Summary
Heavy-oil recovery under solution-gas drive is affected by several
interacting factors including pressure-decline rate and pressure gradients. It
has been suggested that a high pressure decline rate (dp/dt)
generates larger supersaturation and faster nucleation that leads to
more-dispersed gas bubbles, while a high pressure gradient (∇p )
increases the viscous forces acting on the gas phase, enhancing bubble break up
and gas dispersion. Both effects lead to lower gas mobility, affecting oil
recovery; however, the relative importance of each is not known. Finding this
is important to develop mathematical models and to allow extrapolation of
experimental results to field conditions, where the relative importance of
these factors changes with time and space. Previous experimental studies were
affected by a combination of the two effects.
In this paper, we distinguish between the effect of the pressure-decline
rate and pressure gradient on gas mobility and oil recovery by varying these
independently. In the experimental work reported in this paper, change in
confining pressure is used to create a change in pressure-decline rate, and a
change in production rate is used to change the pressure gradient. Several
depletion experiments at varying pressure-decline rates and production rates
are reported here.
At a constant pressure-decline rate, the recovery factor tripled when the
flow rate was increased by one order of magnitude. Similar experiments were
conducted when the pressure-decline rate was increased by one order of
magnitude but the flow rate was kept constant. In this case, the recovery
factor did not change significantly. The results of this study clearly indicate
that the pressure gradient has a much greater effect on gas mobility and oil
recovery than pressure-decline rate has. This paper presents the experimental
results and their analysis, along with the implications of these findings on
modeling of solution-gas drive in heavy oils.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
28 January 2008
- Meeting paper published:
29 March 2008
- Revised manuscript received:
9 October 2008
- Manuscript approved:
10 October 2008
- Published online:
1 June 2009
- Version of record:
1 June 2009
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