Role of mineralogical, structural and hydrodynamic rock properties in conduits formation in three distinct carbonate rock types

Abstract

Adopting sustainable strategies to manage water resources of karst reservoirs requires to understand the carbonate rock reactivity that is responsible for the formation of these reservoirs, as well as the role of the rock mineralogy in the conduit formation. To this end, three carbonate rocks (chalk, crinoidal limestone and dolomite) with different mineralogy and internal structure are submitted to laboratory dissolution experiments by injecting an acid solution under atmospheric conditions and various hydrodynamic conditions with a homemade experimental device. The core samples are characterized by petrophysical investigations with laboratory and imagery techniques before and after the experiments, and the changes in chemical and hydraulic properties are recorded during the experiment. The resulting carbonate dissolution leads to the formation of preferential conduits and the increase in porosity and permeability of the samples. For the three rock types, the dissolution rate mainly depends on the mineral composition, the flow conditions and the initial structural properties. In addition, we observe that (i) for each rock type, the lower Péclet conditions applied to the samples, the lower global dissolution rate, (ii) for each hydrodynamic condition, chalk and crinoidal limestone have the highest and smallest dissolution rates, respectively, (iii) for a similar amount of injected acid, the dissolution is lower in dolomite than in calcite, and (iv) high rock heterogeneities in the chalk samples are responsible for high renewal rates that induce high dissolution rates.