Predicting fracture distribution is a key issue during development of unconventional reservoirs. HTI anisotropy due to natural fractures affects the AVO gradient, amplitude, and travel times. According to this, in the last 25 years geophysicists have developed methods to retrieve reservoir fracture model from seismic data by means of inversion techniques (AVAZ, VVAZ and azimuthal inversion). More recently, another anisotropic effect on seismic response due to natural fractures appealed geophysicists: fractures also affect absorption: a propagation perpendicular to fracture suffers higher attenuation due to friction in medium. The phenomenon is called FVAZ, and we accepted the challenge of testing it in the context of Vaca Muerta to compare the results with those obtained from other methods.
Previous works settled for qualitative results and did not consider that attenuation is an effective magnitude. To overcome this, a "layer-by-layer" inversion was implemented. Processing sequence was revisited to ensure the preservation of frequency changes. The reservoir was mapped into a stratigraphic grid in time and normalized quotients of spectral decay were obtained by FFT. Based on statistical methods, a complete stage of data conditioning was run. An error minimizing problem on the theoretical attenuation was stated on a layer-by-layer basis for three unknowns (reservoir parameters): fracture orientation, fracture intensity and isotropic absorption (Q factor). Finally, the obtained maps for each level and for each parameter were put together into single volumes.
A comparison between the results of three methodologies is presented. Fracture patterns from FVAZ were very encouraging, with detailed image of the high organic content level known as the Cocina. Despite the limited organic formation thickness, several thin level maps could be extracted showing multiple patterns with clear trends. Isotropic absorption –an innovative parameter- is also remarkable as it shows a suggesting correlation with anisotropic parameters from azimuthal seismic inversion. Although the application of FVAZ is more complex than alternate methods, it is a valuable contribution to consolidate a predictive geomechanical model key for drilling risk minimization.
Obtained anisotropic parameter led to correlations that enable asset team to estimate drilling risks. Elastic wave velocity and anisotropy results presented herein, together with effective stress, were the key for a better understanding of the pressure behavior within the Quintuco-Vaca Muerta system and thus for stablishing more realistic drilling operational limits. The high gas kicks risk observed around the area of study suggests that an extra casing is required for certain areas on the Top of Vaca Muerta formation to isolate losses on Quintuco environment. This methodology creates a systematic approach to well planning that reduces drilling risks during the development of a complex area.
