Laboratory and imaging petrophysical characterization of three types of carbonate rocks for karstification purposes

Abstract:

As water is an indispensable resource for human development, geological water reservoirs study is essential. Karst reservoirs are the most interesting in terms of water resources and more studies are required to determine conduits and flow location. A focus on their formation during dissolution process at pore scale is needed.

For this purpose, 3 blocks of carbonate rocks (chalk, limestone and dolomite) were cored to obtain about 30 samples with diameters of 18 mm and lengths of 25 mm. They were characterized from pore to core scale with laboratory tools which can be used at several scales. These technics, coupled with X-ray tomography, allow to quantify hydrodynamic properties (porosity, permeability), elastic and structural properties (by acoustic and electrical measurements), pore distribution (by centrifugation and calculations).

The 3 rocks have properties similar to classical homogeneous carbonate rocks but have specific characteristics depending on the rock type. In the same rock family, samples properties are different and links have been established between the some measured properties. As an example, samples with same hydrodynamic (porosity, permeability) and structural (formation factor, electrical tortuosity) characteristics may have different elastic properties, linked to their cohesion, which depends itself on pore size distributions.

These established links between pore-scale properties are then related to existing large-scale links, to validate rock laboratory characterization and thus facilitate reservoir properties analysis. Differences in properties found between the 3 rocks types show that the study of microstructure is essential for a better understanding of the initial characteristics of rocks. Dissolution of these carbonate rocks by reactive percolation will make possible to observe the evolution of these parameters during a pore-scale karstification process, and thus better understand the location of conduits and flows in karstic reservoirs.