Experimental Observation of Flow, Dissolution and Precipitation in Fractured Carbonate Rocks

A series of laboratory experiments was carried out in order to study permeability fields and flow patterns induced by simultaneous precipitation and dissolution processes in fractured carbonate rocks (calcareous sandstone and dolomite). In particular, the influence of fracture orientation relative to the direction of flow was studied; isolated and through-flow fractures were each considered. Dissolution and precipitation are two of the most important processes affecting groundwater chemistry, and they can significantly modify the physical and chemical properties of fractured media. Two types of linear flow experiments were performed: (1) a 2D flow cell containing 1 cm thick rock slices was used for single fracture experiments; and (2) 3D rock cores containing fractures were investigated to study the interplay of flow between fractures and porous host rock. In both experimental systems, different acid mixtures were injected at various flow rates. Changes in head gradient were recorded at specific time intervals during the experiments, and the overall hydraulic conductivity was then calculated as a function of time. The outlet effluent was collected and analyzed for different ion concentrations in order to calculate porosity changes (based on molar volume of the components) during the experiments. The evolution of hydraulic conductivity in the fractured rock samples, due to competition among flow, precipitation and dissolution processes, was found to display a variety of possible behaviors. In all cases, the flow through the system ends either by clogging of the porous host rock and/or the fracture(s) (due to precipitation and mechanical movement of particles) or by collapse of the rock sample (due to dissolution). Different structures of precipitate were found in the porous host rock, and in both isolated fractures and through-flow fractures.