Modified Method of Characteristics for Generating Enhanced Oil Recovery Curves

Abstract

Summary Gas injection is a frequently used technique for enhanced oil recovery. If the gas injection is carried out at a pressure at or above the minimum miscibility pressure (MMP), it is possible to achieve a recovery that approaches 100%. However, it is not always possible to inject the gas at such a high pressure, which makes it interesting to also know what recoveries can be obtained at pressures below the MMP. The recovery as a function of pressure can be found experimentally by conducting a slimtube experiment or by simulating such an experiment. Jessen et al. (2001) has, as an alternative to a traditional 1D or cell-to-cell slimtube simulation techniques, proposed a very fast simulation method based on the method of characteristics (MoC). It constructs the composition zones developing in a slimtube during gas injection as well as providing gas and oil saturation profiles and compositions throughout the slimtube. From this information, the recovery can readily be calculated. The method in Jessen et al.(2001) relies on volumetric gas and oil saturations evaluated at the tie-line intersections which are molar-based. Molar vapor fractions at the tie-line intersections will be below 0 or above 1 and will therefore represent a fluid located outside the two-phase region. With negative molar fractions, the total volume may become negative or zero. When the total volume becomes zero, the volumetric saturations (volume fractions of each phase) cannot be calculated from the molar phase fractions due to zero-division, and the saturation profile calculation cannot be performed. This paper presents a modified MoC that does not have this weakness and will not encounter a zero-division caused by zero total volume. The paper further presents a scaling method that will ensure that the modified method gives results close to those obtained with the method in Jessen et al. (2001) when the latter does not stop due to a zero-division. The result is a fast, safe, and reliable method for calculating the recovery from an oil field with gas injection conducted at a pressure between the reservoir fluid saturation pressure and the MMP.