ABSTRACT

This study investigates the influence of fracture properties on Dense Non-Aqueous Phase Liquids (DNAPL)-water flow in rock fractures. The aperture distributions of fractures in sandstone and shale specimens of Alberta Paskapoo Formation were obtained from the X-ray computed tomography (CT) technique. Then, the geostatistical parameters of each aperture distribution were estimated, and isotropy or anisotropy nature of the aperture distributions was identified. Equiprobable aperture distributions were generated from stochastic simulation to observe the affect of anisotropy of aperture distribution on DNAPL migration parameters. One set of fractures was generated using geostatistical properties similar to that of measured aperture distribution and the other set was generated by interchanging the highest and the lowest spatial correlation directions of the measured aperture distribution. DNAPL-water flow process was simulated in each generated fracture by utilizing the invasion percolation approach. The affects of anisotropy of aperture distribution on the main flow parameter, the capillary pressure–saturation relationship, was determined from the result of the simulated flow process.

1 INTRODUCTION

Rock fracture is a crucial geological feature in subsurface contaminant migration process. The affects of fracture characteristics on migration process of Dense Non-Aqueous Phase Liquids (DNAPL), a critical ground water contaminant in many industrialized cities, are the main focus of this study. The investigation is restricted for single rock fractures, which act as the fundamental elements in understanding the flow behavior in fracture networks.

1.1 Fracture aperture distribution

Determination of aperture distribution is essential to estimate the fracture characteristics and flow behavior of a single fracture. Some early researches proposed to estimate the aperture distribution by obtaining a cast of the void space. In several other studies fracture replicas were first constructed from a transparent material and the aperture distribution was determined by means of light transmission. The use of a profilometer or a nuclear magnetic resonance imaging (NMRI) technique is another method of measuring the fracture apertures. X-ray computer tomography (CT) is an attractive technique for determination of morphology of rock fractures (Johns et al., 1993; Keller, 1997; Bertels et al., 2001; Muralidharan et al., 2004; Walters, 1995) due to its convenience of use and non-destructive nature.

Goestatistics have been used in studies of single fractures for characterization of fracture surfaces (Marache et al., 2002) or for generation of aperture distributions (Moreno et al., 1988; Tsang and Tsang, 1989; Pruess and Tsang, 1990). The spatial continuity is an important property of a fracture aperture distribution and it can be quantitatively estimated by geostatistical parameters.

1.2 DNAPL-water flow in rock fracture

The DNAPLs migrate deep into the subsurface due to their high densities and accumulate on the bedrock. If the bedrock is fractured, DNAPLs can enter into the fractures. However, for entry, the DNAPL pressure have to be higher than the capillary forces between water and DNAPL at the fracture entrance (Kueper and McWhorter, 1991). Once entered, the migration of DNAPL in variable aperture fractures is controlled by the capillary forces (Reitsma and Kueper, 1994).

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