Integrated seismic reservoir chacterization of Vaca Muerta shale

Abstract

Summary The Vaca Muerta (VM) formation, covering approximately 30,000 square kilometers in Argentina's Neuquen Basin, is a developing unconventional resource play. The giant Vaca Muerta shale oil and gas field has an estimated resource potential of 600+ billion barrels of oil and 1,000+ TCF of natural gas. Like other unconventional shale plays, horizontal drilling and hydraulic fracture stimulation are essential for economic viability. Main factors that can strongly influence the effectiveness of hydraulic stimulation are the maximum horizontal stress azimuth, the in-situ stress anisotropy, natural fracture system, and geomechanical rock properties. This paper describes a case study in which azimuthal velocity analysis, seismic inversion, and an integrated geomechanical/petrophysical study are utilized to help improve horizontal well placement and completion designs for the Vaca Muerta formation. First, a feasibility study is performed using full suites of log data to investigate potential rock properties that can be used to characterize petrophysical properties and kerogen and other mineral composition. Then P- and S-impedance and density volumes are generated using prestack seismic inversion. The inverted elastic property volumes, integrated and calibrated with well data, are used to generated geo-mechanical volumes and petrophysical volumes such as kerogen, porosity, facies, etc. Finally, FMI and production data are integrated with 3D stress, structure and rock properties for horizontal well planning consideration. In the following sections, we detail the workflow. Reprocessing Project The study lies in the eastern portion of the onshore Neuquen Basin in Argentina. In 2004 a 3D seismic survey was acquired for conventional purposes. In 2012, prior to the integrated (VM) geomechanical/petrophysical study, this survey was reprocessed to optimally condition the data for seismic inversion and derive anisotropy information for use in stress-field analysis and fracture characterization. Two types of anisotropy commonly exist in nearly all land and hard rock marine basins. The first and most common is vertical transverse isotropy (VTI). VTI anisotropy is often caused by fine horizontal layering within the subsurface, resulting in the well- known hockey-stick or non-hyperbolic moveout effect in the data gathers. The second type is horizontal transverse isotropy (HTI). HTI anisotropy is most often caused by the presence of vertically aligned fractures or unequal horizontal stresses in the subsurface causing azimuthal travel time variations in the data. Accounting for these two types of anisotropy is now widely recognized as an important step in seismic data processing. The reprocessing effort for the seismic survey follows a standard land processing workflow with the exception of the implementation of an anisotropic offset vector tile (OVT) PSTM scheme to account for VTI and HTI effects in the data. Using a methodology similar to that proposed by Jenner (2011), Vnmo and η are obtained, smoothed and used as the input velocity field for an anisotropic OVT PSTM. Since OVT PSTM is an offset and azimuth preserving migration scheme, post-migration HTI analysis is possible without having to azimuth sector the data. A proprietary high density, full azimuth (non-sectored) surface-fitting technique is used post-migration to measure and correct for the HTI effects in the data. This workflow results in improved gathers for input to seismic inversion, and produces azimuthal velocity anisotropy attributes for stress field analysis and fracture characterization.