Abstract
The deep geothermal reservoirs in the Larderello-Travale field (southern Tuscany) are found in intensively fractured portions of intrusive/metamorphic rocks. Therefore, the geothermal exploration has been in search of possible fracture signatures that could be retrieved from the analysis of geophysical data. In the present work we assess the feasibility of finding seismic markers in the pre-stack domain which may pinpoint fractured levels. Thanks to the availability of data from boreholes that ENEL GreenPower drilled in the deep intrusive basement of this geothermal field, we derived the expected amplitude versus angle (AVA) responses of the vapour reservoirs found in some intensely, but very localized, fractured volumes within the massive rocks. The information we have available limit us to build 1D elastic and isotropic models only and thus anisotropy effects related to the presence of fractures cannot be properly modelled.We analysed the velocities and the density logs pertaining to three wells which reached five deep fractured zones in the basement. The AVA response of the fractured intervals is modelled downscaling the log data to seismic scale and comparing the analytical AVA response (computed with the Aki and Richards approximation) and the AVA extracted from a synthetic common mid point (calculated making use of a reflectivity algorithm). The results show that the amplitude of the reflections from the fractured level is characterized by negative values at vertical incidence and by decreasing absolute amplitudes with the increase of the source to receiver offset. This contrasts with many observations from hydrocarbon exploration in clastic reservoirs where gas-sand reflections often exhibit negative amplitudes at short offsets but increasing absolute amplitudes for increasing source to receiver offsets. Thereby, some common AVA attributes considered in silicoclastic lithologies would lead to erroneous fracture localization. For this reason we propose a modified AVA indicator which may highlight fracture locations in this peculiar rock type.
Highlights
The Larderello-Travale geothermal field is located in southern Tuscany, in a region known for centuries for its mines and, more recently, for the exploitation of endogenous fluids which are localized in fracture systems inside the reservoir rocks (Casini et al 2010)
We first focus our attention on amplitude versus angle (AVA) analysis, which has become an important tool for hydrocarbon prospecting and it is used in geothermal exploration (Mazzotti et al 2002, Casini et al 2009)
The deepest geothermal reservoirs in the Larderello-Travale area are hosted in localized fractured zones within the intrusive basement
Summary
The Larderello-Travale geothermal field is located in southern Tuscany, in a region known for centuries for its mines and, more recently, for the exploitation of endogenous fluids which are localized in fracture systems inside the reservoir rocks (Casini et al 2010). We verify whether a complex system of interconnected fractures hosting a geothermal reservoir gives rise to a distinct and discernible seismic AVA signature. The AVA response of a massive intrusive rock, locally characterized by interconnected fractures, as it is the case of the deep basement rocks of the geothermal field in the Larderello-Travale area, is largely unknown. The goals of this work are to verify if we can extend the linearized AVA approach to the seismic exploration of this deep fractured geothermal reservoir and, possibly, to derive a specific indicator which may help in the identification of fractured zones within massive intrusive rocks
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