Guha Roy, Debanjan

Molecular simulations are efficient tools in differentiating individual effects of fluid-fluid interactions and pore-fluid interactions on thermophysical properties of confined fluids; e.g. the molecular packing, adsorption mechanics and availability of accessible pore volume for confined fluids and therefore, indicate the rock fracturing phenomena as a function of geological conditions, fracking fluids nature, its composition and rock mineralogy. Presently, we have deployed the classical GCMC molecular simulations to quantify the adsorption of pure nitrogen and N2–H2O mixture (50%–50% and 30%–70%) inside porous silica rocks. While we found that adsorption and molecular packing of pure nitrogen inside silica slit pores are only a function of pore height, which quantifies the pore-fluid interactions; however, for N2–H2O mixture adsorption and molecular packing of N2 inside silica slit pores has been additionally affected by the water content in the equilibrium bulk mixture that as well describes fluid-fluid interactions inside pores. It is interestingly noted that water in N2–H2O mixture results in water-assisted nitrogen adsorption inside hydrophilic silica slit pores, which has been further proven through the radial distribution function data calculations inside each slit pore. Also, the hydrophilic nature of silica increases water adsorption and hence reduces N2 adsorption inside the smallest pore of H = 20 Å. Such a reduction in N2 adsorption density below its bulk density without layering effect, inside 20 Å pore, further initiates the possibility of negative excess adsorption density.

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. © 2026 American Chemical Society