EVAPORATIVE RECOVERY OF TRAPPED OIL IN FRACTURED RESERVOIRS: A PORE-NETWORK LEVEL ANALYSIS OF CAPILLARY AND GRAVITY EFFECTS

The recovery of volatile oils from the matrix of fractured porous media can be significantly aided through gas injection. Within certain assumptions, this process operates much like the isothermal drying of porous media. For example, recovery is controlled by mechanisms involving capillary, viscous and gravity forces in the liquid and the liquid-gas interfaces, and mass diffusion and convection in the gas phase.

In this work the effect of capillarity-gravity driven viscous flow through macroscopic liquid films during the isothermal drying of porous materials is studied. We use a mathematical model based on a 2-D pore-network representation of porous media. The problem is characterized by two dimensionless parameters, a diffusion-based capillary number, Ca, and the gravity Bond number, B, in addition to the various geometrical parameters of the pore network. The simulator accounts for evaporation and diffusion of a volatile liquid, trapped in a heterogeneous pore network.

Results on the evolution of the liquid saturation, the isolated liquid clusters and the drying rates are obtained as a function of time and the dimensionless parameters in the two cases when gravity opposes or aids the process (corresponding to positive or negative Bond numbers, respectively). In the first case, the fronts are stabilized, while in the second they are destabilized. It is shown that gravity-controlled film flow is a major transport mechanism in the drying of porous media, its effect being dominant when gravity and capillarity control the process. Under strong capillarity conditions, the films span across the whole block, enhancing significantly liquid flow from distant clusters and improving recovery rates. An upscaling of the pore-network findings to macroscopic quantities is also presented.

The paper finds application to the recovery of volatile oils from fractured or heterogeneous reservoirs by gas injection.