Fluid distribution in kaolinite- or illite-bearing cores: cryo-SEM observations versus bulk measurements

Cryo-SEM has become a powerful tool to show the distribution of oil and brine in cores, which may be highly influenced by wettability properties. Previous studies have shown special affinity between kaolinite and oil after aging. The present work clarifies the behaviour of illite- or kaolinite-bearing cores, and compares with the measurements made at the plug scale (`bulkʹ measurements). After aging, fibrous illite has still no affinity for oil, kaolinite has an affinity for oil, but platy illite behaves more like kaolinite. However, a minimal quantity of kaolinite-like surfaces is needed to change the wettability of the bulk sample towards oil-wetness. The `bulkʹ wettability depends thus, although not linearly, on the nature, morphology, amount and distribution of clay, but one cannot ignore phenomena occurring at larger scales such as trapping. A qualitative agreement is found between the amount of oil and brine, depending on the state of saturation, and the bulk measurements (wettability index, Swi, Sor).