SUMMARY An analysis of two rock samples, hyaloclastites and basalts, at in-situ reservoir conditions has been done to identify the role of temperature on the seismic velocity and attenuation. The goal is to establish a temperature-dependent fluid substitution analysis of geothermal rocks using Gassmann equation within the framework of Biot''s poroelasticity. The analysis of temperature-dependent wave attenuation is shown for hyaloclastites. The results show that the general decreasing trend of seismic velocity towards temperature may be related to the thermophysical characteristics of fluid. Using Gassmann equation it has been shown that the presence of steam bubbles can reduce the effective elastic property of rocks which indirectly demonstrates the role of temperature to the seismic velocity. The Q factor, i.e., inverse of attenuation, behaves surprisingly almost in the same way as the seismic velocity with temperature, except in the lower temperature range. The Q factor increase with the temperature is supposed to be a quick viscosity decrease. The later decrease of Q factor may indicate the presence of steam bubbles due to the further temperature increase. This finding demonstrates that the application of temperature-dependent fluid substitution modelling using Gassmann equation can be applied for the characterization of geothermal reservoir systems.
INTRODUCTION In geothermal reservoirs, fluid-steam phase transition, fluid pressure and temperature are some crucial factors that potentially produce and/ or contribute to seismic anomalies. When interpreting such anomalies, realistic assumptions based on validated rock physics models are important (Jones et al., 1980; Boitnott and Bonner, 1994).
A laboratory measurement of temperature dependent seismic velocities of rocks at high temperature reservoir conditions has been referred to, for example in (Kern, 1978; Kern et al., 2001; Punturo et al., 2005; Scheu et al., 2006). However, these laboratory experiments have been mainly employed on dry samples and under deep mantle rock condition, i.e., very high pressures (up to 600 MPa with 50 MPa interval) and very high temperatures (up to 1000°C with about 100°C interval). Meanwhile, many geothermal reservoirs, as the case of Icelandic reservoir being investigated, are characterized by temperature range up to 200-300°C and pore pressure around 10 MPa with the pore water being in the liquid phase (Flóvenz et al., 2005). A controlled petrophysical laboratory experiment simulating those conditions becomes important for the evaluation of such a geothermal resource. An analysis of core scale properties of rock sample at in-situ reservoir conditions is useful to identify the role of temperature on the seismic velocity and attenuation. The goal of this work is to present the result of using Gassmann equation within the framework of Biot''s poroelasticity for a fluid substitution analysis of temperature-dependent geothermal rocks. For that, the measurement of ultrasonic transmission wave has been performed on two samples of volcanic geothermal rocks with different alterations (Bruhn et al., 2007; Jaya et al., 2007). Gassmann equation is then used to relate the effect of temperature on the fluid and on the effective elastic property of saturated rock. In addition, the temperature-dependent wave attenuation is shown for the hyaloclastite sample.