Abstract

We performed a series of well-controlled laboratory experiments investigating the infiltration and subsequent immobilization of nonaqueous-phase liquid (NAPL) in saturated heterogeneous media. A system of two distinct aquifer zones separated by a dipping interface was considered. Heterogeneity was represented by a spatially correlated random field with known geostatistical parameters in one zone in combination with a homogenous packing of the other zone. The effects of heterogeneity on NAPL flow and entrapment in each of the two zones were investigated. The time-varying NAPL saturations were continuously monitored in space and the final static entrapment–saturation distribution was accurately measured. The immobilized-NAPL distribution contributes to plume generation from source zones. The results show that capillary barriers produced by the small-scale heterogeneity strongly influenced the migration paths and the final distribution of NAPL both in space and across different saturation ranges. The NAPL was immobilized both by snap-off to discontinuous blobs and ganglia and by capillary barriers at textural interfaces. Heterogeneity generally increased entrapment, because spatial variations in capillary properties caused NAPL to be entrapped at higher saturations. Heterogeneity in the finer formation provided points of entry into this formation where the NAPL subsequently could spread as the pressure built up. The NAPL was immobilized at high saturations because high displacement pressures in the fine materials inhibited flow at low saturations. The accessibility for water flow through NAPL occurrences and thereby also the dissolution of NAPL is limited by (i) high entrapped NAPL saturations that decrease the aqueous-phase relative permeability and (ii) the location of NAPL inside a formation with low average permeability.

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