Experimental mechanical compaction of sands and sand–clay mixtures: a study to investigate evolution of rock properties with full control on mineralogy and rock texture

Abstract

ABSTRACTDevelopment of rock physical properties in well‐sorted and poorly‐sorted unconsolidated mono‐quartz sands and sand–clay mixtures as a function of effective stress in both dry and brine‐saturated conditions is assessed in this study. The tested samples were prepared with full control on their mineralogy, grain size, grain shape, sorting, and fabric. The experiments were performed in a high‐stress uniaxial oedometer up to a maximum of 30 MPa vertical effective stress. Sand–clay samples were a mixture of sand grains and clay particles (kaolinite or smectite) in different proportions. The maximum clay volume fraction used in the experiments was at most 30%. The initial bulk density of the tested sand‐dominated samples was adjusted to be close to the maximum index density expected for natural sediments (sand–clay mixtures) during deposition.In pure sand samples, finer grained sand show higher initial porosity than relatively coarser grained sands. Moreover, sand–clay mixtures have lower initial porosity than pure sands. Porosity decreases as a function of increasing clay content. The poorly‐sorted sand samples are less compaction prone than the well‐sorted sand samples. Among well‐sorted sand samples, coarser grained sands are more compressible than finer grained sands. At a given effective stress level, sand–clay mixtures are more compaction prone compared with their sand component alone. Pure sands and clay‐poor sand–clay mixtures (either sand–kaolinite or sand–smectite) show almost the same degree of compaction when tested in both dry and brine‐saturated conditions. In contrast, clay‐rich sand–kaolinite and sand–smectite mixtures (clay volume >20%) are significantly more compact in brine‐saturated condition. The Vp values of brine‐saturated sand–kaolinite mixtures shows a positive correlation with the kaolinite content, whereas Vp starts to decrease substantially when the volume fraction of smectite exceeds 10% of the whole sand–smectite samples.Saturated bulk moduli estimated by Gassmann's fluid substitution agree with measurements for brine‐saturated clay‐poor sand samples. However, the model does not yield proper predictions for sand–clay samples containing 20% clay volume and above, particularly when the clay is mainly smectite. The acoustic and physical properties derived from experimental compaction of pure sands and sand–clay mixtures show a good agreement with rock properties derived from well logs of mechanically compacted pure sands and shaly sands in progressively subsided basins such as Viking Graben in the North Sea. Thus, the outcome of this study can provide reliable constraints for rock physical properties of sands and shaly sands within the mechanical compaction domain and contribute to improved basin modelling and identification of hydrocarbon presence, overconsolidation, and/or undercompaction.