Permeability Evolution by Dissolution and Precipitation Processes in Porous Carbonate Rocks
Abstract
Precipitation and dissolution processes, mediated by the flow rate, can significantly modify the physical and chemical properties of porous media. The evolution of permeability structure and flow patterns, controlled by simultaneous precipitation and dissolution in porous rocks, is examined in a series of laboratory experiments. Linear flow experiments were performed in columns of crushed calcareous sandstone, by injecting different concentrations of HCl/H{2}SO{4} mixtures at various flow rates. The effect of simultaneous calcium carbonate dissolution and gypsum precipitation was analyzed. Changes in head gradient (measured in manometers) were recorded at specific time intervals during the experiments. The hydraulic conductivity of the column was calculated as a function of time using these data according to Darcy's law. The effluent acid from outlet was collected and analyzed for Ca2+ and SO{4}2- concentrations in order to calculate porosity changes (based on molar volume of the components) during the experiments. After each experiment the rock sample was retrieved and sectioned in order to study the pore space geometry, micromorphology, and mineral concentrations. A range of injected H+/SO{4}- ratios and flow rates was identified which leads to oscillations in the effective hydraulic conductivity of the carbonate rock samples. Since the dissolution of calcium carbonate is a mass-transfer limited process, higher flow rates cause a more rapid dissolution of the porous medium; in such cases, the dominant process is dissolution and formation of highly conductive flow wormholes was observed. At slower flow rates, no wormhole formation was observed, but the porosity varied in different parts of the columns. Analysis of the sectioned parts of the column, after each experiment, showed that total porosity increased significantly by dissolution of carbonate near the inlet of the column and decreased along the interior length of the column by gypsum precipitation. These findings are in qualitative accordance with theoretical understanding of such phenomena.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.H52E..05S
- Keywords:
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- 1829 Groundwater hydrology;
- 1832 Groundwater transport;
- 1869 Stochastic processes;
- 5114 Permeability and porosity