Coupling changes in densities and porosity to fluid pressure variations in reactive porous fluid flow: Local thermodynamic equilibrium

Abstract

AbstractMineralogical 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.