--> VSP Survey in the Thonex Geothermal Well - New Characterization of Potential Carbonatic Geothermal Reservoirs
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AAPG ACE 2018

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VSP Survey in the Thonex Geothermal Well - New Characterization of Potential Carbonatic Geothermal Reservoirs

Abstract

In the framework of the Geothermie 2020 program being developed in the Geneva Canton and the FP7 IMAGE project, a VSP survey was carried out down to a depth of 1500m in order to recontruct a detailed velocity model of the region and characterize the carbonatic reservoir for future potential exploitation for heat storage and production.

One Near Offset (NO), a walk-above (WAB) and 4 addiotanal offsets (VSP1-4) were collected to costrain the fracture condition and their anisotropic distribution.

Overall quality of data is good, with evidence of good direct arrivals and reflections, in particular below 700-800 m in depth.

The presence of a strong high velocity resonant system makes it impossible to pick reliable first breaks on the vertical section of VSPs 1, 2 , 3 and on the Near offset VSP. On VSP 4, although the first breaks are of better quality, the upgoing wavefield is also contaminated by the said resonant system.

A Kirchoff migration algorithm was applied and Full waveform inversion (FWI) was tested on VSP4. A detailed starting model based on the interval velocities was beneficial for the successful application of FWI. Final elastic-parameter models were obtained that resolve small-scale velocity structures and result in seismograms that very closely match the actual field data.

Intepretations show that the Molasse strata appear well visible on the VSP profile, as well as the top of the Mesozoic because of its high contrast of acoustic impedance. On the other hand, the Cretaceous and Upper Malm limestones are very transparent on the VSP. This could be explained by the fact that the acoustic impedance contrasts within these limestone units are much lower than those generated by the sandstone alternations of the Molasse or that generated by the Molasse-limestone contact. Additionally, the Cretaceous shows a variety of acoustic impedance, which can be interpreted as response to the well-known variability in lithology (i.e. limestone and marls alternations) and facies within this interval. In particular the high acoustic impedance at 1450 m has been calibrated with logs and core indicating that the strong seismic response is caused by the occurrence of a complex open and closed fracture system subsequently affected by infill (sedimentary dykes?) and dissolution (karst?) processes, which can represent favourable conditions for potential development of heat storage and production projects.