This study investigates the effects of grain size on petrophysical and acoustic properties of experimentally compacted synthetic mudstones. Four brine-saturated kaolinite aggregates sorted by the grain size were compacted in the laboratory under vertical effective stress up to 50 MPa. Results show that the kaolinite aggregates compacted differently and have atypical acoustic and petrophysical properties as a function of grain size. The maximum compaction was observed in a mixture which contained mixed grain sizes (composite aggregates) whereas the minimum compaction was found in the mixture containing the smallest grain size (<2 microns). The composite aggregate compacted to 27% porosity at 20 MPa effective stress while the aggregates containing the <2 micron grains retained porosity of about 34% at same effective stress. This could be explained by the distribution of effective stress over a larger number of grain contacts in the fine-grained mixture compared to coarser-grained mixtures. The finest grained sample has the lowest density, permeability and velocity compared to the other mixtures. To our knowledge this is the first time grain size related changes in physical properties during compaction has been demonstrated in kaolinite aggregates. These results will have practical application in rock physics, seismic and well log interpretation.


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