Anomalous softening signal in a clastic reservoir explained by pore collapse

We present an example of a weakly cemented sandstone reservoir that presents unusual 4D seismic softening signals as a response to pore pressure decrease. With a detailed multidisciplinary interpretation, we assess all possible explanations for this signal showing that explanations based on fluid saturation changes or elastic stress variations fail to explain the full characteristic of the observed anomalies. We propose an inelastic rock damage process as an alternative hypothesis to explain the observed anomalies. We show that this hypothesis is the only one that explains all the characteristics of the observed anomalies: their lateral and vertical extents, as well as their magnitude and temporal evolution. We then conduct a theoretical modelling study to estimate the amount of rock damage needed to produce the observed 4D seismic signals. This feasibility study suggests that the effective pressure achieved during field production may be enough to cause the cracking of cement and weak grains, which we estimate through rock physics modelling to be a sufficient mechanism for producing the observed softening anomalies. We highlight important considerations regarding plans of CO₂ storage into depleted reservoirs, as the possibility of rock damage during production would complicate the monitorability of the injected CO₂ plume.