In many field applications of hydrocarbon miscible flood, the water alternating gas (WAG) process is used to control solvent mobility. In this process mobile water saturation may occur in the vicinity of miscible displacement front. This water saturation can influence the microscopic unit displacement of oil by solvent to some degree. For example, the pore size distribution available for miscible flood may be altered in the presence of water saturation or some of the oil can be blocked by water such that it is not flowable and is bypassed by solvent. In some cases, blocking of solvent by water may occur. In all of these cases, the 1ength of the mixing zone is1arger as compared to the case where the oil saturation is 100%.

It is a common practice to use the dispersion theory for determination of minimum solvent slug size. In applying this theory, it is assumed that oil saturation is 100%. However, additional solvent may be required for the influence of mobile water saturation on the length of mixing zone. The purpose of this paper is to explain clearly how mobile water saturation affects solvent slug size determination. A procedure was developed to provide standard formulas using theconcept of dispersion - mass transfer theory to determine solvent slug size required in the presence of mobile water saturation. Examples were used to illustrate the calculation procedures.


In miscible displacement of oil by solvent, the poor sweep efficiency is generally caused by the high mobility of solvent relative to the mobility of the oil bank. To control solvent mobility Caudle and Dyes1 suggested the use of a water alternating gas (WAG) process. In this process the higher water saturation caused by water injection decreases the relative permeability f the reservoir rock to solvent, thus reducing the mobility of solvent bank relative tothe oil bank. The reduction in mobility ofsolvent by water injection stabilizes the flood front and reduces the contact area between the solvent and oil. This may reduce the solvent requirement for the flood. Figure la and b presents schematically the flood front for a stable and unstable system. However, as suggested by Thomas and Countryman the higher water saturation caused by water injection may ave two possible affects on the microscopic nit displacement of oil by solvent. First the pore size distribution may be altered as a result of high water saturation. This can change the magnitude of transverse and longitudinal dispersion coefficients3. Secondly, a substantial amount of oil may be trapped by mobile water.

The effect of mobile water saturation on miscible displacement has been investigated by many researchers4–10. Ralmond, and Torcaso4 conducted several experiments in naturally consolidated, artificially consolidated, and crushed Berea sandstone cores. In these experiments the oil and brine were injected simultaneously at various desired ratio until steady state conditions of saturation were reached. At these conditions the injection of oil was switched to solvent without any interruption.

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