Capillary Control of Free Phase Oil Migration in the Shallow Subsurface: Review with Case Histories

Abstract

Abstract When contamination, such as crude oil, condensate, or gasoline, is detected in the shallow subsurface, investigators usually assume that it will migrate down the water table gradient. This is not always the case. Frequently, when a spill or leak occurs, much of the oil is adsorbed onto the soil and held as residual. If enough is spilled that free phase oil can be detected on the water table, the water table gradient is only one factor to consider in determining whether or not oil will flow or migrate laterally. Movement of light nonaqueous phase liquids (LNAPLs) such as oil through water-wet granular material is frequently controlled by capillary pressures. Capillary pressures must be overcome before oil will flow. Parameters that control how much pressure head is necessary for LNAPL to overcome capillary pressures are (1) the interfacial tension (σ) between the wetting fluid and the nonwetting fluid, (2) the radius (r) of the water-filled pore that the LNAPL must enter (or the fracture aperture [b]), and (3) the contact angle (ϕ) between the fluids and the formation or media. If an LNAPL has enough pressure head to enter the pores of adjacent material, lateral movement is then affected by relative permeability. Relative permeability is always expressed by a number less than one, because putting two immiscible fluids, such as oil and water, in a porous material reduces the permeability to both.