Interfacial energies of supercritical CO2 and water with 2:1 layered silicate surfaces: A density functional theory study

Abstract

Interfacial energies between supercritical CO2 (scCO2) and water with a model clay surface under conditions related to hydraulic fracturing have been investigated using density functional theory (DFT) methods. Planewave DFT methods in the program VASP (Kresse and Furthmüller, 1996) were used to perform molecular dynamics (MD) simulations at a temperature of 333.33 K to calculate these interfacial energies and molecular structures. The calculated interfacial energies between scCO2 and water with the K+-bearing model clay surface were 0.022 and 0.0422 J/m2, respectively. The lower interfacial energy between scCO2 and model clay suggests that scCO2 would create a better fracturing fluid because it can enter the nanopores of clay-dominated shales more readily than water. This entrance into nanopores before fracturing occurs could create a more complex fracture network. The orientations of the scCO2 molecules with respect to the silicate layer also suggest that scCO2 is not influenced by a charged ion surface on the surface of the silicate whereas the water is. These interfacial structures also provide insight into why scCO2 would have a lower interfacial energy with silicate than water.