Acoustic and petrophysical properties of mechanically compacted overconsolidated sands: Part 2 - Rock physics modelling and applications

Part one of this paper reported results from experimental compaction measurements of unconsolidated natural sand samples with different mineralogical compositions and textures. The experimental setup was designed with several cycles of stress loading and unloading applied to the samples. The setup was aimed to simulate a stress condition where sediments underwent episodes of compaction, uplift and erosion. P‐ and S‐wave velocities and corresponding petrophysical (porosity and density) properties were reported. In this second part of the paper, rock physics modelling utilising existing rock physics models to evaluate the model validity for measured data from part one were presented. The results show that a friable sand model which was established for normally compacted sediments is also capable of describing overconsolidated sediments. The velocity‐porosity data plotted along the friable sand lines not only describe sorting deterioration, as has been traditionally explained by other studies, but also variations in preconsolidation stress or degree of stress release. The deviation of the overconsolidated sands away from the normal compaction trend on the VP/VS ‐ AI space shows that various stress paths can be predicted on this domain when utilising rock physics templates (RPTs). Fluid saturation sensitivity is found to be lower in overconsolidated sands compared to normally consolidated sands. The sensitivity decreases with increasing preconsolidation stress. This means detectability for 4D fluid saturation changes can be affected if sediments were pre‐stressd and unloaded. Well log data from the Barents Sea show similar patterns to the experimental sand data. The findings allow the development of better rock physics diagnostics of unloaded sediments, and the understanding of expected 4D seismic response during time‐lapse seismic monitoring of uplifted basins. The studied outcomes also reveal an insight into the friable sand model that its diagnostic value is not only for describing sorting microtextures, but also preconsolidation stress history. The outcome extends the model application for preconsolidation stress estimation, for any unconsolidated sands experiencing similar unloading stress conditions to this study