Shear-wave splitting measured for permanent reservoir monitoring systems: an example from the Snorre field

Joseph Asplet, John-Michael Kendall, Annie Jerkins, Tom Kettlety

Microseismic monitoring of offshore CO2 storage projects is likely to include some deployment of offshore sensors. To improve the value proposition of this monitoring infrastructure, it is important to consider what other information can be gained about the CO2 storage complex and the surrounding region. Shear-wave splitting is one potential source of added value to microseismic monitoring of CO2 storage operations at minimal additional cost, if factored in during network design. Shear-wave splitting provides a means to passively monitor the in situ horizontal maximum stress azimuth and potentially the magnitude of differential horizontal stresses. We demonstrate this for offshore monitoring of reservoirs using data recorded by the permanent reservoir monitoring (PRM) network at the Snorre field. We measure shear-wave splitting for the MW 5.1 Tampen Spur earthquake and subsequent microseismic aftershocks. Our results show that high-quality shear-wave splitting measurements can be made for microseismicity, with ML ≥ 0.7, recorded by seafloor instruments. At Snorre, the average shear-wave splitting fast polarisation direction φf = 92±15◦ and percentage anisotropy ξ = 2.68 ± 0.26. This is consistent with microcracks preferentially aligned with the maximum horizontal stress azimuth. At Snorre we estimate this as 108 ± 4◦ using data from the World Stress Map. The shear-wave splitting results contain two groups of fast polarisation directions. The four westernmost stations cluster around φf = 68 ± 13◦ with the remaining clustering around φf = 113 ± 4◦. This variation may be due to the depletion history of the reservoir. Incorporating shear-wave splitting into microseismic monitoring plans potentially allows for semi-continuous measurements of the changes to the stress field in the storage complex and surrounding region, provided there is sufficient microseismicity. This demonstrates that shear-wave splitting is a valuable dataset for monitoring the offshore subsurface stress field, which should be considered when planning offshore passive seismic monitoring.