When two fluids of differing densities, such as liquid water and steam vapor, flow through circular conduits, they tend to segregate.
Investigators of two-phase flow in steam injection wells have documented the occurrence of two-phase annular flow in the injection tubing when steam quality is moderate to high. Most of the liquid flows as an annular film adjacent to the tubing wall; the center of the injection tubing is filled with vapor and a small amount of entrained liquid.(l) Upon reaching the injection interval, the segregated liquid and vapor will not be evenly distributed to the formation. Elson (2) observed nearly complete segregation of air and liquid water in a perforated acrylic column used to simulate wet steam injection. Irrespective of the tubing size and location employed, vapor preferentially entered the uppermost perforations.
Kasraie and Farouq Ali (3) used a numerical model of wellbore heat transfer to investigate the effect of multiple off take points on steam quality. They concluded that quality declined in lower segments of the tubing due to steam off take. Kumar and Hong (4) studied the effect of vapor and liquid segregation on steamflood performance. They found that the assumption of uniform quality steam injection across the injection interval led to optimistic oil recoveries.
Nguyen and Steven(5) employed radioactive inert gas tracers to profile the vapor phase during steam injection. In a field test, all of the vapor exited through the upper two of five sets of perforations; standing liquid prevented vapor from entering the lower three sets of perforations. They concluded that a large amount of phase segregation occurs in steam injection wells.
Conventional thermal simulations ignore the segregation of liquid and vapor. Variations in quality across the injection interval result solely from the effect of pressure on the liquid-vapor ratio and are, in consequence, small. This does, however, simplify the modeling.
In this paper, segregation of liquid water and steam vapor in the injection interval is incorporated into the modeling of stimulation of a prototypical heavy oil reservoir. A novel technique, location of two wells at the same point with the steam injection well completed above the hot water well, is used to include steam segregation. The distribution of steam in the reservoir and the effect on production following uniform and segregated injection are compared.
A prototypical heavy oil reservoir 15.2 m (50 ft) thick was used for the study. The reservoir was divided into 9 vertical layers and 15 logarithmically-spaced radial blocks. The wellbore radius was 0.125 m (hole diameter of 9–7/8 inches). The mid-point of the first grid block was at a distance of 1.22 m (4 ft) from the wellbore centerline and the outer boundary of the 15th grid block was 1000 m from the wellbore centerline. The top of the reservoir was 622 m (2040 ft) subsurface and the reservoir was horizontal. A schematic of the reservoir grid system is shown in Figure 1.