Effects of Pressure, Temperature, Fluid-Rock Interactions, and Phase Changes on the Physical Properties of Geothermal Reservoir Rocks: the Experimental Perspective

Abstract

Within the GFZ, Section “Reservoir Technologies”, there is a long-standing tradition in experimental rock physics. We maintain several high pressure (100 MPa) and high temperature (200°C) flow-through apparatuses to address questions arising in geothermal reservoir characterization, evolution and sustainability. Various research programs are conducted both site specific and process oriented. So far, the types of fluid-rock combinations explored, relate to the geothermal wells in Groß Schönebeck (Germany), Hengill and Krafla (Iceland), and Anqua and Radicondoli (Italy), respectively. With regard to particular parameters and processes related to these reservoirs we experimentally investigated:
1) The pressure dependence of rock transport properties and their interrelations (Germany), where three different models relating permeability and electrical conductivity were tested and the appropriateness of an individual model showed to be rock-type dependent.
2) The effect of dissolution-precipitation reactions on rock permeability (Germany), where in a series of long-term flow-through experiments at various chemical fluid compositions even after six months of flow no significant change in permeability in neither direction was observed.
3) The temperature dependence of electrical rock conductivity and seismic wave velocities (Iceland and Italy), where the values derived for the temperature coefficient α, were in the range 0.027-0.160 1/°C indicating the predominance of interface conduction regardless of the respective alteration stage. P-wave velocities systematically decreased with temperature and were in the range of 4.4 km/s (25°C) and 3.4 km/s (250°C).
4) The evolution of electrical rock conductivity in a fluid-rock disequilibrium (Germany and Iceland), where observed transient conductivity changes are interpreted as alterations of the fluid-rock interface properties and/or of pore fluid compositions resulting from dissolution processes.
5) The petrophysical signature of a water-steam phase transition within the pore space (Germany and Iceland), where it showed that the conduction mechanism (fluid vs. interface conduction) controls the pattern of electrical conductivity variations as steam saturation changes.
Here, we provide an outline of the technical features of our apparatuses, background information on the particular geothermal setting as well as experimental details and results of the individual projects outlined above.