Aging of oil/gas-bearing sediments, their compressibility, and subsidence

The in situ stiffness and apparent maximum preconsolidation stress of many soils and sediments appear to be higher than in the laboratory tests. We seek to verify experimentally whether this also holds for deep marine sediments. We also discuss an alternative explanation for this effect to the classical one, implying the sample damage during coring. We test numerically the explanation, suggesting possible unaccounted changes in stiffness, occurring in sediments in situ when subjected to aging, or secondary compression for geological scale time periods. Results of "aging tests" on sandy and clayey sediments are presented, involving secondary compression at the constant in situ stress level, during which strain develops together with other changes in properties. Only two weeks of aging produced a notable increase in the apparent maximum preconsolidation stress and in the stiffness below it, and above the in situ stress. A framework for a mathematical model is proposed, based on the supposition that during aging the sediment develops a secondary microstructure through reactions of local dissolution/precipitation of less stable minerals.