Nano Geochemistry of Low Salinity Enhanced Oil Recovery

In this thesis a wide range of analytical techniques were used to characterise several petrophysical properties of Berea sandstone, including the mineral distribution at the pore surface and its pore structure, both playing a crucial role in determining its response to low salinity enhanced oil recovery (EOR) investigations. In addition, the role of different cations in affecting the wettability state of pure quartz and Berea sandstone was experimentally investigated in order to gain an insight on the behaviour of sandstone reservoirs during low salinity waterflooding EOR. Results from the multi-technique, multi-scale characterisation of Berea indicate that the mineralogy exposed to the pore spaces is highly heterogeneous across different length scales, going down to the often-neglected nanoscale were significant amounts of phases identified as grain coatings. In addition, analysis of the porosity and pore-connectivity also requires a multi-length approach for its full characterisation to be realised. Both aspects are crucial to understand the role of mineral surface chemistry in determining oil/water and oil/minerals interactions in both experiments and field conditions. Investigations on wettability alteration using environmental scanning electron microscopy (ESEM) and contact angle measurements on ideal quartz surfaces showed that reduced salinity leads to a more water-wet state. These measurements were complemented with atomic force microscopy adhesion measurements on quartz surfaces, the results giving further insight into the role of nano-scale roughness on quartz surfaces in wettability alteration by increasing the amount of oil retained on the surface. Finally, similar wettability alteration experiments were performed on Berea sections. The effect of brine was consistent, reproducible and reversible and again showed a low salinity effect, i.e. a change to more water-wet conditions with lower salinity. The results also demonstrate that quartz surfaces always contributes at least in part to the low salinity effect, decreasing oil wettability when salinity is low. In addition, we demonstrate that the ESEM can be an essential tool in studying the wettability alteration of rocks and minerals