Natural conditions for more limited osmotic abnormal fluid pressures in sedimentary basins

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Abstract
Chemical osmosis is considered a plausible cause of abnormal pressures in shale formations of sedimentary basins. A set of experimental data on chemical osmosis was recently obtained for different shales, mainly in the framework of studies on radioactive waste repositories in deep argillaceous formations. Based on these data, large, osmotically induced overpressures up to tens of MPa were predicted by Neuzil and Provost [2009] under appropriate conditions. However, such large overpressures have not been found in sedimentary basins such that the reasons for this disparity between predictions and observations need to be clarified. Accordingly, two natural causes for lower than expected osmotic pressure were investigated: the effect of the complex composition of natural waters, including both monovalent and divalent cations, on the osmotic efficiency and the resulting abnormal pressures, and the presence of steady state rather than transient‐state salinity distributions. For this purpose, an electrical triple‐layer model accounting for multi‐ionic solutions was developed and used to calculate the osmotic efficiency at different proportions of monovalent and divalent cations. The calculated decrease of the osmotic efficiency when Ca2+ is introduced in a Na+‐clay system yields a noticeable decrease in the ability of the shale to generate overpressures. A discussion addresses the generation of abnormal pressures at steady state conditions found in sedimentary basins, i.e., with a linear distribution of the concentration across the formation. The persistence of moderate overpressures was predicted because of the nonlinearity associated with the dependence of the chemo‐osmotic efficiency on the concentration and the porosity. Finally, a case study of the moderate excess hydraulic head measured in the Toarcian/Domerian argillaceous formation of Tournemire (SE of France) was investigated. The analysis indicated an osmotic origin for the excess head and illustrated the influence of the pore water composition.

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