Shear‐Wave Splitting Measured for Permanent Reservoir Monitoring Systems: An Example From the Snorre Field

ABSTRACT Microseismic monitoring of offshore 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 storage complex and the surrounding region. Shear‐wave splitting is one potential source of added value to microseismic monitoring of 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 network at the Snorre field. We measure shear‐wave splitting for the M W 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 0.7, recorded by seafloor instruments. At Snorre, the average shear‐wave splitting fast polarisation direction and percentage anisotropy . This is consistent with microcracks preferentially aligned with the maximum horizontal stress azimuth. At Snorre, we estimate this as 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 with the remaining clustering around . 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.