Network modeling of imbibition in fractured porous media

The fundamentals of multiphase flow in fractured media are poorly understood with some debate in the literature over the appropriate form for the relative permeability of a single fracture. Empirical rules, with little physical basis, are used to model the transfer of fluids between the fracture and matrix.; In this thesis we use a pore-level network model to study multiphase flow in fractured media. We assume that a single fracture, with a variable aperture distribution, may be modeled as a two-dimensional porous medium in contact with a three-dimensional matrix of lower permeability. We use a perturbative approach to model the effects of viscous forces. The model captures the competition between flow in wetting layers and piston-like advance and can study the effects of contact angle, aperture distribution and the interplay of viscous and capillary forces.; We use our model to interpret the different generic types of behavior seen in experiments on single fractures. We identify five types of displacement pattern with characteristic relative permeabilities and residual saturations. We quantify the range of flow rates and physical properties under which the different regimes are seen. We can understand why multiphase flow in fractures may be percolation-like, with very little two-phase flow, to a frontal advance, with very little trapping and straight-line relative permeabilities. This same model is used to study rate effects and capillary desaturation in normal granular media. We then use the model to simulate flow through an aperture distribution measured on a single natural fracture. The model predictions of the displacement profile compare well with experimental measurements. Lastly, we show how flow through the matrix and fracture compete in a three-dimensional simulation of matrix/fracture transfer, and demonstrate the importance of the matrix immediately adjacent to the fracture to the overall recovery.