Occurrence state and nuclear magnetic resonance relaxation characteristics of confined water in quartz nanopores

Abstract

Water in the shale obstructs the enrichment and transportation of shale gas, so it is crucial to identify the occurrence mechanism of water in shale. In this work, molecular dynamics (MD) simulation had been performed to explore the occurrence state, dynamic characteristics, and nuclear magnetic resonance (NMR) relaxation properties of confined water in shale inorganic nanopores (quartz). The results show that the number of adsorbed layers, diffusion coefficients (D) and rotational correlation time (τ c) of water in quartz pores are strongly influenced by pore width (H). Layer analysis of water held in confinement indicates that the D value increases and the τc value decreases as the water approaches the centre of the pore. Furthermore, varying occurrence states lead to different NMR relaxation mechanisms. With the increase of H, the transverse relaxation time of adsorbed water is basically stable at 10−1 s. Finally, an exponential formula for evaluating pore size distribution is established. The total thickness of the water layer with the ratio of the intramolecular relaxation rate to the total relaxation rate less than 40% is defined as the thickness of the adsorbed water film.