Computational Simulation of Three-Dimensional Behaviour of Chemical Dissolution-Front Instability in Fluid-Saturated Porous Media

Abstract

Numerical simulation of chemical dissolution-front instability in fluid-saturated porous media is an important topic in the field of computational geosciences, which is a beautiful marriage between the contemporary computational mechanics and traditional geosciences. If fresh water is injected into a fluid-saturated porous medium with the solute (i.e. chemical-species) being in an equilibrium state, it can break the equilibrium state of the solute so that the whole system becomes chemically far from equilibrium. To drive the system towards a new chemical equilibrium state, the solid part of the solute is dissolved within the porous medium. This process causes an increase in the porosity of the porous medium. The resulting porosity increase can cause a corresponding increase in the permeability of the medium so that the pore-fluid flow can be enhanced within the porous medium. This means that both permeability and diffusivity are dependent on the porosity of a porous medium (Bear 1972). When the injected fresh water flow is relatively weak, the chemical dissolution front is stable so that a planar chemical dissolution-front remains the planar shape during its propagation within the porous medium. However, when the injected fresh water flow is strong enough, the chemical dissolution front becomes unstable. In this case, a planar chemical dissolution-front can be changed into a complicated and complex morphology during its propagation within the porous medium. This is the scientific problem, known as the chemical dissolution-front instability problem in the fluid-saturated porous medium, to be considered in this investigation. It is noted that there is another mechanism that can also cause pore-fluid channeling instability due to the rheological asymmetry associated with compaction and decompaction in ductile rocks (Connolly and Podladchikov 2007). This instability is closely associated with porosity waves and can be referred to as the mechanical instability. Owing to a significant mechanism difference between a chemical dissolution instability problem and a mechanical instability one, this chapter focuses on the consideration of chemical dissolution instability problems. However, a possible interaction between these two different mechanisms may need to be considered in a future investigation.