STATUS REPORT ON MISCIBLE/IMMISCIBLE GAS FLOODING

Bath, P.G.H., 1989. Status report on miscible/immiscible gas flooding. J. Pet. Sci. Eng., 2: 103-117. The gases used for flooding a reservoir - hydrocarbons, carbon dioxide and nitrogen - are injected in conditions varying from immiscibility to full miscibility. The high mobility of gas is always a problem, and difficulties are exacerbated by miscibility: the maximum stable rate is reduced below the immiscible critical rate, and the integrity of slugs can be difficult to maintain, especially in stratified systems. Water and gas are often injected in alternating slugs, and useful work has been done recently in clarifying the influence of the parameters controlling the process. Rock wettability appears to be important in flooding by carbon dioxide: there may be cases where single-slug CO, will give better results than WAG. Research has identified some tracers for tracking carbon dioxide fronts, as well as classes of chemicals useful as foam additives. Carbon dioxide has useful properties, but disadvantages too. Nitrogen is worth considering as an alternative gas for secondary and tertiary floods in light-oil reservoirs of good permeability. Full miscibility would occur only at high reservoir pressure, but gravity drainage combined with vaporising enrichment of the gas stream may lead to useful recovery at lower pressures. Three-phase relative permeability is a controlling parameter in tertiary flooding, and recent experiments suggest that it may be more favourable in some cases than is predicted by existing methods.