The effect of changes in wettability on nonaqueous phase liquid (NAPL) flow in single natural fractures

Non-aqueous phase liquid (NAPL) flow and residual distribution within a fracture is controlled by two key properties, the void geometry of the fracture network and the surface wettability of the fracture. By using translucent epoxy replicas of natural single fractures filled with dyed liquids it is possible to investigate the aperture distribution and to directly observe NAPL flow. However, the surface properties of epoxy, which is hydrophobic, are quite unlike those of natural rock, which is generally assumed to be hydrophilic. The goals of this study are to experimentally observe NAPL flow in single fracture models with differing surface wettabilities, and to specifically examine the validity of using epoxy replicas as analogues for rock surfaces in two-phase flow experiments.; Aperture distribution is measured using a light attenuation method prior to the injection of both light non-aqueous phase and non-toxic dense non-aqueous phase test liquids. In order to evaluate the effect of changes in surface wettability on NAPL behavior, a range of surfaces were tested; strongly hydrophobic polystyrene surfaces, with sessile water contact angles of between 90° and 100°, strongly hydrophilic surfaces where polystyrene coated models were altered by exposure to radio frequency glow discharge (RFGD) plasma resulting in water contact angles between 0° and 30°, and plain untreated epoxy surfaces, with water contact angles of about 63°.; Changes in surface wettability result in dramatically different two-phase flow behavior and residual distributions. In hydrophobic replicas the NAPL flows freely in well developed channels, displacing the water and filling all of the pore space. In hydrophilic replicas the invading NAPL is confined to the largest aperture pathways and flow frequently occurs in pulses, with no stable channel development, resulting in isolated blobs with limited accessible surface area. If channels do develop, they can be abandoned if the pressure distribution within the fracture changes. The pulsing and channel abandonment behaviors described are significantly different from those assumed in current modeling practice. A detailed characterization of the epoxy revealed that it is not a sufficiently good analogue to natural rock to be used in most two-phase flow studies.