ABSTRAKT

Lean gas injection has been considered as a process to improve the recovery of residual volatile hydrocarbons from fractured petroleum reservoirs. The characterization and modeling of flow and mass transfer in fractured reservoirs are challenging tasks due to the complexity of the pore space, the anisotropy in the permeability of the rock, as well as the complex interplay between capillary, viscous, and buoyancy forces. In this contribution we develop a three-dimensional pore-network model that accounts for evaporation and diffusion of volatile liquids trapped in anisotropic pore networks. We investigate the effect of permeability gradients on the saturation profiles, the recovery rates, the evaporation patterns, and the stability of the receding evaporation fronts. It is shown that permeability gradients affect the stability of the evaporation front. When the permeability decreases in the direction of the receding evaporation front, then the front is stable and recedes in a piston-like manner, where a two-phase region of finite size develops early in the drying process. The size of this region depends on a permeability-based bond number defined in this paper. In the opposite case, where the permeability increases in the direction of the receding evaporation front, the liquid-gas interface becomes unstable and produces finger-like patterns. The thickness of these fingers is a function of the permeability-based bond number.