INTRODUCTION
A stratified continuum model is used to generate fracture void geometries to investigate the effect of trapping on immiscible flow through a fracture. For a wetting phase invading a fracture initially saturated with a non-wetting phase, trapping of the non- wetting phase occurs in local maxima that are surrounded by smaller apertures. Trapping of the non-wetting phase results in high residual saturation and low values of wetting phase relative permeability. An epoxy cast of a natural fracture was used to visually observe trapping in a fracture.
Energy sources such as coalbed methane, geothermal springs, and oil fields often occur in fractured reservoirs and involve the flow of two phases in fractures. Multiphase flow through fractures is also a concern for contaminant transport and the isolation of radioactive waste. An underlying question of the movement of two fluids through a fracture is how the fracture geometry affects residual saturation and relative permeabilities. In reviewing the literature, few experimental measurements have been made of relative permeability in fractures (Barton, 1972; Merrill, 1975; Bawden & Rogiers, 1985). These experiments were performed on artificial fractures, or fractures represented by parallel glass plates. Several investigators have undertaken theoretical investigations of multiphase flow in fractures. Some models involve the use of capillary theory to study multiphase flow through fractures idealized as parallel plates (Evans, 1983; Evans & Huang, 1983; Rasmussen et al., 1985) or wedge-shaped fractures with continuously varying apertures [Rasmussen, 1987]. Pruess & Tsang (1990) numerically analyzed relative permeabilities of a rough-walled fracture for a lognormal aperture distribution and various spatial correlations. They found that relative permeabilities are sensitive to the nature and range of spatial correlation of the apertures. Pyrak-Noltet al. (1990) examined unsaturated flow in single fractures for the case of a non-wetting phase invading a wetting phase fluid (such as mercury injected into a fracture saturated with air in rock).
Pruess & Tsang (1990) investigated numerically the effect of different aperture distributions on two-phase flow through a fracture using global accessibility. Accessibility determines which apertures will be occupied by which phase. For wetting- phase invasion, global accessibility allows all sites to be occupied even if they are not connected to the inlet. This paper will examine the effect of global accessibility compared with inlet accessibility with trapping for a wetting phase invading a fracture initially saturated with a non-wetting phase. Numerical and experimental results will be presented.
