Banca de DEFESA: Haris Raza

Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
STUDENT : Haris Raza
DATE: 18/12/2023
TIME: 14:00
LOCAL: Sala de Videoconferência e Plataforma Teams
TITLE:

Investigations of the Reservoir-triggered Seismicity of Nova Ponte and Irapé, Brazil

 

KEY WORDS:

Brazil, Reservoir-triggered earthquake, Irapé, Nova Ponte, Coupled poromechanical processes, permeability, porosity

PAGES: 94
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUMMARY:

The rise of large hydropower plants with extensive reservoirs has raised the need for a deeper understanding of reservoir-triggered seismicity, a phenomenon that causes significant challenges for safe reservoir management. Reservoir-triggered seismicity is an increasingly relevant topic of interest throughout the world. The global distribution cases involve a large number of hydroelectric power plants as well as future planned dams and reservoirs, with many documented cases for more than six decades. In this scenario, Brazil has experienced 29 cases of RTS in which the mechanism of magnitude 4.0 Nova Ponte and M3.0 Irape is not well understood. The M4.0 RTS mechanism at Nova Ponte is still important to understand, the second-largest known case of Reservoir reservoir-triggered earthquake in Brazil. The main event occurred in the year 1998 along predominantly reverse faulting and roughly oriented NE-SW. But it still remains to be understood whether it occurred by state pore pressure or undrained response of excess loading of the reservoir is such low permeable rocks that are needed to trigger RTS. If the pore pressure diffusion could not propagate deep into vertical low permeable rocks it can take hundreds of years to reach the depth of the earthquakes. So, in such above conditions, the two major events of triggered earthquakes occurred in 1995 and 1998 just 1.5 years after the impoundment of the reservoir when the water reached the highest level. Thus, in this study, we made an attempt in this area to explore the causal mechanism by creating the 3D Reservoir model and its effect on the subsurface both in terms of pore pressure and stress changes with the help of numerical modeling. We numerically simulate the poromechanical subsurface response to reservoir impoundment using a 3D model that includes three geological rock layers down to 10 km depth. From the proposed potential nodal planes of the 1995 M3.5 earthquake, we show that the earthquake has most likely occurred on a vertical, E-W-oriented strike-slip fault with a reverse-displacement component. The deviatoric stresses generated by the water column loading on the surface, superimposed by undrained pore pressure enhancement in deep low-permeability layers can explain the fault reactivation. We find that for the 1998 M4.0 earthquake to occur, conductive flow pathways with permeability as high as 6.6·10-15 m2 should exist to transmit pore pressure to a deep critically oriented fault. The analysis owned by us raises the importance of accounting for coupled poromechanical mechanisms controlling fault stability, hydromechanical properties of different rock layers, and realistic shape of the reservoir to accurately assess the potential for reservoir-triggered seismicity. We remarked that our findings can explain the triggered seismicity by the undrained response of excess loading of the reservoir filling due to pore pressure perturbation such as in deep underground low permeable rocks. The second case study Irapé in Brazil is a prominent RTS site where seismicity surged after reservoir filling, with a maximum event of M3.0 in May 2006. We attempt to understand the potential causes of RTS at Irapé dam, Brazil, which is the highest dam in Brazil with about 208 m, and the second highest in South America. Despite more than a decade, the factors governing these earthquakes and their connection to subsurface rock properties remain poorly understood. We have done permeability and porosity tests of cylindrical cores from hard and intact samples of rocks, which have been extracted (by pitting 10 cm from the surface) near the RTS zone to determine the role of permeability and porosity on the triggering of the Irapé earthquakes. We report porosity values ranging from 6.340 to 14.734% and most of the permeability values are <0.002 mD. According to our analytical calculations, the reservoir-water level increased by 137 m, resulting in a 0.96 MPa pore pressure rise. The resulting vertical effective stress increased by 0.41 MPa and the horizontal effective stress decreased by 0.08 MPa. Thus, the deviatoric stress would increase in the faulting stress regime, destabilizing the fault and causing RTS. Below the reservoir, there is an instantaneous increase in pore pressure in the subsurface due to elastic compression, which brings potential faults located below the reservoir closer to failure conditions. The laboratory measurements and analytical calculations corroborate the hypothesis that the initial seismic activity was induced by the undrained subsurface response to the reservoir loading at Irapé.

COMMITTEE MEMBERS:
Presidente - 1542246 - GEORGE SAND LEAO ARAUJO DE FRANCA
Externa ao Programa - 3165215 - MONICA GIANNOCCARO VON HUELSEN - UnBExterna ao Programa - 1780820 - SUSANNE TAINA RAMALHO MACIEL - UnBExterna à Instituição - ALANNA COSTA DUTRA - UFBA
Externo à Instituição - CRISTOBAL CONDORI QUISPE - UNSA