Assessment of non-linear pressure sensitivity using frequently acquired seismic data

The pressure sensitivity of time-shift signals in a North Sea chalk reservoir is assessed during both production and injection. By analysing permanent reservoir monitoring (PRM) seismic data from 20 surveys, we investigate the influence of pressure cycling on velocity changes. Observed time-shifts are measured using the nonlinear inversion (NLI) method, and pressure estimates are obtained from flow simulation and bottom hole pressure (BHP) data. The analysis focuses on a producer and an injector in an area with expected pressure fluctuations, examining time-shifts and pressure changes between survey sequences. To estimate reservoir stress sensitivity, we apply the S-factor equation by Bauer et al. (2008) and utilize techniques to reduce noise levels and isolate pressure-driven signals. The results confirm that pressure increases show greater stress sensitivity compared to pressure decreases. However, we also observe that the S-factor at the producer is 0.07% per MPa, while for the injector it is 0.04% per MPa, demonstrating stronger pressure sensitivity at the producer. One explanation is that compaction following initial water flooding has reduced the chalk matrix's sensitivity to pressure changes around the injector. The study concludes that a bi-linear model appears most appropriate to represent pressure sensitivity in the reservoir.