Subproject No 4
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FREQUENCY DEPENDENCE OF POROELASTIC PROPERTIES OF HETEROGENEOUS ROCKS

Doctorate Researchers: Benedikt Ahrens, Jana Musialak
Principal Investigators: Jörg Renner, Holger Steeb


The effective material properties of fractured and fluid saturated rocks are controlled by the strong interaction of the viscous pore fluids and the damaged skeletal matrix. Especially in the brittle crust, these interactions are responsible for the control of mechanical and chemical processes, such as relevant for geothermal heat and mass transport in reservoir rocks. Rocks can show heterogeneities on various characteristic length scales, ranging from micrometer to meter and even kilometer scale. Heterogeneities can typically be related to rock frame differences like micro-cracks, joints or faults. Additionally, pore space and heterogeneities can be filled with one or more fluids in various proportions, resulting in fully or patchy saturation. The length scales of fluid-saturation and rock-frame heterogeneities have a large influence on the effective (hydro-)mechanical/poroelastic properties, e.g. P- and S-wave attenuation.


We investigate experimentally the effective poroelastic properties of fractured and partially saturated rocks in the seismic and ultrasonic frequency range up to 1 kHz and above 100 kHz, respectively. In order to understand the effect of rock heterogeneities, we developed three experimental setups for cyclic loading tests, covering both kinds of heterogeneities and frequency ranges:
(1) Torsion loading tests (f < 100 Hz) on homogeneous samples with an idealized fault
(2) Cyclic axial loading tests (f < 1 kHz) on patchy saturated samples under confining pressure
(3) Cyclic axial loading tests (f < 10 Hz) with simultaneous ultrasonic measurements (f > 100 kHz) on homogeneous samples with an idealized fault


For our investigations we use defined fracture roughnesses, various pore fluids and control the degree of saturation to understand their frequency and stress dependency. Our experiments provide effective time- and frequency-dependent elastic material parameters of heterogeneous rocks for numerical implementation in other subprojects, e.g. for large-scale quasi-static stimulation simulations (UP5) and transient seismic simulations (UP3).


Conference Proceedings

Conference Date Title Author
6th European Geothermal PhD Day, Delft, The Netherlands February 25–27 2015 Characterization of partially saturated reservoir rocks by dynamic testing Jana Musialak, Jörg Renner and Holger Steeb