Seismic Delineation of a Geothermal Reservoir in the Monteverdi Area from VSP Data

Abstract

Surface seismic reflection surveys and VSP's have been extensively carried out in the Monteverdi area, within the Larderello region. Calibration of these seismic observations with data from numerous deep wells allowed the interpretation of the 2D traverses so that a good geologic-structural reconstruction for the whole area was achieved. Many important reflectors were detected inside the Metamorphic basement by VSP's acquired in several wells in the area. These reflections were attributed to changes in petrophysical characteristics of the medium, particularly changes in fracture density, rather than to contrasts in lithology. Since at this time the main goal of the exploration in the Larderello region is the location of producing layers inside the basement, a major effort was applied to acquiring and modeling numerous VSP's in existing wells. In particular, a multioffset VSP was carried out in the COLLA 2 well with the specific goal of detecting fractured horizons within the reservoir of the teverdi area. Because of the three-dimensionality of the seismic wave propagation in such a complicated geological structure, the standard one-dimensional VSP modeling is not reliable. Therefore, 2D and 3D methods of modeling were applied. The modeling of the VSP data predicts important reflections at the depths corresponding to the main fractured zones intersected by drilling. These fractured zones should show zontal distribution and thickness of tens of meters