Two-phase flow in a rough fracture: experiment and modeling

To develop and test theory-based procedures for modeling two-phase flow through fractures, it is important to be able to compare computational results for a fracture with experiments performed on the exact same fracture. Unfortunately for real fractures, any attempt to image the fracture and to produce a numerical model of the fracture accessible to computer modeling unavoidably results in a coarsening of the resolution, with the very small-scale features of the imaged fracture averaged to produce the numerical representation used in modeling. Contrary to the hope that these high-resolution features would be unimportant, several modeling efforts have shown that such changes in resolution do affect the flow. Therefore, the numerical representation is different from the real fracture because of this unavoidable coarsening of the resolution. To remove the problems caused by the use of different fractures in the experiment and in the model, the fracture used in our experiments was stereographically constructed from the same numerical representation used in the modeling so that the only difference between the experimental "fracture" and the modeling "fracture" is a manufacturing error of approximately 3% or less in the aperture sizes of the manufactured experimental model. Using several models not unlike others in the literature, we modeled injection of air into the water-saturated fracture. The modeling results are compared to experimental results for injection of air into the water-saturated stereolithographically constructed fracture. A comparison between modeling and experimental results for the essentially identical fractures shows a much better detailed agreement than obtained in other studies, which compared experimental flows on the real fracture with modeling results for a lower resolution representation of the real fracture. This suggests that many of the differences between experiment and modeling in previous work resulted from the differences between the experimental and modeling fractures. For our low capillary-number cases, the best agreement with experiment is for a modification of invasion percolation with trapping (IPwt) that included approximations to viscosity ratio effects and to the interfacial tension effects in reducing very short-range curvature.