Summary
We have carried out an extensive set of tests on solution-gas drive for a
heavy oil to study the effects of initial water saturation, temperature, and
gas-oil-ratio (GOR). The viscosity and the API gravity of the stock-tank oil
from Hamaca field (in Venezuela) used in our tests are 560,000 cp (at 24°C) and
8.7, respectively. The solution-gas drive tests were conducted using live oils
with solution GOR of 6.5, 9.0, and 12.2 (vol/vol at standard conditions). In
two tests, initial water saturations of 4.0 and 5.2% were established. Five
tests were conducted at a temperature of 35°C; one test was conducted at 46°C.
The duration for each test was approximately 3 months. The following
conclusions are drawn based on the results from all the tests. Initial water
saturation decreases gas-bubble density. The recovery efficiency decreases
because of an increase in gas mobility with increase in initial water
saturation. As the temperature increases from 35 to 46°C, the gas relative
permeability increases one order of magnitude, which in turn results in a
reduction of recovery efficiency. The temperature effect on recovery is in
agreement with our previous work with temperature increase from 24 to 35°C.
Increase in solution GOR has a significant effect on the gas-bubble nucleation
process, mainly because of change in interfacial tension. For all the tests
conducted, the oil recovery by solution-gas drive at test termination was
approximately 16% and higher.
Introduction
Solution-gas drive from some heavy oil reservoirs in Canada, Venezuela, and
more recently in China1 has shown efficient recovery. There is a wide
range of opinions on the unexpected recovery efficiency. In an overview paper
on the subject,2 we have argued that the high recovery efficiency may be the
result of high oil viscosity. From a practical point of view for reservoir
engineering applications, one can divide the existing models into two
categories. In the pseudosingle-phase model,3 two groups of parameters govern
the efficiency of solution-gas drive: 1) compressibility and viscosity, and 2)
parameters of the nonequilibrium phenomenon. In another approach, the
conventional two-phase flow model is suggested4,5 wherein gas- and oil-phase
relative permeabilities and oil viscosity govern the phase mobilities. In
the latter, an attempt is made to reduce the nonequilibrium phenomena in the
laboratory measurements to apply the results to reservoir conditions where
there may exist small supersaturation away from the wellbore.
In the past, we have mainly studied gas and oil relative permeabilities for
heavy oils using a constant GOR without the initial water saturation. We have
also limited our measurements to two temperatures of approximately 24 and 35°C.
In this work, we present the results of a comprehensive study on the effect of
initial water saturation, solution GOR, and temperature. We used Hamaca
stock-tank crude (from Venezuela) and mixed it with methane in various
proportions to obtain GOR values of 6.5, 9.0, and 12.2 vol/vol (at standard
conditions). Five different tests were conducted at T = 35°C, and one test
at T = 46°C. The temperature of 46°C is close to the reservoir temperature
of 49°C. In this paper, we will first present the experimental setup and
the depletion procedure, followed by the experimental results. In the last
part of the paper, we will discuss the results, followed by a number of
conclusions.
© 2005. Society of Petroleum Engineers.
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History
- Original manuscript received:
30 October 2001
- Revised manuscript received:
16 November 2004
- Manuscript approved:
14 January 2005
- Version of record:
15 March 2005
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