Adsorption Isotherms Analysis of Liquid Nitrogen inside Montmorillonite and Kaolinite Rock Pores using Molecular Simulations

Hydraulic fracturing or fracking for shale gas and oil production is a water intensive and environmentally damaging process. It is often blamed for groundwater contamination and artificial earthquakes. Therefore, cryogenic fracking using liquid nitrogen (LN₂) has recently emerged as much safer and greener alternative. This novel process enhances rock permeability by creating thermal fractures by subjecting hydrocarbon-bearing rocks to repeated freeze-thaw cycles. However, the extent and efficiency of this process depends on the constituent minerals of the rock. Clay minerals such as montmorillonite and kaolinite constitute on average approximately 30 to 35 % of shale. These phyllosilicate group of minerals, despite their common layered structures, vary in composition and arrangement, resulting in distinct properties. Therefore, total adsorption of LN₂ in the clay minerals of shale must combine insights on their individual adsorption responses. It is essential to estimate the total and residual LN₂ volumes trapped in pores that will impact the mobility of hydrocarbons. In this work, we studied adsorption behaviour of nitrogen inside the montmorillonite and kaolinite nanopores using Grand Canonical Monte Carlo (GCMC) simulations. The slit pores, with 5 nm, 8 nm, and 12 nm opening were simulated for reservoir pressures ranging from 50 to 95 MPa and temperatures from 300 to 355 K. The influence of pore size, composition, pressure, temperature, and fluid type were studied to understand the relationship between adsorption isotherms and excess properties. The Canonical Ensemble simulations were performed in conjunction with Widom’s insertion technique performed to estimate the average chemical potential and interaction among the N₂ molecules calculated via pair distribution function. The N₂ was precisely represented by TraPPE force field model. The simulated bulk phase densities of N₂ were observed to be in good agreement with literature values. As shown by the pair correlation function obtained from the Canonical Ensemble simulations, the bulk phase N₂ was in a supercritical thermodynamic state at rock-fluid equilibrium temperature and pressure. The results indicated that the pore volume on both surfaces played a crucial role in the behaviour of N₂ adsorption. It was observed that the adsorption capacity of N₂ was affected by the amount of available pore space, revealing insights into interactions between pore surface and adsorbed N₂. Additionally, the study also confirmed that the extent of adsorption was dependent on surface area and morphology of the material. The adsorption isotherms exhibited a well-defined relationship with excess properties of adsorbed N₂. Further, these simulations analysed the thermodynamic nature of adsorbed fluid within the pores using the molecular density distribution profiles across the height of the pore.