International Journal of Rock Mechanics & Mining Sciences 42 (2005) 924–932 Poroelastic behaviour of physical properties in Rotliegend sandstones under uniaxial strain U. Trautwein , E. Huenges GeoForschungsZentrum, Potsdam, Germany Accepted 3 May 2005 Available online 15 July 2005 Abstract Laboratory experiments simulating reservoir depletion and reinjection of fluids have been conducted on Rotliegend reservoir sandstones. Pore pressure dependence of petrophysical properties have been measured under uniaxial strain boundary conditions, i.e., zero lateral strain at constant overburden pressure. Permeability, formation resistivity factor, and compressional as well as shear wave velocities were recorded simultaneously and continuously during deformation in direction of maximum principal stress. The results show that the stress development is specific for different sandstones depending on the efficiency of pore pressure. The stress anisotropy increases with decreasing pore pressure, leading to stress-induced anisotropy of pore structure with preferred closure of horizontally oriented pore space. Permeability decline of initially lower permeability sandstones indicate the opening of axially oriented pores at lower pore pressure. r 2005 Elsevier Ltd. All rights reserved. 1. Introduction Permeability is a key parameter describing the quality of geothermal reservoirs. It depends strongly on pore structure, which is assumed to change with effective stress. Therefore, understanding the pore pressure- induced alteration of rock structure and permeability is crucial for reservoir exploration and exploitation. The concurrent measurement of additional petrophy- sical properties, e.g., electrical or mechanical properties, parallel with permeability measurement during defor- mation, provides important information for the under- standing of permeability development from the microstructural point of view. It also helps to find correlations as well as discrepancies between these properties. Reliable correlations are required for log interpretation as well as for 4D monitoring of hydro- carbon and geothermal reservoirs. Development of permeability depends on the stress path applied to the rock, defined as the change in effective horizontal stress to change in effective over- burden stress (see [1–4]). The importance of stress anisotropy especially for permeability but also for other structure-sensitive parameters is well documented in the literature (e.g. [5,6]). Heiland [7] recently compiled the extensive amount of experimental work done on stress- dependent permeability for hydromechanical coupling. Uniaxial strain boundary conditions are usually assumed for reservoir compaction (e.g. [1,8]), i.e., no lateral strain occurs due to crustal confinement, and overburden stress does not change during production of the reservoir. Therefore, changes of reservoir fluid pressure will lead to changes in effective stress and consequently to reservoir compaction and alteration of petrophysical properties. In laboratory experiments, we varied the pore pressure under uniaxial strain boundary conditions to simulate reservoir depletion and reinjection of fluids. Stress, strain, permeability, formation resistivity factor and compressional as well as shear wave velocities are ARTICLE IN PRESS www.elsevier.com/locate/ijrmms 1365-1609/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmms.2005.05.016 Corresponding author. Geological Institute RWTH Wu¨llnerstraXe 2, D-52062 Aachen, Germany. Tel.: +49 241 80 95732. E-mail address: trautwein@geol.rwth-aachen.de (U. Trautwein).