Elastic wave velocities and permeability evolution during compaction of Bleurswiller sandstone

Field observations and laboratory experiments have recently documented the formation of compaction bands in porous sandstones [Mollema and Antonellini, Tectonophysics 1996;267:209-28; Olsson and Holcomb, Geophys Res Lett 2000;27:3537-40; Bésuelle, J Geophys Res 2001;106:13435-42; Klein et al., Phys Chem Earth 2001;26:21-5]. It has been observed experimentally [Wong et al., J Geophys Res 2001;28:2521-4; Baud et al., J Geophys Res 2003, submitted; Fortin et al., 2003, EGS-AGU Nice] that under axisymmetric compression, compaction bands develop sub-perpendicular to the main compressive stress which is predicted theoretically in the framework of strain localization theory [Bésuelle, J Geophys Res 2001;106:13435-42; Issen and Rudnicki, J Geophys Res 2000;105:21529-36]. Volumetric strain, fluid transport and elastic properties are intimately coupled to one another, for they all depend on a few intrinsic parameters such as the porosity, the crack density, and the matrix and fluid elastic properties. On the one hand, Scott et al. [Rock Mech Min Sci Geomech 1993;30:763-9] showed that elastic wave velocities were clearly affected during the deformation of porous sandstones. On the other hand, Zhu and Wong [J Geophys Res 1997;102:3027-41] showed that the relation between the evolution of permeability and volumetric strain during compaction of sandstones was not straightforward. In this study, we present for the first time the simultaneous evolution of volumetric strain, elastic wave velocities and permeability for a set of deformation experiments of Bleurswiller sandstone. We show that, although very coherent to one another, those three sets are not systematically correlated. Indeed, inelastic compaction, whether it is distributed or localized, is accompanied by a drastic decrease of elastic wave velocities due to grain crushing, a decrease of permeability and porosity due to pore collapse. Using simple statistical physics concepts based on the study of Kachanov [Adv Appl Mech 1993;30:259-445] and Guéguen and Dienes [Math Geol 1989;21:1-13], we try to understand and address the issue of coupling/decoupling between volumetric strain (mainly sensitive to equant porosity variations), elastic properties (mainly sensitive to crack density) and permeability (theoretically sensitive to both) during the formation of compaction bands. Finally, we show that the mineral composition of a sandstone is a key parameter controlling the effective pressure at which the onset of pore collapse P* takes place.