Geopressures modeling has become a critical task in unconventional plays since the high pore pressures found in some areas makes drilling and completion operations particularly challenging. This field study helped to quantify the pore pressure anomalies and understanding the overpressure mechanisms acting in the tight gas reservoir of Rio Neuquen field. To achieve this objective a pore pressure model was built based on seismic attributes and multi well data analysis. This model also helped to establish possible relationships between reservoir performance and those pore pressure anomalies.
The mathematical model used seismic velocities from PSDM tomography and simultaneous inversion for establishing relationships between the elastic rock response and pore pressure, through a series of effective stress-velocity transforms. The model also considers a multi variable analysis to account for other variables affecting seismic response, including porosity, VSH and effective stress, like that proposed by Sayers (2003). Field data from several wells was used to tie the seismic velocities to well logs, while laboratory tests and pressure measurements were used as calibration points for the model, supporting the results of the study. The analysis showed that interval velocities from seismic inversion led to a more representative and accurate pore pressure model than that obtained from the PSDM velocities. The analysis showed that velocity reversals from seismic had a strong stress influence while as confirmed later by ultrasonic lab test analysis and observed pressure data. There were also some lithological and porosity effects in the elastic response of seismic, as evidenced by well logs. This, however, could not be modeled at field scale due to limitations in the available 3D seismic data.
Pore pressure anomalies are expected to be highly influenced by the presence of a nearby overpressurized source rock. The study suggests indeed that that multiple type II overpressure mechanisms can be present in the area, including lateral transfer, HC generation and tectonic load. The model showed that the pressure anomalies are higher as closer is the reservoir to the source rock. Four new wells drilled in the recent campaign probed the accuracy and robustness of the pore pressure predictions.
This work is an excellent case study that shows how pore pressure can be modeled in some tight sands that exhibits a stress sensitive behavior. The pore pressure model helped to evaluate not only gas reserves but also sweet spots, making the 3D pore pressure prediction a powerful tool for exploration, drilling and field development planning.