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Carbon dioxide-enhanced metal release from kerogen

Heavy metals released from kerogen to produced water during oil/gas extraction have caused major enviromental concerns. To curtail water usage and production in an operation and to use the same process for carbon sequestration, supercritical CO2 (scCO2) has been suggested as a fracking fluid or an oil/gas recovery agent. It has been shown previously that injection of scCO2 into a reservoir may cause several chemical and physical changes to the reservoir properties including pore surface wettability, gas sorption capacity, and transport properties. Using molecular dynamics simulations, we here demonstrate that injection of scCO2 might lead to desorption of physically adsorbed metals from kerogen structures. This process on one hand may impact the quality of produced water. On the other hand, it may enhance metal recovery if this process is used for in-situ extraction of critical metals from shale or other organic carbon-rich formations such as coal.


Scientific Reports
| (2022) 12:15196 | https://doi.org/10.1038/s41598-022-19564-z www.nature.com/scientificreports/ been proposed as a fracking fluid or an enhanced oil/gas recovery agent. Upon injection, scCO 2 adsorbs onto kerogen structures and displaces CH 4 and oil 30 . The adsorbed scCO 2 remains locked in nanoporous kerogen structures. Many studies have demonstrated that injected scCO 2 may cause dramatic changes in wettability of kerogen 26,31 . In this communication, we will investigate how the scCO 2 injected would affect metal adsorption on kerogen surfaces. We will conduct molecular dynamics simulations for the metal adsorption on overmature and top of the oil window kerogen (type IID and IIB, respectively) 32 in the presence or absence of scCO 2 . We will show that injection of scCO 2 may greatly enhance the release of adsorbed metals from kerogen surface. The work presented below will provide the first assessment of the impact of scCO 2 on the ion adsorption on kerogen and highlight the importance of kerogen-metal interactions in controlling the quality of produced water and the efficiency of potential in-situ extraction of critical metals from shale or other organic carbon-rich formations such as coal. Different from the current research theme related to kerogen, which focuses mainly on oil/gas adsorption and transport, the work presented will emphasize metal-kerogen interactions under an influence of scCO 2 .

Method
Simulation snapshots provided in Fig. 1 illustrate the model setup for simulating Cu 2+ , Cs + , Cl − , and OH − adsorption onto a porous kerogen surface in the presence or absence of scCO 2 . Cu 2+ and Cs + ions were selected to represent common metal cations found in produced water 4 . Overmature and top of the oil window kerogen structural models (type IID and IIB, respectively) 32 were used in our simulations. The chemical formulas for kerogen IIB and IID are C 234 H 263 O 14 N 5 S 2 and C 175 H 102 O 9 N 4 S 2 , respectively. The kerogen surfaces in Fig. 1 were constructed in our previous work 14 . There are -OH functional groups in kerogen. However, for simplicity, no protonation/deprotonation would be allowed in the simulation (i.e., the kerogen surfaces remained to be charge neutral). No information is available on surface protonation/deprotonation of kerogen. The point of zero charge of other alike natural carbon materials such as algae and coal charcoals were found to be close to neutral pH 33 . Therefore, the assumption of no surface protonation/deprotonation may be a reasobale approximation of an actual system. The composition, number of molecules, and simulation box size for all simulations are reported in Table 1. Water molecules were initially placed near the surfaces and ions were randomly distributed in water. For the systems without scCO 2 (Fig. 1A), simulations were conducted in the NVT (constant number of atoms, volume, and temperature) ensemble, with a vacuum volume in the simulation box. CO 2 molecules were then filled in the vacuum volume to create the systems with scCO 2 (Fig. 1B) to study the effect of scCO 2 on ion adsorption.
With the presence of scCO 2 , the simulations were run in the NPT (constant number of atoms, pressure, and temperature) ensemble with a 200 atm pressure imposed in the z dimension. The temperature was set at 300 K for all simulations. The temperature and pressure were controlled using the Nose-Hoover thermostat 34,35 . All systems were simulated until an equilibrium condition reaches (e.g., the number of ions adsorbed on a surface is constant). Accordingly, the simulations without CO 2 were run for 35 ns, while the simulations with CO 2 were run for 60 ns to 90 ns. Water molecules were simulated using a flexible SPC water model 36 . Cu 2+ ion parameters were taken from Babu and Lim 37 , which accurately reproduce hydration energies. Cs + and Cl − ions were described using Smith and Dang models 38,39 . Lennard-Jones (LJ) parameters for OH − ions are similar to those of a SPC water model, and O charge is − 1.41e and H charge is 0.41e 40,41 . CO 2 molecules were modeled using the TRaPPE force field 42 . The rigidity of a CO 2 molecule was maintained by using the algorithm proposed by Kamberaj 43 . The CVFF force  Table 1). Color codes: kerogen-silver, water-red, Cu 2+ -blue, Cl − -cyan, and CO 2 -green. Simulation box size and number of molecules simulated for each system are reported in Table 1. Some water and CO 2 molecules can adsorb deeply inside the kerogen porous structure. However, no ion is observed inside the structure due to small pore size. www.nature.com/scientificreports/ field 44 was used for kerogen (a LAMMPS 45 data file containing all force field parameters for the kerogen molecule IID can be found in our previous paper 30 ). The pairwise LJ potential energy was expressed as: where r is the distance between two atoms, ε and σ are the depth of the potential energy well and the distance at which the LJ potential is zero, respectively. LJ interactions among atoms were calculated using the Lorentz-Berthelot mixing rules ε ij = √ ε ii ε jj and σ ij = (σ ii + σ jj )/2 . Short range interactions were calculated using a cut-off distance of 10 Å. Long range electrostatic interactions were computed using the PPPM (particle-particle-particle-mesh) solver 46 . All simulations were conducted using the LAMMPS code 45 .

Results
Metal adsorption on kerogen surfaces. Figure 2A reports the number of Cu 2+ and Cl − ions and water molecules as a function of distance to the closest kerogen atoms. These data are obtained for the IID-CuCl 2 and IIB-CuCl 2 systems (Table 1). Because the kerogen surface is very rough 31 , the profile of the number of each species from the closest kerogen atoms (instead of the density profile) is the appropriate selection to quantify the adsorption. The results indicate that Cu 2+ ions prefer to adsorb as outer sphere complexes (the first Cu 2+ peak locates at ~ 4.5 Å away from kerogen atoms, between the first and second water peaks, and the second Cu 2+ peak locates at ~ 7.1 Å away from kerogen atoms, beyond the second water peak). The adsorption of Cu 2+ ions depends on the interactions of Cu 2+ ions with water molecules and with kerogen surfaces. Because the kerogen Table 1. Simulation box size and number of molecules in each simulation system. In the IID-Cu(OH)Cl-CO 2 system, kerogen type IID is simulated with Cu 2+ , OH − , Cl − , CO 2 , and H 2 O. In the IIB-CsCl system, kerogen type IIB is simulated with Cs + , Cl − , and H 2 O.    47 ) that makes it difficult to strip water molecules from the hydration shell to form an inner sphere complex, thus different from its inner-sphere adsorption on silica and alumina surfaces (inner spheres) 48,49 .
The results in Fig. 2A also suggest that some Cl − ions adsorb closer to kerogen atoms, i.e., as inner sphere complexes (Fig. 3A), as indicated by the first Cl − peak locating at 3.75 Å away from kerogen atoms, closer than that for Cu 2+ ions, which is consistent with the lower hydration energy of Cl − (− 81.2 kcal/mol) 47 . However, the majortiy of Cl − ions adsorb still as outer-sphere complexes as the predominant Cl − peak locates at 6.2 Å away from the kerogen surface. The results in Fig. 2A also indicate that there is not any significant difference in the ion adsorption between kerogen IIB and kerogen IID. Compared with kerogen IID, kerogen IIB is less matured and has more funtional groups (e.g., higher O/C, S/C, and N/C ratios) 32 . Note that these ratios are generally small (e.g., 0.1 for O/C) 28 , and thus the atoms that an adsorbed ion can "see" on the kerogen surface are mainly C and H. This may be the reason why we do not observe a significant effect of kerogen maturity on ion physical adsorption.
In Fig. 2B, C, we compare Cu 2+ and Cs + adsorption on kerogen IID and IIB surfaces. The results for Cs + ions are obtained for the IID-CsCl and IIB-CsCl systems (Table 1). We observe a low intensity Cs + peak at 3.75 Å away from the kerogen atoms, suggesting inner sphere adsorption (see a snapshot in Fig. 3B). The Cs + hydration energy is about − 60 kcal/mol 47 , much smaller than that of Cu 2+ (− 480.4 kcal/mol), thus making it easier to strip water molecules to form inner sphere complexes. However, since Cs + ions weakly interact with the neutral kerogen surface, the majority of Cs + ions prefer to locate at the same position of the second water layer on both kerogen IIB and IID. In contrast, Cu 2+ ions prefer to avoid the dense water layers and adsorb between the first and the second water layers, or beyond the second water layer.
In Fig. 2D we compare Cu 2+ adsorption onto kerogen IID surface from solutions with or without OH − ions for the IID-CuCl 2 and IID-Cu(OH)Cl systems ( Table 1). The results indicate that the first Cu 2+ peak observed for IID-CuCl 2 system (red lines) diminishes due to the presence of OH − ions (green lines). Because of very limited amount of OH − ions found near the kerogen surface, compared to Cl − ions (Fig. 2E), and because of Cu 2+ -OH − ions paring (Fig. 2F, i.e., more Cu 2+ ions pair with OH − ions than with Cl − ions), the adsorption of Cu 2+ ions can be considered as the adsorption of Cu 2+ -OH − pairs. These complexes affect the amount of Cu 2+ ions accumulate near the surfaces (e.g., the first Cu 2+ peak), but do not affect Cu 2+ accumulation far away from the surface (second Cu 2+ peak). Fig. 4A we report the results for IID-CuCl 2 -CO 2 system (Table 1) to eluciate the effect of supercritical CO 2 on ion adsorption. Note that CO 2 molecules are initially added to the vacuum space in Fig. 1A. During the simulation CO 2 molecules diffuse through water and adsorb onto kerogen structure (Figs. 1B, 4B). The results indicate that after CO 2 is added, the first and second Cu 2+ peaks for the system without CO 2 (i.e., IID-CuCl 2 system) diminish (red vs. green lines), suggesting that Cu 2+ ions desorb from the kerogen atoms. When Cu 2+ ions within 6 Å from the kerogen atoms (i.e., the first minimum on the red line, Fig. 4A) are considered, about 78% of the adsorbed cations desorbs from kerogen in the presence of scCO 2 . When Cu 2+ ions within 8.2 Å from kerogen atoms (i.e., the second miniumum on the red line, Fig. 4A) are considered, about 60% of the cations desorbs after scCO 2 is introduced. In other words, injection of scCO 2 causes the adsorbed Cu 2+ ions to desorb from kerogen surfaces. The adsorption of scCO 2 on kerogen surface is indicated by a CO 2 peak at 3 Å away from kerogen atoms (purple line, Fig. 4A). The purple profile for CO 2 also demonstrates the formation of a monolayer of CO 2 on a kerogen surface and a futher decrease in the number of CO 2 away from the surface due to the limited CO 2 solubility in water. When CO 2 molecules accumulate near the surface, they partly replace water molecules, leading to the lower intensity water peak (blue line, Fig. 4A vs. blue line, Fig. 2A) and desorption of adsorbed ions (Fig. 4C-F). The desorption of water from kerogen surface due to scCO 2 adsorption was initially reported in our previous work 31 . The adsorbed layer of scCO 2 between water and kerogen surfaces acting like a lubricant to facilitate water flow on the kerogen surfaces. The main reason for a CO 2 molecule substitution for a H 2 O molecule to adsorb on the surface is because CO 2 interacts with kerogen surface more strongly than H 2 O (− 6.2 kcal/mol for CO 2 vs. − 4.7 kcal/mol for water) 26 . The adsorbtion of CO 2 also causes the change in wettability of kerogen (i.e., increases hydrophobicity) 26 . These phenomena were computationally confirmed by other groups 50 . Note that increasing hydrophobicity of kerogen upon injection of scCO 2 can enhance water exclusion, and therefore might futher increase water release (and hence heavy metals). Our current work provides the first assessment of the impact of scCO 2 on the ion adsorption, which requires futher experimental investigation.

Conclusions
Using molecular dynamics simulations, we investigated ion adsorption on kerogen surface in the presence or absence of scCO 2 . Due to weak interactions of ions with neutral kerogen surfaces, the majority of Cu 2+ , Cs + , Cl − , and OH − ions adsorb as outer sphere complexes. Some Cs + and Cl − ions adsorb as inner sphere complexes. We also found that the presence of OH − ions reduces the number of Cu 2+ ions adsorbed due to ion paring. All ions were observed to be desorbed when scCO 2 was introduced to the system. For the conditions simulated in this work, we observed that about 60% of Cu 2+ , 50% Cs + , and 55% Cl − within ~ 8 Å from the kerogen atoms desorb when introducing scCO 2 into the system. This process on one hand may impact the quality of produced water. On the other hand, it may enhance metal recovery if this process is used for in-situ critical metal extraction from shale or other organic carbon-rich formations such as coal. The work presented here can be extended and validated through adsorption and leaching experiments as well as by quantum-based calculations to further determine the kinetics and thermodynamics of metal adsorption onto kerogen under various scCO 2 pressure, environmental temperature, and kerogen maturity.

Data availability
The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.  . Number of Cu 2+ as a function of distance from the closest kerogen IID surface atoms for the IID-CuCl 2 (red line) and IID-IID-CuCl 2 -CO 2 (green line) systems (A). Distributions of water and CO 2 molecules are also shown for the IID-CuCl 2 -CO 2 system. The simulation snapshot demonstrates the adsorption of CO 2 (green) on kerogen (silver) in aqueous solution (water: red, Cu 2+ : blue, Cl − : cyan) (B). Distribution of Cs + (C) and Cl − (D) ions on kerogen IID in the presence/absence of CO 2 . Distribution of Cu 2+ (C) and OH − (D) ions on kerogen IID in the presence/absence of CO 2 for IID-Cu(OH)Cl-CO 2 system.