Strain characteristics and permeability evolution of faults under stress disturbance monitoring by fibre bragg grating sensing and pressure pulses

The sealing and structural stability of faults must be considered in many underground engineering endeavours, such as underground energy extraction and CO2 geological storage. In order to simultaneously study the evolution laws of fault sealing and structural stability evolution law under stress disturbance, the fault samples were first consolidated under previous effective stress (P-pre = 10 MPa) for 62 h to imitate the fault being under the ground stress; then, the effective stress (P-e) was reduced to 0.5 MPa and gradually changed to 20 MPa to imitate the stress disturbance. Permeability and strain were measured throughout the stress disturbance by the pressure pulse method and fibre Bragg grating (FBG) sensors. In particular, high-precision FBG sensors were applied inside the fault to obtain the internal strains. The test results show that: (1) when P-e < P-pre, the permeability rapidly decreased with increasing P-e, and a risk of leakage occurred. Conversely, when P-e > P-pre, fault permeability changed little as P-e increased; but the permeability was still 4.91 times that of the undisturbed state. (2) The internal and external strain monitoring results revealed that the geometric centre of fault showed tensile strain, while other monitoring points showed compressive strain most of the time. The reason for this phenomenon is the end effect, which causes the gouge particles in the middle to move to both ends. (3) The normal deformation of fault gouge not only controls the fault volumetric strain but also controls the evolution of permeability. This study provides technical and theoretical support for the evaluation of fault sealing and structural stability when faults are disturbed in underground engineering development and construction.

Automated summary

This study investigates the evolution of fault sealing and structural stability under stress disturbance by conducting experiments on fault samples. The results show that fault permeability is significantly affected by stress changes, and internal strain is influenced by the end effect. The normal deformation of fault gouge plays a crucial role in controlling both fault volumetric strain and permeability evolution.