Carbon dioxide-enhanced metal release from kerogen

Abstract

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.

Figure 1

Simulation snapshots illustrating IID-CuCl₂ (A) and IID-CuCl₂-CO₂ (B) model systems (see Table 1). Color codes: kerogen—silver, water—red, Cu²⁺—blue, Cl⁻—cyan, and CO₂—green. Simulation box size and number of molecules simulated for each system are reported in Table 1. Some water and CO₂ molecules can adsorb deeply inside the kerogen porous structure. However, no ion is observed inside the structure due to small pore size.

Figure 2

Number of ions (Cu²⁺ and Cl⁻) and water molecules as a function of the distance to the closest kerogen atoms (A). Comparison of Cu²⁺ and Cs⁺ adsorption between kerogen IID (B) and IIB (C) surfaces. Comparison of Cu²⁺ ion adsorption for the systems with and without OH⁻ ions, i.e., for IID-CuCl₂ (red) and IID-Cu(OH)Cl (green) systems (D). Comparison of OH⁻ and Cl⁻ ion adsorption on kerogen IID obtained for IID-Cu(OH)Cl system (E). Cu²⁺–Cl⁻ and Cu²⁺–OH⁻ paring calculated from IID-Cu(OH)Cl system (F).

Figure 3

Simulation snapshots demonstrating the inner sphere complexes of Cl⁻ (A) and Cs⁺ (B) on kerogen IID surface. See Fig. 1 for the color codes.

Figure 4

Number of Cu²⁺ as a function of distance from the closest kerogen IID surface atoms for the IID-CuCl₂ (red line) and IID-IID-CuCl₂-CO₂ (green line) systems (A). Distributions of water and CO₂ molecules are also shown for the IID-CuCl₂-CO₂ system. The simulation snapshot demonstrates the adsorption of CO₂ (green) on kerogen (silver) in aqueous solution (water: red, Cu²⁺: blue, Cl⁻: cyan) (B). Distribution of Cs⁺ (C) and Cl⁻ (D) ions on kerogen IID in the presence/absence of CO₂. Distribution of Cu²⁺ (C) and OH⁻ (D) ions on kerogen IID in the presence/absence of CO₂ for IID-Cu(OH)Cl-CO₂ system.