Anisotropic seismic tomography of a potential hot dry rock reservoir before and during induced pressurization

Abstract

Abstract. To assess the effects of changing fluid pressure in a potential hot dry rock reservoir, two crosshole seismic surveys were conducted: one at ambient pressures and one with the investigated volume hydraulically overpressured by ~3.3 MPa. Sources and receivers were located in two straight subvertical boreholes ~100 m apart. Traveltime data were inverted for velocity information in two phases. During the first phase the seismic velocities were constrained to be isotropic, and during the second they were allowed to be anisotropic. Accounting for anisotropy enhanced greatly the interpretability of the tomograms. Before pressurization, a low-velocity anomaly at 455-475 m depth correlated with a subhorizontal zone of hydraulically permeable fractures connecting the two boreholes. Although the absolute magnitude of anisotropy is not well constrained, the dominant orientation of the TIaxes is consistent with the local stress field. Hydraulic over-pressurization resulted in a pronounced extension of the low-velocity zone, but had no significant effects on the velocity structure elsewhere. The results of this study suggest that anisotropic seismic tomography may be a key method for characterizing hot dry rock reservoirs.

Microseismic investigation of an unstable mountain slope in the Swiss Alps

“…Microseismic monitoring can be used to track the movements of various types of material. It is a well established method in studies of volcanoes [ Vilardo et al , 1996; De Natale et al , 1998; Lomax et al , 2001; Pezzo et al , 2004; Presti et al , 2004; Lippitsch et al , 2005], fracturing and fluid flow in hydrocarbon reservoirs and hot dry rock systems [ Rutledge et al , 1998; Vécsey et al , 1998; Oye and Roth , 2003; Evans et al , 2005], mining‐induced earthquakes [ Trifu and Urbancic , 1996; Iannacchione et al , 2005], and major tectonic faulting [ Malin et al , 1989; Schorlemmer and Wiemer , 2005]. It is starting to play an increasingly important role in investigations of unstable hill and mountain slopes [ Rouse et al , 1991; Eberhardt et al , 2004b; Amitrano et al , 2005; Brückl and Parotidis , 2005; Merrien‐Soukatchoff et al , 2005; Roth et al , 2006].…”

Advanced seismic processing/imaging techniques and their potential for geothermal exploration

“…Several approaches have been proposed to handle anisotropy in crosshole seismic tomography, differing in simplifying assumptions and parameterization of anisotropy (Chapman and Pratt, 1992;Zhou et al, 2008). Vécsey et al (1998) demonstrate that accounting for anisotropy in crosshole measurements at a potential hot dry rock reservoir to monitor effects of changing fluid pressure greatly enhanced the interpretability of the tomograms.…”