Comparative study of the impacts of soil wettability during entrapped LNAPL removal by surfactant flooding in two different sand media

Abstract

Purpose

Non-aqueous phase liquid (NAPL) spills may cause soil wettability changes. Soils traditionally water-wet may become NAPL-wet or fractionally wet. Since soil wettability alterations may impact entrapped light NAPL (LNAPL) removal by surfactant flooding, the effects of soil wettability and NAPL contact time were investigated for graded sand and compared to uniform sand. The graded medium was expected to have higher entrapped LNAPL saturations and lower entrapped LNAPL removal after surfactant flooding.

Materials and methods

Capillary pressure–saturation (P _c–S) tests (water-LNAPL) and waterflood and surfactant-flood column tests were performed using fine graded sand and compared with results previously reported for the uniform coarse-grained sand. Heptane and LNAPL collected from observation wells at a LNAPL-contaminated oil refinery were used as the ideal NAPL and field LNAPL, respectively. Four experimental conditions were investigated: (1) clean water-wet sand and heptane, (2) clean water-wet sand and field LNAPL, (3) fractionally wet sand and heptane and (4) fractionally wet sand and field LNAPL. LNAPL entrapment after waterflooding and the enhanced entrapped NAPL removal (surfactant-flood minus waterflood saturations) were evaluated after 2 h and 5 days of contact time. Two anionic surfactants, sodium dodecylbenzenesulfonate and a field-site surfactant, were applied at the submicellar concentration of 0.5 g/L.

Results and discussion

The P _c–S curves obtained for heptane and the field LNAPL, for a given experimental condition, closely resembled each other owing to the similar interfacial tension behaviour observed for these LNAPLs. In contrast to the results reported previously for the uniform sand, the average entrapped saturations after waterflooding were variable for the finer graded medium; contact time had a significant effect on LNAPL entrapment for the graded clean sand medium with an increase of the entrapped LNAPL saturation from 17 to 28 %. Although the removal for the fractionally wet medium was similar for both sand types, with average removals as high as 45 %, the surfactant-flood column tests with the clean water-wet graded medium did not result in a consistent reduction of the entrapped saturations.

Conclusions

The P _c–S relationships demonstrated that heptane and the field LNAPL behave similarly during the P _c–S tests. The column experiments revealed differences in entrapped LNAPL saturations among the two sand types as a result of contact time and fractional wettability. Pore structure heterogeneity impacted LNAPL saturations and entrapped LNAPL removal. Overall, soil wettability should be characterized prior to surfactant flooding. The use of anionic surfactants at submicellar concentrations may be beneficial at sites with fractionally wet media.