Summary
Foamy-oil viscosity is a controversial topic among researchers regarding
what happens to the oil viscosity when the solution gas starts coming out of
solution because of decreasing pressure and the released gas starts migrating
with the oil in the form of dispersed gas bubbles. For conventional oils, below
the true bubblepoint pressure, the oil viscosity increases as the gas freely
evolves from the oil. For foamy oils, it has been suggested that the apparent
oil viscosity remains relatively constant or perhaps declines slightly between
the true bubblepoint and a characteristic lower pressure, called
pseudobubblepoint, which is the pressure at which the gas starts separating
from the oil. Below this pressure, the viscosity increases, reaching the
dead-oil value at atmospheric pressure. However, it is a well-known fact in
dispersion rheology that the viscosity of dispersion is higher than the
viscosity of the continuous phase. Therefore, the concept of foamy-oil
viscosity being lower than the oil viscosity is counterintuitive. It is likely
that the apparent viscosity for flow of foamy oil in porous media is not the
true dispersion viscosity because of the size of dispersed bubbles being
comparable to the pore sizes.
This study investigates this issue by measuring the foamy-oil viscosity
under varied conditions. The effect of several parameters, such as flow rate,
gas volume fraction, and type of viscometer employed, on foamy-oil viscosity
was evaluated experimentally. Three different viscosity-measurement techniques,
including Cambridge falling-needle viscometer, capillary tube, and a slimtube
packed with sand, were used to measure the apparent viscosity of gas-in-oil
dispersions. The results show that the type of measuring device used has a
significant effect. The results obtained with Cambridge falling-needle
viscometer correlate better with the observed behaviour in the sand-packed
slimtube than the capillary viscometer results. Overall, the apparent viscosity
of foamy oil was found to be similar to live-oil viscosity for a range of gas
volume fractions.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
12 August 2010
- Meeting paper published:
19 October 2010
- Revised manuscript received:
14 June 2011
- Manuscript approved:
27 July 2011
- Published online:
9 January 2012
- Version of record:
30 January 2012
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