Anisotropy of volume change and permeability evolution of hard sandstones under triaxial stress conditions

Volumetric strain and permeability are strictly interconnected properties and important controlling parameters for deformation patterns in rock masses. Under reservoir conditions, stresses may be highly inhomogeneous and anisotropic, leading to porosity changes and consequently affecting fluid flow. Therefore, it turns out be a challenging issue in rock mechanics to evaluate volume change based on traditional soil mechanics background, originally intended for soft materials under low and mostly isotropic pressures. In this respect, triaxial compression tests were carried out to describe the interplay between physical properties, volume change and permeability of two hard sandstones by quantifying porefluid volume change with fully water saturated rock specimens (14 cm length and 7 cm radius). The investigated sedimentary rocks are (1) the greyish Trendelburg beds, a silica cemented subarkose Bunter Sandstone of Triassic age (porosity of ca. 12%), and (2) the red-brownish Rotliegend Sandstone (Bebertal), a carbonate and silica cemented sandstone of Permian age, clearly less porous (ca. 6% of effective porosity) and less permeable (3.5 x 10(-10) m/s) than the Bunter Sandstone. Both materials present a pronounced brittle behaviour influenced by coring direction: permeability, volumetric strain and fracture pattern are direction-dependent. Effective porosity and pore pressure level affect the fracturing development, which therefore influences the permeability after stress fall. For the Bunter Sandstone, increasing porefluid pressure leads to an earlier microcraking stage, which shortens the forestage of compaction and induces a more pronounced dilatant behaviour with decreased compressive strength. For the Rotliegend, the increase of pore fluid pressure enhances compaction. Altogether, this examination is valuable to understand the combined effects of pore pressure change and pore space quality on the mechanical behaviour of rock masses.