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Nanoparticles (NPs) have been proposed as a promising agent for groundwater aquifer remediation, greenhouse geological storage, and enhanced oil recovery (EOR) applications. Nevertheless, in many of these cases, NPs applications require long transport distances, which may be limited by NPs retention in porous media, especially at high saline conditions as is the case with some oil reservoirs, causing a reduction in rock permeability and subsequent formation damage. Therefore, understanding of NPs transport through porous media and their stability behavior are significant challenges before their large-scale applications, which has been less explored in previous works. This paper investigates silica NPs transport through dolomite rocks, which are the host rocks of many sub-surface reservoirs. To this end, the stability of silica nanofluids with different concentrations in the presence of different ionic strength and ion types was explored by zeta potential measurement and particle size analysis. NPs adsorption on rock surface was examined by SEM images. The DLVO analysis was conducted to determine the interaction energies between NPs and the rock, and investigate the reversibility of NPs adsorption on the rock. Single-phase coreflooding experiments aided by NPs effluent analysis were performed to obtain breakthrough curves. Results indicated that increasing silica NPs concentration from 0.1 to 0.5 wt% leads to a reduction in nanofluid stability in which the average size of silica NPs increased from 110 to 220 nm. Higher ionic strength (50,000 ppm) decreased nanofluids stability and this negative effect was more pronounced for nanofluids containing MgCl2 salt. It was found that less stable nanofluids showed higher NPs retention through porous media. Highly stable nanofluids exhibited a higher probability of NPs to be detached during the brine post-flushing stage. As to the results of silica nanofluid injection in dolomite rock, increase of the silica NPs concentration and decrease of their stability in suspension were two causes of the rock permeability impairment; a 10% reduction in rock permeability was observed by 0.1 wt% salt-free silica nanofluid, whereas it decreased by 87% using 0.5 wt% of saline nanofluids prepared by 50,000 ppm MgCl2. According to the interpretations based on the deep bed filtration model, the higher salinity of nanofluid is, the higher filtration coefficient would be, meaning that the higher rate of particle capture at higher salinities. Also, no internal/external filter cake was formed and instead, the pore filling process was found to be the major mechanism of the permeability impairment by silica NPs. |
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