Icelandic GeoSurvey ISOR

Reykjavík, Iceland

Icelandic GeoSurvey ISOR

Reykjavík, Iceland

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Jaya M.S.,Free University of Berlin | Shapiro S.A.,Free University of Berlin | Kristinsdottir L.H.,Icelandic GeoSurvey ISOR | Bruhn D.,Helmholtz Center Potsdam | And 2 more authors.
Geothermics | Year: 2010

Petrophysical experiments on two Icelandic geothermal rock samples at simulated in situ reservoir conditions are analysed to delineate the effect of temperature on seismic velocity and attenuation. A goal of the present work is to predict the effect of the saturating pore fluid on seismic velocity using the Gassman equation, which has been modified for this purpose. To include the temperature effect in the equation, two assumptions are made: (1) the grain/mineral and dry bulk moduli are independent of temperature; and (2) the temperature dependence follows solely from the thermophysical characteristics of the saturating fluid through the fluid bulk modulus and fluid density. Laboratory measurements show that P-wave velocities decrease with increasing temperature. This change is related to the thermophysical characteristics of the saturating fluid; at higher temperatures bubbles and thermal microfractures are formed affecting seismic velocities. The measurements also show that at low temperatures seismic attenuation decreases with temperature due to the rapid decrease in the fluid viscosity. On the other hand, at higher temperatures the attenuation increases because of the generation of bubbles and thermal microfractures. Although having data from only two samples and that no measurements on dry samples were done, thus limiting the generality of the claims that can be made, the study presents a plausible approach to relate changes in seismic properties to geothermal system temperatures. © 2009 Elsevier Ltd. All rights reserved.


Kristinsdottir L.H.,Icelandic GeoSurvey ISOR | Flovenz O.G.,Icelandic GeoSurvey ISOR | Arnason K.,Icelandic GeoSurvey ISOR | Bruhn D.,Helmholtz Center Potsdam | And 3 more authors.
Geothermics | Year: 2010

Measurements of electrical conductivity and P-wave velocity of seven rock samples were made in the laboratory under inferred in situ conditions. The samples were collected from smectite and chlorite alteration zones in boreholes from the Krafla and Hengill, Iceland, geothermal areas. The measurements were done in the 25-250 °C range, with pore pressure and confining pressure equal to inferred in situ hydrostatic and lithostatic pressures, respectively. Conductivity increases linearly with temperature over the 30-170 °C range; that rise is considerably smaller above 170 °C. Time-dependent effects on conductivity occur above approximately 100 °C. These effects may be related to ion exchange between the clay minerals or the Stern layer, and the pore fluid. The temperature coefficient of conductivity is found to be considerably higher than attributed to pore fluid conduction alone, indicating interface conduction in an electrical double layer on the mineral-water interface in the pores. The results also show that there is no distinction in electrical conduction mechanism in the smectite and chlorite alteration zones; both are dominated by interface conductivity under in situ conditions. The sharp decrease in conductivity at the top of the chlorite alteration zone, commonly observed in resistivity surveys in high-temperature geothermal systems, is most likely due to the lower cation exchange capacity of chlorite compared to that of smectite. © 2009 Elsevier Ltd. All rights reserved.


Milsch H.,Helmholtz Center Potsdam | Kristinsdottir L.H.,Icelandic GeoSurvey ISOR | Spangenberg E.,Helmholtz Center Potsdam | Bruhn D.,Helmholtz Center Potsdam | Flovenz O.G.,Icelandic GeoSurvey ISOR
Geothermics | Year: 2010

The effect of the water-steam phase transition on electrical conductivity was experimentally investigated in volcanic and sandstone samples to support the interpretation of resistivity data to determine changes in steam saturation in geothermal reservoirs. The measurements were performed at simulated in situ conditions with controlled pore fluid chemistry, temperature, and confining and pore pressures. At constant temperature (150 °C) and confining pressure, pore fluid was withdrawn from the sample by steadily increasing the volume of the pore fluid system. At the vapor saturation pressure, the pore water progressively boiled to steam, resulting in a continuous conductivity decrease by a factor of approximately 20. The study showed that: (1) for rocks in which conduction is controlled by the pore fluid, the concurrent changes in both electrical conductivity and pore (vapor) pressure are defined by the pore size distribution; the changes in liquid-steam saturation are approximately proportional to those in conductivity and can thus be quantified; and (2) for rocks in which surface conduction is predominant there is no direct relation between conductivity, pore pressure and drained fluid volume; this implies that the conduction mechanism controls the pattern of electrical conductivity variations as steam saturation changes. © 2009 Elsevier Ltd. All rights reserved.

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