Abstract
Carbonate rocks constitute a very important part of the rocks at the Earth surface. Associated reservoirs represent essential resources for human societies, whether direct (water, gas, hydrocarbons) or indirect (geothermal energy, storage of CO2).The high reactivity of carbonate rocks is responsible for karstification, a reaction process due to the thermodynamic imbalance between the rock and the water circulating in the reservoir. This results in the formation of karstic aquifers, characterized by strong structural heterogeneities going with a complex hydrological behavior. This makes it a vulnerable system, but also a very productive one.In order to better manage these reservoirs and the water resource they contain, a better understanding of the formation and location of karst conduits is necessary.For this purpose, laboratory experiments can be performed to reproduce the karstification phenomenon at small scale. Rock samples from three types of carbonate rocks are extracted from homogeneous rock blocks in order to be dissolved. The rocks studied are a chalk, a crinoidal limestone and a dolomite, and the samples are considered to be REV.Before the experiments, the samples are characterized by laboratory and imaging methods, in order to know their structural, elastic, mechanical, mineral and hydrodynamic properties.They are then submitted to an injection of acidic fluid in an experimental device developed during the thesis. For chalk samples, two fluids with different acid concentrations are used, unlike crinoidal limestone and dolomite, where only one acid is used. Different flow rates, associated with different Péclet conditions, are applied to the samples. During the dissolutions, hydrodynamic and hydrochemical data are recorded continuously.After the experiments, the same measurements as before the experiments are performed.The characterization of the samples has shown that the microstructure controls the rock properties, and that the relationships established between the petrophysical properties at the sample scale are found at large scale reservoirs.For all rocks, the injection of acidic fluid into the samples causes rock dissolution, leading to the formation of preferential conduits, associated with an increase in permeability and porosity. Moreover, the observed dissolution regimes are directly correlated to the concentration of the injected acid and its flow rate, but also to the initial structural properties of the rock.For the experiments conducted on chalk, which is a very heterogeneous rock due to its high proportion of micropores, heterogeneities are responsible for the dissolution patterns observed, in particular the formation of channels while the experimental conditions suggest an uniform dissolution.For experiments involving all three rock types, the dolomite-containing rock shows a lower dissolution rate than the rocks without dolomite, which is due to the lower reaction kinetics of dolomite compared to calcite. With its high microporosity, chalk is the rock with the highest dissolution rate. Moreover, the conduits created in dolomite are much more localized and linear than the conduits in the other rocks. For the same amount of acid injected, the mineralogy of the rock, associated with its structure, is therefore mainly responsible for the dissolution figures in the rock.