Measurement of Capillary Pressure Curve of DNAPL in a Water-Saturated Sandstone Fracture

Abstract

This paper describes a methodology for the measurement of the capillary pressure curve of dense nonaqueous phase liquids (DNAPLs) in a water-saturated sandstone induced fracture. Details of specimen preparation, capillary barrier design, and capillary pressure measurement setup, procedure, and results are given. The aperture distribution of the induced fracture was estimated using the X-ray computed tomography (CT) technique. The primary drainage or invasion of DNAPLs in the water-saturated induced fracture was simulated using the invasion percolation (IP) approach. It was found that the invasion process was highly dependent on the spatial distribution of the fracture apertures. The IP model matched the experimentally measured entry pressure very well but deviated from the measured capillary pressure curve at the midrange and high nonwetting phase saturations. The IP model tended to yield an L-shaped function with a fairly flat portion at the midrange saturation and a sharp rise at the high end. The measured capillary pressure curve was observed to follow the well-known Brooks-Corey porous media function. These discrepancies between the measured and predicted results could be attributed to the fact that the IP model did not consider the effects of fracture surface roughness and undulation. For a fracture with small apertures, these two factors could make the fracture behave more as a porous medium than a parallel-plate channel.