Previous Research

Previous research^[1,2,3]  at Açu used observations of Reservoir-Induced Seismicity (RIS) beneath the reservoir to investigate the damage zone permeability of geological faults in crystalline basement rocks. High-resolution digital seismic monitoring of the reservoir provided information on the timing and locations of seismic events. Model simulations, using a 3D decoupled hydromechanical formulation (i.e. a static permeability field decreasing exponentially over depth due to increased confining pressure), showed that flow occurs within permeable fault planes that are embedded within a low permeability host rock. Predictions of the maximum pressure change in these faults at hypocentral depths (~2.5 km) indicate <0.05 kPa is required to trigger seismic events. Further, the observed spatial and temporal variability of earthquakes indicate that these faults must have heterogeneous permeability fields with significant spatial structure; pockets of high and low permeability of the order of 0.5 - 1.5 km in diameter (see figure below-right).

This early research^[1,2,3] demonstrated for the first time that microseismicity data could be successfully employed to image spatial and temporal evolution of fault permeability.

1.  Nascimento, A.F.D., Cowie, P., Lunn, R.J. and Pearce, R., 2004. Spatio-temporal evolution of induced seismicity at Acu Reservoir, NE Brazil. Geophysical Journal International, 158: 1041-1052.
2.  Nascimento, A.F.D., Lunn R.J. and Cowie, P., 2005. Numerical modeling of pore-pressure diffusion in a reservoir-induced seismicity site in NE Brazil. Geophysical Journal International, 160:249-262.
3.  Nascimento, A.F.D., Lunn R.J. and Cowie, P., 2005. 3D numerical modelling of reservoir-induced seismicity: the impact of fault heterogeneity. Journal of Geophysical Research. 110: B09201.