The effect of ionic strength on oil adhesion in sandstone – the search for the low salinity mechanism

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.

A pH-Resolved View of the Low Salinity Effect in Sandstone Reservoirs

“…Although the particles came from the same reservoir core plug, their topography and surface composition were very different. The images were consistent with images taken by other researchers on material from the same core plug and from different samples 13,16 . Fig.…”

“…We see this range in adhesion, in this work and others 13, 16 but we have also seen that the general adhesion properties are consistent across all of our work. Regardless of the initial adhesion force, which is a result of the interaction between the organic material on the surface and the tip, the force changes consistently when the salinity of the solution in contact changes and these changes are reversible through several cycles of solution exchange 16 . This provides confidence that changes in the adhesion resulting from changing pH would be observable regardless of the absolute adhesion, which 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 7 would depend on the character of the adsorbed organic material.…”

“…The samples were prepared for the AFM experiments according to the procedure described in Hassenkam et al 16 . Briefly, a glass cover slip was treated in an ozone plasma chamber for 20 min to remove adventitious organic compound contamination.…”

“…This has been investigated by changing salinity while measuring the adhesion between sandstone grains and a model for an oil droplet, in AFM chemical force mapping experiments 16 . The result of such work can be interpreted by DLVO theory 17 where a double layer force is described in terms of a screened interaction between two charged surfaces and interactions between the solution ions and the two surfaces.…”

Efficiency of enhanced oil recovery using polymer-augmented low salinity flooding

“…Extensive studies have been conducted to determine the LSW recovery mechanisms, and debatable conclusions were proposed. Among those suggested is fines migration (Tang and Morrow 1999a), multi-ion exchange (MIE) and doublelayer expansion , pH effect (McGuire et al 2005), and wettability alteration (Tang and Morrow 1999a, b;Austad et al 2010;Alotaibi et al 2010;Hilner et al 2015).…”