Molecular Simulation Analysis of Nitrogen-Based Fracking Fluid Behavior in Silica-Kerogen Composite Rock Pores

This study focuses on quantifying the adsorption of pure N2 and N2/H2O mixtures (energized fluids) of two different compositions inside porous silica–kerogen composite porous rocks (SKC-Sp), via classical Grand Canonical Monte Carlo (GCMC) simulations at geological reservoir thermodynamic conditions (P = 52–93 MPa and T = 312.61 K to 356.58 K). Hence, we aim at performing waterless fracking for complex rock pores using pure N2 and N2/H2O mixtures, which are becoming viable candidates in field applications. Pure N2 adsorption is expected to be higher inside SKC-Sp than the mixture state N2 adsorption from both energized fluids. However, while the pure nitrogen adsorption density decreases in SKC-Sp with an increase in pore height, in the case of the mixture state nitrogen, the adsorption density increases inside wider pores due to water-induced nitrogen adsorption, as also reported and observed inside pristine silica slit pores (PS-Sp). Interestingly, the presence of hydrophobic kerogen in SKC-Sp increases the pure N2 adsorption density by promoting pore filling compared to the layering of pure N2 inside PS-Sp. However, for the mixture state N2, a reverse trend is observed between SKC-SP and PS-Sp due to competitive water-induced nitrogen adsorption, which is primarily governed by the molecular ordering of water inside hydrophobic SKC-Sp. Inside SKC-Sp, mixture state water fills the pores, which causes “sticking” of mixture state N2 at the pore center. In contrast, inside PS-Sp, layering of water alone occurs near the hydrophilic silica pore walls. The estimation of the disjoining pressure across the slit pore heights shows more stable pure nitrogen adsorption inside SKC-Sp, whereas energized fluids are more stable inside hydrophilic PS-Sp. Furthermore, the selectivity (SN2,H2O) and thermodynamic ratio (κ) of water and nitrogen indicate higher adsorption of water inside both SKC-Sp and PS-Sp, though competitive mixture state N2 adsorption is favored in PS-Sp than in SKC-Sp.