Periodic Reporting for period 1 - DEGASS (An experimental approach to understand inDuced sEismicity in GAS Shales)
Periodo di rendicontazione: 2016-01-01 al 2017-12-31
Slip along pre-existing faults can be stable, or unstable, in which case it causes seismicity. This is important in a variety of settings, including during hydrofracturing operations to recover natural gas. Successful, safe and publically accepted global implementation of such operations critically depends on understanding the conditions leading to the different types of fault slip. Such understanding requires information on the frictional properties of the materials within these faults, typically consisting predominantly of clays, i.e. phyllosilicates. The principal objective of this project was to determine the conditions and the microscale deformation mechanisms that lead to stable versus unstable slip of reactivated faults in phyllosilicate-rich materials.
Experiments, microstructural analyses and microphysical modelling work were conducted at the University of Liverpool, UK, to achieve the objective of this study. The results of this work suggest that phyllosilicate frictional behaviour is controlled by bending of the platy phyllosilicate grains into the pore space between the grains and breaking of the grains. Factors such as temperature and fault slip velocity seem to control stable versus unstable fault slip. Another major result of this study is that rock samples taken from sections along a fault with different slip behaviour also show different slip behaviour in the laboratory. This is interesting given that the different samples contain very similar minerals. These results suggest that composition is not the only factor that determines whether a fault slips in a stable or unstable manner.
The microphysical model developed allows for the first time quantitative prediction of the frictional properties of phyllosilicate fault materials. These predictions can be used by modellers to better predict seismicity. The model provides a framework where individual components are testable with laboratory experiments and hence it may be refined in the future to give a physical explanation of friction in phyllosilicate-rich faults.