Fundamentals of Fluid Flow in Porous Media
Multiple Contact Miscibility Processes: Condensing Gas Drive
In this process dynamic miscibility result from the in situ transfer of intermediate molecular weight hydrocarbons, predominantly ethane through butane, from the injected fluid into the reservoir oil. The modified oil then becomes miscible with injected fluid. So the injected fluid should contain significant amount of intermediate components, rather than being a dry gas.
Figure 5-19: Schematic Condensing Gas Drive
Suppose crude oil ‘O’ and injection solvent ‘S’ (Figure 5‑19) are on opposite side of the critical tie line but reversed from the vaporizing gas drive (Figure 5‑16). Oil and injected fluid are not miscible initially as the dilution straight line between them pass through the two phase region. M1 is the first mixture resulted after first contact of ‘S’ and ‘O’. M1 will split to liquid L1 and gas V1 that are in equilibrium at this point in the reservoir. The liquid phase L1 is richer in intermediate components than the original oil ‘O’. Gas phase V1 move faster because of its higher mobility and leaves oil phase L1 to mix with the fresh fluid injected ‘S’ and form mixture M2. The new mixture will split to liquid L2 and gas V2. Liquid L2 lies closer to the critical (plait) point than L1 and is richer in intermediate components. The gas passes the liquid phase and L2 contact with the fresh solvent to form M3 and so forth. By continuing injection of the solvent ‘S’ the composition of the liquid phase is altered progressively in a similar manner along the bubble point curve until it reached the critical point. The plait point fluid is directly miscible with injection fluid ‘S’. the limiting tie line in this process pass through the solvent composition ‘S’ (in contrast to the VGD that pass through the oil composition), so the MMP in this process is defined as the pressure at which the critical tie line coincides with limiting tie line and its extension passes through the solvent composition (Figure 5‑18.a).