Cryo-SEM and ESEM: New techniques to Investigate Phase Interactions within Reservoir Rocks

Abstract

Abstract Recent advances in scanning electron microscopy allow the observation of liquid samples, either under a frozen state in the case of Cryo-SEM (Cryo Scanning Electron Microscopy) or directly with ESEM (Environmental Scanning Electron Microscopy). In the case of sandstones, cryo-SEM studies have pointed out the special affinity between kaolinite and oil after aging, and shown the importance of both amount and distribution of kaolinite on wettability behavior at the core scale. Good correlations were found between observations at the pore scale and wettability indices. Wettability alterations were shown to be very sensitive to thermodynamic conditions (pH, salinity, etc.). It has also been used to study the impact of the spreading coefficient on oil recovery by gas injection. It has been experimentally shown that both wettability and spreading could affect gas injection efficiency by controlling the distribution of the three phases at the pore level. It proved to be efficient in other studies such as emulsion characterization or asphaltene deposition. The main advantage of ESEM is to operate under reduced pressure and to allow the observation of liquid-containing samples without any special preparation. Combined with Energy Dispersive X-ray Spectroscopy (EDS), it also allows elemental analysis. It has successfully been used to investigate the sensitivity of different clays to freshwater, to characterize oil base muds, to analyze formation damage mechanisms and to improve acid stimulation. Dynamic experiments are also possible such as drying, freezing or even capillary flows. ESEM allows wettability estimation by direct visualization of both oil and brine in their liquid state; it has confirmed the results obtained by Cryo-SEM and the correlation between fluid distribution with either pore structure (carbonate) or mineralogy (sandstone).