Early Warning of Fault Reactivation through Passive Acoustic Emission in Samples Analogous to Carbon Storage Reservoir

Climate change drives urgent action in decarbonisation, and carbon capture and storage (CCS) has emerged as a crucial technology in mitigating greenhouse gas emissions. Large-scale subsurface CO₂ injection carries the inherent risk of inducing fault reactivation and microseismic events, which could compromise the project. To optimise CCS projects while mitigating these geological hazards, passive acoustic emission (AE) monitoring offers a real-time method to detect initial fracture activity before failure.

In this study, triaxial compression experiments were conducted on reservoir-analogue sandstone sample plugs. Intact samples were axially loaded under an initial confining pressures (Pci) with continuous passive AE recording. A shear fracture was then induced in each sample, which was subsequently re-sheared under different confining pressure regimes (Pc) to mimic fault reactivation. Two porosity groups (~20% and 26%) were tested to evaluate deformation effects on AE response. Acoustic sensors at the sample ends captured the P-wave signals throughout each loading cycle, and the AE events were analysed in conjunction with the mechanical stress-strain data. From these mechanical data, failure envelopes were derived to assess the applicability of failure criteria. The results show that the Mohr–Coulomb criterion provides good agreement with all tests conducted and that fractured specimens may exhibit friction angles different from intact rock while retaining a non-zero cohesion, which should not be neglected when modelling fractured reservoirs for CCS.

The acoustic emission results reveal clear precursor patterns to fracture slip. For intact samples, axial loading triggered intense AE activity from the outset, reflecting micro-cracking and particle rearrangement. In contrast, samples with pre-existing fractures showed an initially low rate of emissions, increasing significantly just before the peak stress. Notably, higher-porosity samples generated roughly an order of magnitude more emissions than lower-porosity samples during both the initial fracturing and the reactivation phases, and consequently a much higher cumulative acoustic energy release.

Crucially, the cumulative AE record revealed a distinct acoustic precursor to failure. During re-shearing, the cumulative event count initially increased steadily, then underwent a sudden acceleration (an identifiable inflection point) shortly before the peak stress. This surge in event rate consistently occurred when the sample was still below its peak strength, signalling imminent failure. Such a signal could serve as an early warning. In a field injection scenario, detection of this acoustic inflection would allow operators to adjust injection rates or pressures before fault reactivation. Incorporating passive AE monitoring in this way could enhance CCS safety by optimising operations and preventing induced seismicity.