lennard-Jones parameters used for calcite were Table 5. Location and depth of minima for taken from [16] which featured a rather deep and Buckingham interaction between calcite and water narrow well in case of the calcium ion. The goal of Depth Calcite-H O r [Å] this unusual force field was apparently to emphasize 2 min [kJ/mol] the role of short-range contributions to override the normally dominant electrostatic forces [5, 17, 18]. C (calcite) - H (H2O) 3.54 -0.140 Table 3. Lennard-Jones and Buckingham force field C (calcite) - O (H O) 3.73 -0.551 parameters for calcite 2 +2 Ca C O Ca (calcite) - H (H2O) 3.87 -0.339 1.482 -1.118 Charge, q [e] 1.881 Ca (calcite) - O (H2O) 4.11 -1.280 σ [Å] 0.899 3.742 2.851 O (calcite) - H (H O) 3.31 -0.142 ε [kJ/mol] 113.819 0.5021 0.6657 2 A [kJ Å6/mol] 55686.7 2432.71 1123.56 O (calcite) - O (H2O) 3.48 -0.556 B [kJ/mol] 82942.86 369822.7 230230.1 C [1/Å] 2.198 3.6019 3.9602 3 Results and discussion 3.1 "Hybrid" Lennard-Jones -- Buckingham As seen from Table 2, the Lennard-Jones radii and system well depths of water and carbon dioxide oxygen are This system was characterized by the unusually not too dissimilar, which would make the short- strong Lennard-Jones interaction between calcium range interactions of water and carbon dioxide with in calcite and carbon dioxide [19] and regular calcite comparable in strength. In this case one Buckingham potential for water and calcite [2]. would expect the electrostatics to determine the At all three temperatures (277, 388, and 500 K), relative affinities between various substances and CO molecules managed to cross the aqueous layer the mineral surfaces. 2 to reach the calcite slab surface where they were The Buckingham potential between calcite and able to successfully displace the original water water was adapted from [2].