Abstract
Core flood and field tests have demonstrated that decreasing injection water salinity increases oil recovery from sandstone reservoirs. However, the microscopic mechanism behind the effect is still under debate. One hypothesis is that as salinity decreases, expansion of the electrical double layer decreases attraction between organic molecules and pore surfaces. We have developed a method that uses atomic force microscopy (AFM) in chemical force mapping (CFM) mode to explore the relationship between wettability and salinity. We functionalised AFM tips with alkanes and used them to represent tiny nonpolar oil droplets. In repeated measurements, we brought our “oil” close to the surface of sand grains taken from core plugs and we measured the adhesion between the tip and sample. Adhesion was constant in high salinity solutions but below a threshold of 5,000 to 8,000 ppm, adhesion decreased as salinity decreased, rendering the surface less oil wet. The effect was consistent, reproducible and reversible. The threshold for the onset of low salinity response fits remarkably well with observations from core plug experiments and field tests. The results demonstrate that the electric double layer force always contributes at least in part to the low salinity effect, decreasing oil wettability when salinity is low.
Highlights
We have developed a method that uses atomic force microscopy (AFM) in chemical force mapping (CFM) mode to explore the relationship between wettability and salinity
We identified the sand grains as quartz by their hexagonal crystal form, i.e. the angles between crystal faces, observed by optical microscopy and we confirmed their identity by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDXS) after the AFM experiments were completed
Using force mode AFM, we collected adhesion force maps in a series of solutions where salinity was sequentially diluted from artificial formation water to low salinity artificial sea water to mimic a low salinity flood, as is used in enhanced oil recovery’’ (EOR)
Summary
Lager et al.[8] suggested that multicomponent ion exchange (MIE) takes place at the pore surface with low salinity flooding so polar compounds desorp from the surface when the low concentration of ions promotes breaking of mineral-cation-oil component bridges This would change the wettability toward more water wet conditions. Change of wettability of the pore surfaces is, as stated by Lager et al.[8] and Austad et al.[5], is central to the low salinity mechanism Both -CH3 and -COO(H) terminated tips have been used previously, to investigate the effect of changing salinity, on surface adhesion in sandstone and chalk[20,21,22,23]. We decreased ionic strength sequentially and increased it again by mixing aliquots of AFW so we could i) identify the threshold where the low salinity effect begins to appear and ii) elucidate how large a role, the electric double layer expansion plays in the observed adhesion behaviour
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