The role of fractures on coupled dissolution and precipitation patterns in carbonate rocks

A series of laboratory experiments is presented which investigates the influence of fractures on the evolution of hydraulic conductivity and porosity caused by flow, dissolution and precipitation-and the interplay among them-in carbonate rocks. We inject input solutions of HCl and H2SO4, at different flow rates, into carbonate rock samples containing different configurations of fractures. As a consequence, host rock dissolves and gypsum subsequently precipitates. These experiments allowed us to determine effects of fracture orientation, fracture wall roughness, fluid flow rate and chemistry, and coupled dissolution/precipitation reaction mechanisms on overall patterns of hydraulic conductivity and porosity evolution. To separate the relative effects of these parameters, flow experiments used quasi-two-dimensional (2D) rock fractures, three-dimensional (3D) intact rock cores, and 3D rock cores containing different fracture configurations. Changes in pressure gradient along the sample, recorded at specific time intervals during the experiments, were used to calculate the overall evolution of hydraulic conductivity. The effluent acid was analyzed for Ca2+ and SO42=2- concentrations to estimate corresponding porosity changes. After each experiment, the rock sample was retrieved and sectioned in order to study the pore space geometry, micromorphology, and distribution of precipitated and dissolved minerals. We find that fracture sample geometry and chemical composition of the reacting fluid are the two main factors most strongly influencing precipitation and dissolution patterns within a fracture. The interplay of these factors is controlled largely by the flow rate of the injected fluid. In 3D systems, we find that fracture orientation controls whether precipitation or dissolution is the dominant process: a through-flow fracture led to a dissolution-dominated system, in contrast to an isolated fracture which led to a precipitation-dominated system under the same experimental conditions. Moreover, comparison of the hydraulic conductivity and porosity evolution among the intact core, the isolated fracture and (multiple) fracture system experiments demonstrates that, under the same flow conditions, cores containing isolated fractures clog more rapidly than intact cores, while cores with multiple fractures clog even more rapidly than the isolated fracture systems. Finally, in spite of the complex coupling of flow and reaction processes between intact rock and fractures, good agreement was obtained between time-varying estimates and experimentally obtained values of system hydraulic conductivity for a core sample containing a through-flow fracture. (c) 2005 Elsevier Ltd. All rights reserved.