(155d) Pressure-Driven Brine Flow Behaviors in Silica Mesopores from Molecular Perspectives

The pressure-driven water flow behaviors in mesopores are of great importance for subsurface energy extraction and geological carbon sequestration (GCS) in shale/tight formations where pore sizes range from a few nanometers to several micrometers. On one hand, formation water (the so-called brine) has exceptionally-high salinity, up to 35 wt%. Herein, the presence of salt ions and ionic hydration alter hydrogen bonding (HB) network among water molecules. On the other hand, silica is one of the most abundant minerals in reservoir formations, while its surface chemistry is dependent on solution pH. Specially, silica surface deprotonation and charge properties have a profound impact on water-surface HB formation. Despite the extensive researches on brine flow in silica nanochannels, the effect of microstructures (in particular, HB networks and ionic hydration depending on surface chemistry) on brine flow behaviors, especially in the brine-surface interfacial region, remains largely unclear.

To investigate pressure-driven brine flow in silica mesopores under a typical reservoir condition (323 K and 20 MPa), molecular dynamic (MD) simulations are conducted to study the collective effects of nanoconfinement, surface chemistry, and intermolecular interactions. The bulk salinity is set at ~10 wt%, while the pore size is kept at ~5 nm as a representative silica mesopore. The surface chemistry effect is taken into account by varying the deprotonation degrees of silica surfaces.

Water molecules exhibit multilayer structures near the substrate, and surface counter ions accumulate strongly near the fully-deprotonated silica surface. The 1^st and 2^nd water layer (WL) are formed due to surface HBs. The 3^rd WL in fully-protonated silica slits is mainly determined by HBs among H₂O molecules, while the formation of the 3^rd WL in fully-deprotonated ones is also partially due to surface counter ion hydration. Furthermore, three flow regions are observed in the velocity profile in fully-deprotonated silica slits, referred as adsorption region, transition region and bulk region, respectively. However, the transition region is negligible in fully-protonated ones. Strong surface HB and the high degree of hydration of stick ions collectively hinder the mobility of water molecules in the adsorption region. Even in the absence of surface HB, the hydration of stick ions impedes water flow, resulting in a transition region in fully-deprotonated silica slits.

Our work elucidates the effects of surface chemistry and interfacial water structures which are dictated by fluid-fluid and fluid-surface interactions as well as ion hydration on interfacial brine flow behaviors in silica mesopores from molecule perspectives. The fundamental understanding and knowledge from this work can provide important insights into multi-phase multi-component flow behaviors in nanoporous media which are imperative to subsurface energy extraction and gas storage.