Effect of low-salinity waterflooding on rock wettability based on DLVO theory

Key words:  wettability alternation  low-salinity waterflooding  DLVO theory  electric double layer theory  disjoining pressure

Abstract:

To clarify the main controlling factors of wettability alternation mechanism, a mathematical model of disjoining pressure for surface interaction forces was established in terms of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and electric double layer theory of colloid stability. And the sensitivity analysis for disjoining pressure was conducted at the microcosmic level to examine the impact of salinity and cation type. Also, the stable water film thickness and contact angle of water on rock were calculated by combining the Young-Laplace equation and interaction potential theory. The results show that cation type is the main factor of controlling the rock wettability. A small amount of divalent cation can trigger large negative disjoining pressure, forming a thinner water film with a thickness of 0.375 nm and the contact angle of water is maintained at about 20°. With the decrease of monovalent cation salinity, the disjoining pressure increases gradually and produces a higher positive peak value, forming a thick water film with a thickness of 5.334 nm. The rock wettability is altered to complete water-wet state and the contact angle of water is maintained at 0°, which causes crude oil to detach from rock surface eventually. It is found that low salinity waterflooding with monovalent cation has a higher potential for improving oil recovery in oil field development.