A feasibility study on the expected seismic AVA signatures of deep fractured geothermal reservoirs in an intrusive basement

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.

Synthetic Seismic Reflection Modelling in a Supercritical Geothermal System: An Image of the K-Horizon in the Larderello Field (Italy)

“…Vp logs of the Larderello-Travale area, at a depth of between 2400 to 3800 m, indicate a general negative asymmetric distribution around 20-30% of the Vp velocities with respect to a reference Vp value (5000-6000 m/s [73]). Furthermore, data logs indicate that the productive levels at the log scale are characterized by more complex velocity structures at short wavelengths (few meters) not resolvable with surface seismic.…”

“…For the Metamorphic Unit, we adopted a velocity value of 5150 m/s, which is within the velocity range (i.e., 4400-5500 m/s) suggested by several authors (e.g., [6,9,13,18,76]). This velocity value was defined after analyzing the data observed in the Vp logs and in laboratory measurements of unfractured rocks from the Mica-schist complex and Gneiss complex in the depth range 1000-3500 m, as reported in the literature and summarized as follows [6,[71][72][73][74][75].…”

A szeizmikus reflexiós módszerek szerepe a nem szénhidrogénipari kutatásokban Magyarországon – Példák a Magyar Bányászati és Földtani Szolgálat és elődei tevékenységéből

“…Az Amplitude Versus Offset (AVO-) ana lí zist a szénhidrogén-tartalmú porózus kőzetek előrejelzé sére már több mint két évtized óta sikeresen használják a hazai szénhidrogéniparban (TAKÁCS 1996, TAKÁCS et al 2001, LŐRINCZ 2019. Az eljárás geotermikus kutatási cé lok ra tör ténő alkalmazásának lehetőségei azonban még nap jainkban sem teljesen tisztázottak (CAMELI et al 2000, ALEARDI & MAZZOTTI 2012, ebből kifolyólag ritkán alkal mazott vizsgálati módszer.…”

Advanced three‐dimensional seismic imaging of deep supercritical geothermal rocks in Southern Tuscany

“…Other previous seismic studies are also remarkable as they attempted to characterize the geothermal reservoirs in Southern Tuscany. For example, Aleardi and Mazzotti () and Aleardi et al . () carried out rock physics analysis on the same 3D seismic data set using amplitude versus angle and seismic attribute analyses.…”