Upward brine migration resulting from pressure increases in a layered subsurface system

Abstract

Upward displacement of brine from deep geological formations poses a potential threat to near-surface drinking water resources. In this work, the impact of a layered sequence of hydraulically permeable and impermeable layers connected by a vertical fluid pathway like, e.g., a fault is investigated using an idealized scenario and numerical process simulation. Long-term upward brine migration is induced by overpressure in the lowest permeable formation, and the upward migration through the vertical pathway and the interaction with the intermediary permeable layers is investigated. The simulations show that brine displaced upwards through the vertical fluid pathway moves into the intermediary permeable formations, settling into the lower parts of the permeable layers and displacing the resident less salty formation brine from this layer further upwards through the vertical pathway. Thus, formation brine from different depths displaces each other rather than mixing along the pathway or rising along the full length of the vertical pathway. An effect of the gradual upward displacement is a decrease in the salt concentrations along the pathway such that brine intrusion into the groundwater aquifer is reduced. However, if the hydraulic connection between the vertical pathway and the intermediary layers is low, higher-density brine accumulates in the vertical pathway and upward movement of the brine is impeded due to its own weight.