Spontaneous wormhole formation in dissolving rock fractures and its effect on early development on karst conduits

A dissolution front in a single rock fracture is potentially unstable to small variations in local permeability, leading to spontaneous formation of wormhole-like channels in the rock. These channels have a very high flow rate and can carry an under-saturated solution deep into the fracture, thus dramatically increasing the conduit growth rate. We have carried out numerical investigation of dissolution processes in fractures by means of a pore-scale numerical model of fluid flow and chemical transport. The effects of flow rate, mineral dissolution rate and the geometrical properties of the fracture were investigated and optimal conditions for wormhole formation determined. The timescales for solutional widening of initially narrow fissures are compared with those obtained with a simple one-dimensional model of fracture dissolution. One-dimensional models, in which the fracture aperture is assumed to depend only on the distance from the inlet, are frequently used in quantitative assessments of conduit growth. Although analytically tractable, such models cannot account for wormhole formation and thus their results need to be critically reassessed. Wormholing, if present, leads to a dramatic decrease of fissure widening times, since the flow becomes focused in highly permeable channels, which transport the reactants and products very efficiently. These effects must be taken into account not only in the estimation of karstification times but also in the assessement of ground subsidence, dam collapse or toxic seepage risks