3D Seismic Surveys and Deep Target Detection in the Larderello-Travale Geothermal Field (Italy)

Abstract

Deep exploration in the Larderello-Travale area highlighted a single deep geothermal system wider than 400 km2 and with temperature of about 300°C at a depth of 3000 m. The deep reservoir is hosted in metamorphic/intrusive rocks and is characterized by a highly inhomogeneous permeability. This can increase the mining risk and, as a consequence, the drilling costs. A significant contribution in the detection of potential, productive targets for deep drillings can be provided by a detailed reconstruction of high amplitude seismic markers that, as confirmed by recent 3D seismic surveys, often correspond to fractured levels.
In the framework of a deep exploration project, new 3D surveys were performed in three areas of the Larderello-Travale geothermal system for a total full fold area wider than 100 km2. Data processing was aimed at preserving the correct signal amplitude and at detailing the subsurface structures. An accurate joint interpretation of well and geophysical data allowed defining the 3D structure and the high amplitude anomalies of a seismic marker that shows a quite good statistical correlation with fractured and productive levels. This marker, known in literature as “H” horizon, is discontinuously present everywhere in the Larderello-Travale system, but can be originated by different or concomitant causes such as lithology changes and/or presence of fractured levels.
More than 10 directional wells have been drilled up to now to reach 3D seismic targets, but for the above said reason their productivity was different. The best results were obtained in the Travale area (3D Montieri-Chiusdino) where the H horizon corresponds to a highly fractured contact metamorphic aureole and where the correlation between well productivity and seismic marker suggests that more than 70% of steam production originates from depth intervals within this horizon.