Abstract
Foamy oil flow appears to be an important contributing mechanism in the better than expected performance of solution gas drives in many Canadian and Venezuelan heavy oil reservoirs. The foamy flow occurs when the gas released from solution during depletion is able to flow through the sand while remaining dispersed in the oil. In laboratory depletion tests, it occurs when a high enough rate of pressure decline is used. With low rates of pressure decline, the depletion appears to occur by the classical two-phase flow mechanism and results in a low recovery factor.
Two types of laboratory depletion experiments were carried out to further examine the conditions needed for initiating foamy flow. First, a series of depletion tests was completed at different rate of pressure depletion to establish the relationship between the rate and the recovery factor. In the second type of experiments, the depletion was started at a low rate and continued to different depletion levels before increasing the depletion rate to a much higher value.
The results show that in tests done at a constant rate of depletion the recovery factor decreases with the decreasing rate of pressure depletion. In the second type of test involving slow depletion followed by fast rate of pressure decline, the recovery factor depends strongly on the pressure level to which the slow depletion was continued. If the slow depletion was continued to level that resulted in the establishment of continuous gas saturation, the subsequent fast rate of depletion was ineffective in providing a high recovery factor. However, when the depletion rate was increased before the average gas saturation in the sand-pack exceeded the critical gas saturation, the recovery factor was much higher and comparable to the tests involving high rate of depletion from the beginning.
These results tend to strongly suggest that the high recovery factor obtained in fast depletion tests is not related to events that take place during the initial phase of depletion, i.e. during the nucleation and initial growth of gas bubbles. It is what happens at the point when the gas phase is ready to become continuous that controls the depletion performance. A plausible explanation is that when the pressure gradient becomes high enough to mobilize the bubbles before they coalesce into a continuous gas phase, the formation of continuous gas phase is delayed. This keeps the gas mobility low and improves the recovery performance.
