In a previous JPT paper we reported that gravity segregation of different-density fluids in the wellbore strongly affected placement profiles of sequentially injected fluids. That report was based on both experimental and theoretical studies of placement profiles involving immiscible fluids. A significant profiles involving immiscible fluids. A significant finding from the experimental study was that immiscible fluid placement is not affected by tubing tail position nor by injection rate, but by density differences only.In a new study reported here, we observed that miscible and immiscible fluid systems behave differently. Chiefly due to fluid mixing in the wellbore, miscible fluid placement profiles respond to both tubing tail position and injection rate. Hence, injection profiles may be controlled to some extent by proper choices of injection rate and tubing tail proper choices of injection rate and tubing tail position. position. We illustrate the new findings in this paper by applying them to miscible fluid placement problems previously modeled only approximately as equivalent previously modeled only approximately as equivalent immiscible cases. These applications suggest several additional approaches to solving common placement problems. A new 16-mm color film with narration problems. A new 16-mm color film with narration presents results of the current work and those from presents results of the current work and those from prior studies of immiscible fluid placement. prior studies of immiscible fluid placement. Experimental Observations
We conducted six miscible fluid placement tests using the apparatus employed in the previous study, with minor modifications. The modifications included placing refractive index detectors to measure mixture placing refractive index detectors to measure mixture concentrations in the exit lines and metering valves to equalize flow through the three exit tubes. Isopropyl alcohol (sp. gr. 0.78 g/cm3) and a mixture (sp. gr. 0.92 g/cm3) of four parts diesel oil per part chlorothane were used as the two miscible fluids.The following observations are based on these and prior flow tests. prior flow tests. 1. Regardless of miscibility, gravity forces cause injected fluid to rise or fall through the in-wellbore fluid, depending on which fluid is heavier. If the two fluids are miscible, a mixing zone occurs at the discharge of the injection tubing. This zone grows as injection continues.2. For miscible systems, the growth and movement of the mixing zone depend on the injection rate, the injected/in-place fluid density ratio, and the tubing tail position relative to the perforations. In contrast, immiscible fluid systems produce a sharp fluid interface that rises or falls solely according to the density ratio and amount of fluid injected; the tubing tail position has no effect for immiscible systems. position has no effect for immiscible systems. 3. In miscible systems the interface between the mixing zone, with its constantly changing composition, and the in-place fluid reaches different perforations at different times; consequently, a perforations at different times; consequently, a different concentration of treatment fluid may be injected through each perforation. With thick intervals and small treatment volumes, some perforations may not receive any treatment fluid. perforations may not receive any treatment fluid. 4. Where a fluid is injected into a well filled with a heavier miscible fluid, injection rate and turbulence can cause the heavier rathole fluid to be displaced from the wellbore. Complete displacement depends on the rate and length of injection, the tubing tail position, and the depth of the rathole. For position, and the depth of the rathole. For immiscible systems, it is not possible to displace a rathole fluid during injection of a lighter fluid.5. Regardless of miscibility, when a heavier fluid is used to displace another fluid and the tubing tail is set at or below the top perforation, the fluid in the dead space above the top perforation cannot be displaced.
JPT
P. 1451
