Spatiotemporal variation in upper crustal seismic anisotropy and VP/VS ratio in Groningen gas field, Netherlands: insights from shear wave splitting

Abstract

SUMMARY We investigated pore fluid characteristics of reservoir at the Groningen gas field, Netherlands using seismic anisotropy from shear wave splitting. We obtained 663 high-quality splitting parameters, such as delay time (δt) and fast-axis direction (φ), from local induced earthquakes that occurred during 2014–2020. We then conducted δt tomography and spatial averaging of φ to probe the spatiotemporal changes in the parameters. Our results demonstrate that crustal anisotropy in Groningen is closely related to changes in fluid flow and compaction of the reservoir. Low anisotropy strength (normalized delay time, δtn) was detected near shut-in wells where considerable reservoir compaction has occurred, whereas high anisotropy strength occurred near operating wells with less compaction. The temporal variation in δtn further shows that the long-term trend is distinctly different between the shut-in and production wells: while δtn near the shut-in wells remained nearly constant at <6 ms km–1 during the entire period, δtn near the operating wells notably showed high values exceeding 10 ms km–1 in 2015 and a decreasing trend until 2017. In addition to this long-term variation, short-term δtn variation was observed, which can be related to fluid flow perturbations owing to seasonal gas production fluctuations. The φ values along the NNW–SSE, WNW–ESE and E–W directions could be related to not only the in situ maximum horizontal stress direction but also the gas flow direction near the extraction wells. The observed negative relationship between the spatial distribution of P-to-S velocity ratio and the anisotropy strength can potentially indicate that gas-filled pore spaces are dominant in the reservoir.