Passive Monitoring of in situ Stress Using Shear-Wave Splitting: Applications to CO2 Storage and Beyond

Joseph Asplet, Mark Fellgett, Tom Kettlety, John-Michael Kendall

Characterising the in situ stress field is important for a range of industrial applications, such as geological CO₂ storage, geothermal projects, hydrocarbon extraction, geological disposal facilities, and for natural seismic hazard assessment. Shear-wave splitting is a passive measurement which can be used to passively monitor changes in the in situ stress field. We combine borehole stress and passive seismic data, primarily from microseismicity, across Great Britain to assess the potential of shear-wave splitting to be used to monitor the in situ stress field with several in-depth case studies. At Preston New Road, Lancashire, the measured shear-wave splitting fast polarisations (ϕf) have a circular mean of −14° and are consistent with the regional Sₕmax. At Newdigate, Surrey, the measured shear-wave splitting is best explained by stress-controlled seismic anisotropy, with fast polarisations consistent with regional Sₕmax (ϕ̄f = −51°) to the north of the Newdigate fault. However, we observed a consistent 90° polarisation flip (ϕ̄f = 40°) at a station to the south of the fault, which is likely due to overpressured pore fluids. In South Wales, there is structure-controlled anisotropy from the Neath Disturbance, with ϕf measurements near the fault parallel to the fault strike. These examples demonstrate the potential of shear-wave splitting as a passive measure of in situ stress and the challenges that remain. We find that shear-wave splitting, measured from microseismicity, could be used to monitor the in situ stress field with sufficient monitoring infrastructure and geological context to constrain interpretations. For CO₂ storage projects, incorporating shear-wave splitting into the design of microseismic monitoring networks will enable valuable additional data to be collected.