Temperature-Dependent Seismic Properties of Geothermal Core Samples at In-Situ Reservoir Conditions
Authors: Jaya, M. S., Shapiro, S., Kristindóttir, L., Bruhn, D., Milsch, H. and Spangenberg, E
Keywords: Seismic properties, temperature dependence, Gassmann relationship, fluid substitution analysis, geothermal cores
Conference: World Geothermal Congress Session: 13. Geophysics
Year: 2010 Language: English
Abstract: The goal of the present work is to predict the effect of saturating pore fluid on seismic properties using a rock-physics model in two Icelandic core samples (hyaloclastite and basalt) saturated with a low-saline fluid.
To achieve this goal, the fluid substitution analysis of seismic properties (velocity and attenuation) as a function of temperature under simulated reservoir conditions using the Gassmann equation has been employed. To include the temperature effect in the Gassmann equation, assumptions have been made: 1) the grain/mineral and dry bulk moduli (initially in the absence of microfractures) are independent of temperature; 2) the temperature dependence follows solely from the thermophysical characteristics of the saturating fluid through the fluid bulk modulus and fluid density. Therewith, a temperature-dependent fluid substitution modeling using the Gassmann relationship has been realized.
Laboratory measurement results show that P- wave velocities decrease with increasing temperature. This trend in seismic velocity with temperatures is related to the thermophysical characteristics of the fluid. The Q factor behaves almost in the same way with temperature as the seismic velocity, except in the lower temperature range.
The results shown here indicate that both velocity and attenuation may be diagnostic for the temperature conditions of a reservoir. The Gassmann equation applies reasonably well for the sample material investigated in the present study and can help explain the contribution of grain/mineral and fluid bulk modulus change with temperatures to the velocity and attenuation. The way how such approaches are implemented in the current study may give the modality for the prediction of a temperature dependence of seismic properties and demonstrates that the application of a fluid substitution modeling using the Gassmann relationship may be useful for the characterization and interpretation of geothermal reservoir systems.
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