Analysis of Velocity Variation with Azimuth (VVAZ) for natural fracture and stress characterization, Vaca Muerta Formation, Neuquén Basin, Argentina

Abstract

The upper Tithonian – lower Valanginian Vaca Muerta Formation, located in the Neuquén Basin, Argentina, is one of the most prolific unconventional shale plays in the world. It spans an area of around 30,000 km2, covering four different provinces, Neuquén, Mendoza, La Pampa, and Rio Negro. Because the unit requires multistage horizontal wells in order to produce commercially, detailed analysis of the regional stress and natural fractures of the area is needed. Previous studies have shown that both the Neuquén Basin and the Vaca Muerta Formation are highly affected by stress due to their proximity to the Andes chain, and also that the unit may present a high density of natural fractures. This study is located in a block operated by Wintershall Holding GmbH, where previous research projects focused on natural fractures analysis from well images, seismic inversions for mechanical parameters prediction, and well based anisotropic geomechanical models. Using well log and wide-azimuth seismic data, analysis for natural fractures and stress characterization was performed. Well based anisotropic geomechanical models were built for the three wells in the area using well logs, laboratory measurements and completion data. Stresses were calculated and calibrated with fracture data. For the studied block, the stress regime in the Vaca Muerta Formation is mainly strike-slip (SH>SV>Sh), with local areas showing normal (SV>SH>Sh) and thrust (SH>Sh>SV) regimes. In this thesis, the Lower Vaca Muerta section is recommended as a possible landing zone based on stress and fracability analysis, and considering previous studies and current practices in the basin. Anisotropy and azimuthal analysis from wide-azimuth seismic data showed that maximum horizontal stress is oriented between 105º and 120º. It also proved the presence of two sets of extensional fractures oriented at 50º and 150º, formed at different stages during the development of the basin. All these observations show a reasonable calibration with log and microseismic data. Finally, following the azimuthal analysis and results from the geomechanical models, new drilling areas and landing points are suggested for the studied block.