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Abstract
Fines migration is considered one of the mechanisms of permeability alteration associated with water injection in aquifers and oil reservoirs. Numerous studies have argued fines migration as a mobility-control enhanced oil recovery (EOR) mechanism during low-salinity waterflooding (LSW). However, enhanced oil recovery by LSW has been shown to result from the combined effect of various mechanisms. The dependence of oil recovery on fines migration remains unclear. The purpose of this thesis is: (1) to present the conclusive evidence for fines migration and (2) to investigate oil recovery mechanisms associated with fines migration during low-salinity waterflooding. In this thesis, laboratory experiments were designed to render inactive all but the fines migration mechanism. Then, waterflooding results and detailed characterizations of core plugs and produced fluids were used to analyse fines migration-induced formation damage and retention effects on displacement efficiency.
Single-phase and two-phase flow experiments were performed on Berea and Obernkirchener sandstones. In the single-phase experiments, core plugs were subjected to waterflooding with monotonically reducing salinities from 40 to 0 g/L NaCl. 80% to 99% decrease in permeability was observed, which were accompanied by large concentration of fines in the produced-water samples. Scanning Electron Microscope (SEM) images of the pores before and after injection showed fines retention and blockage. Produced fines were analysed using SEM integrated with Energy Dispersive X-ray Spectroscopy (SEM-EDS), analysis shows that clay particles were produced during LSW. Micro-CT images were taken and Nuclear Magnetic Resonance (NMR) responses were recorded before and after the single-phase experiments. Micro-CT analysis and NMR responses support the fines migration. Micro-CT images show that clay fines were strained near the core outlet of the core plug.
There were three two-phase LSW experiments performed in total. In all of these two-phase LSW experiments, clay migration results were consistent with those of single-phase LSW experiments. Furthermore, the two-phase experiments showed 50% reduction in water relative permeability after LSW. Two out of the three experiments show 3% higher oil recovery in LSW than high-salinity waterflooding. Because no other mechanism was active, the increase in oil recovery was solely a result of enhanced displacement due to fines migration. However, one of the two-phase experiments showed 16% lower oil recovery in LSW than high-salinity waterflooding. It is suggested that mobilized fines caused blockage which restricted the direct contact between oil and water. More experiments are needed to find out conditions under which LSW has positive effect on oil recovery.