Nanometer-scale pore fluid distribution and drying damage in preserved clay cores from Belgian clay formations inferred by BIB-cryo-SEM

•BIB-cryo-SEM method allows imaging fluid-filled pores in preserved, hydrated clays.•Nm- scale pore fluid distribution in preserved clay cores from Belgium.•Water measured by weight loss is only located within the clay aggregates.•The original clay cores are not fully wet-preserved.•Identification and description of nm-scale damages in clay induced by drying.•Statistical BIB-SEM studies of porosity on dried samples are representative.

This contribution reports on the distribution of in situ pore-water in preserved clay cores from Belgium (Boom Clay at HADES URF level and clay from the Pittem Member of the Gent Formation) and identification of typical pore-scale damage induced by drying.BIB–cryo-SEM (Broad Ion beam–cryogenic-SEM) shows that the majority of pores visible at the resolution of SEM (> 50 nm) does not contain water, suggesting that the water content measured by weight loss is located in clay aggregates, in pores < 50 nm.Because water-free pores are not expecting in situ clay formation, we conclude that the procedure used to preserve the original clay cores is not fully efficient. Observations of evolution of microstructures before and after drying by sublimation of water point to four types of drying-damages driven by the shrinkage of clay particles. On the one hand, quantification of pore size and pore morphologies before and after drying indicates no significant change in statistical pore characterization, suggesting that quantitative pore morphology studies on a large number of pores performed on dried samples are representative of the preserved core samples. On the other hand, the drying damage observed in this contribution is proposed to be responsible of the changes of fluid flow properties typically measured by other authors on preserved and dried specimens of clays.