Multi-Scale Investigation of Low Salinity Water Flooding for Enhanced Oil Recovery

Injecting low salinity water in sandstone reservoirs is the most practical example of smart water technology. However, the feasibility of low salinity flooding for enhanced oil recovery is controversial for carbonates due to the poor understanding of the displacement mechanisms, which should be potentially responsible for low salinity effect.
This project presents a novel concept to study the complicated interactions in solid/brine/crude oil systems at multiple scales with more insight on the physicochemical mechanisms affecting the wettability trend and hence, the potential of low salinity flooding. Different microscopic and macroscopic apparatuses were used including atomic force microscopy (AFM), quartz crystal microbalance (QCM), microfluidic system, goniometer, and core flooding setup.
Studying the rock/brine/oil interactions at a macroscopic-scale shows that the salinity effect is more salient at the rock/liquid interface than the liquid/liquid interface, and the response to the brine composition is dominated by the chemical composition of crude oil with respect to its content of polar organic components, as well as the rock mineralogy. These results have been corroborated by a series of macroscopic core flooding experiments conducted at in-situ reservoir conditions. Molecular level QCM study also shows that increasing the content of negative polar components in crude oil leads to less desorption from calcite surface compared to the silica surface upon exposure to low saline solutions, verifying the macroscopic core flooding findings. Two times diluted seawater yielded the highest desorption efficiency as a result of a reduction in the adhesion forces, as detected by AFM study.
Investigating the potential of enhancing oil recovery by low salinity flooding at the pore-scale, however, did not show any positive effect on the microscopic sweeping efficiency for the oil-wet system compared to the water-wet system. No change in the in-situ wettability was observed during a sequential low salinity injection in a hydrophobic microstructure, and the pore surfaces stay within a strongly oil-wet condition.
The work described in this thesis revealed that there is a critical brine concentration for EOR in carbonates that should be considered, after which no measurable effect is detected. Low salinity flooding is an inappropriate technique for enhanced oil recovery for the strongly oil-wet formations saturated with heavy-polar crude oils. In addition, while the expansion of the electric double layer at lower salt is likely to be responsible for reduced oil adhesion on carbonates, it is modulated by surface ions binding. Therefore, a combination of these two mechanisms, as well as the salting-out phenomenon has a dominating effect on low salinity flooding performance. The pore-flow of brine in the water-wet system is different from that of a completely oil-wet system, and thereby the potential and associated mechanisms of low salinity flooding are expected to be different.