| Title | Petrophysical and Geophysical Characterization of an Icelandic Geothermal Reservoir, Case Study on the Reykjanes Peninsula |
|---|---|
| Authors | Adelinet, Mathilde; Fortin, Jerome; Schubnel, Alexandre; Le Ravalec, Mickaele; Dorbath, Catherine; Gueguen, Yves; Geoffroy, Laurent |
| Year | 2011 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Geothermal; Reykjanes; petrophysics; basalt; acoustic emissions; bulk modulus; MHz frequency gap; squirt-flow effect; elastic properties; Kleifarvatn Lake |
| Abstract | Nearly 60 % of the Earth surface is composed of basaltic rocks which form the upper part of the oceanic crust. This paper discusses the elastic properties of basalt at several spatial scales, from the sample and its microstructure to the reservoir scale. Firstly we present experimental data obtained on the frequency relating to measurement of the bulk modulus of a fresh and young basalt sampled in Iceland. We show that a gap of about 20 % exists between the bulk modulus calculated at 0.01 Hz and the one measured at 1 MHz. We relate this effect to the microstructure and to a squirt-flow effect. Secondly we investigate the modes of deformation of basalt by performing triaxial tests and recording acoustic emissions. The most significant result is the one obtained for high confining pressure. Indeed for such pressure, an axial loading induces a localization of deformation under the form of subhorizontal bands in the sample. In these bands, focal mechanisms of acoustic emissions reveal the co-existence of two processes, pure compaction and pure shear. It is the first time that such a mode of deformation is observed in a low-porosity rock (< 10 %). Finally, we studied the elastic properties of the upper Icelandic crust by using a mechanical approach on tomography data derived from microseismicity activity. We interpreted the evolution of P- and S- waves velocities in terms of crack density and fluid state according to depth. Our results show that at 6 km depth a large part of the Icelandic crust is saturated with a supercritical fluid phase, while at 2 km depth, only the most hydrothermal areas present such a fluid signature. Moreover, crack density tends to increase with depth under these areas leading to the assumption of a hydrofracturation process due to supercritical fluid overpressures. |