Challenges in Coupling Acidity and Salinity Transport in Porous Media

Salinity is an increasingly prescient
issue in reactive transport,
from low salinity water flooding to fracking brine leakage. Of primary
concern is the effect of salinity on surface chemistry. Transport
and batch experiments show a strong coupling of salinity and acidity
through chemical interactions at the mineral–liquid interface.
This coupling is ascribed to the combined effects of ionic strength
on electrostatic behavior of the interface and competitive sorption
between protons and other cations for binding sites on the surface.
The effect of these mechanisms is well studied in batch settings and
readily describes observed behavior. In contrast, the transport literature
is sparse, primarily applied to synthetic materials, and offers only
qualitative agreement with observations. To address, this gap in knowledge,
we conduct a suite of column flood experiments through silica sand,
systematically varying salinity and acidity conditions. Experiments
are compared to a reactive transport model incorporating the proposed
coupling mechanisms. The results highlight the difficulty in applying
such models to realistic media under both basic and acidic conditions
with a single set of parameters. The analysis and experimental results
show the observed error is the result of electrostatic assumptions
within the surface chemistry model and provide a strong constraint
on further model development.