Yves Bernabé

Permeability-porosity Relationships in Rocks Subjected to Various Evolution Processes

It is well known that there is no “universal” permeability-porosity relationship valid in all porous media. However, the evolution of permeability and porosity in rocks can be constrained provided that the processes changing the pore space are known. In this paper, we review observations of the relationship between permeability and porosity during rock evolution and interpret them in terms of creation/destruction of effectively and non-effectively conducting pore space. We focus on laboratory processes, namely, plastic compaction of aggregates, elastic-brittle deformation of granular rocks, dilatant and thermal microcracking of dense rocks, chemically driven processes, as a way to approach naturally occurring geological processes. In particular, the chemically driven processes and their corresponding evolution permeability-porosity relationships are discussed in relation to sedimentary rocks diagenesis

Preparation of synthetic sandstones with variable cementation for studying the physical properties of granular rocks

In this article, we report a new set of procedures to fabricate synthetic analogues of granular rocks. These procedures permit accurate control of the most important structural parameters (i.e., grain size, porosity, cement content). We were thus able to prepare two varieties of synthetic sandstones in which only the cement content significantly varied. Our procedures were also particularly successful in producing materials that were very similar to natural rocks. To demonstrate this similarity, we compared the microstructure, the mechanical properties (i.e., strength, elastic moduli) and the mechanical behaviour (i.e., brittle or ductile) of the synthetic materials to those of various natural sandstones. Following this approach, we were able to gain some insight into the influence of cement on the mechanical behaviour of granular rocks. One important conclusion was the identification of the heterogeneity of the cement distribution as a significant factor controlling the mechanical behaviour.

Laboratory measurements of electrical potential in rock during high-temperature water flow and chemical reactions

In previous laboratory experiments [Mok et al., J. Geophys. Res. 107 (2002) ECV 41] we investigated the effect of high-temperature flow of a reactive fluid (i.e. water) on the transport properties of rock. Here we ran a similar experiment during which we performed transient flow measurements while monitoring the electrical potential across the rock sample. The transient flow tests were carried out at temperatures ranging from 25 to 225 °C. We systematically varied the sign and magnitude of the pressure pulses used. This allowed us to check the scaling properties of the recorded electrical signals and their symmetry with respect to fluid flow reversal. Together with the time dynamics of the electric signals, these scaling and symmetry properties provide very powerful tools to identify the mechanisms generating the electrical potential (e.g. electro-kinetic coupling). We observed that the electrical potential exhibited sharply different phenomenological behavior depending on temperature. At low temperature (i.e. up to 70 °C), electro-kinetic coupling was the most likely cause of the electric signals, whereas electro-dispersion and electro-diffusion couplings predominated at temperatures higher than 100 °C.