Coupling changes in densities and porosity to fluid pressure variations in reactive porous fluid flow: Local thermodynamic equilibrium.
- Geochem. Geophys. Geosyst.16. doi:10.1002/2015GC006019.
Abstract
Mineralogical reactions which generate or consume fluids play a key
role during fluid flow in porous media. Such reactions are linked to
changes in density, porosity, permeability, and fluid pressure which
influence fluid flow and rock deformation. To understand such a
coupled system, equations were derived from mass conservation and
local thermodynamic equilibrium. The presented mass conservative
modeling approach describes the relationships among evolving fluid
pressure, porosity, fluid and solid density, and devolatilization
reactions in multicomponent systems with solid solutions. This first
step serves as a framework for future models including aqueous
speciation and transport. The complexity of univariant and
multivariant reactions is treated by calculating lookup tables from
thermodynamic equilibrium calculations. Simplified cases were also
investigated to understand previously studied formulations. For
nondeforming systems or systems divided into phases of constant
density, the equations can be reduced to porosity wave equations with
addition of a reactive term taking the volume change of reaction into
account. For closed systems, an expression for the volume change of
reaction and the associated pressure increase can be obtained. The key
equations were solved numerically for the case of devolatilization of
three different rock types that may enter a subduction zone. Reactions
with positive Clapeyron slope lead to an increase in porosity and
permeability with decreasing fluid pressure resulting in sharp fluid
pressure gradients around a negative pressure anomaly. The opposite
trend is obtained for reactions having a negative Clapeyron slope
during which sharp fluid pressure gradients were only generated around
a positive pressure anomaly. Coupling of reaction with elastic
deformation induces a more efficient fluid flow for reactions with
negative Clapeyron slope than for reactions with positive Clapeyron
slope.