In this paper, we present results of an experimental investiga-tion of the
effects of variations in interfacial tension (IFT) on three-phase relative
permeability. We report results that dem-onstrate the effect of low IFT between
two of three phases on the three-phase relative permeabilities.
To create three-phase systems in which IFT can be con-trolled
systematically, we used a quaternary liquid system composed of hexadecane
(C16), n-butanol (NBA), water (H2O), and isopropanol (IPA). Measured
equilibrium phase compositions and IFTs are reported. The reported phase
be-havior of the quaternary system shows that the H2O-rich phase should
represent the “gas” phase, the NBA-rich phase should represent the “oil” phase,
and the C16-rich phase should repre-sent the “aqueous” phase. Therefore, we
used oil-wet Teflon (PTFE) bead packs to simulate the fluid flow in a water-wet
oil reservoir. We determined phase saturations and three-phase relative
permeabilities from recovery and pressure-drop data using an extension of the
combined Welge/Johnson-Bossler-Naumann (JBN) method to three-phase flow.
Measured three-phase relative permeabilities are reported.
The experimental results indicate that the wetting-phase relative
permeability was not affected by IFT variation, whereas the other two-phase
relative permeabilities were clearly affected. As IFT decreases, the oil and
gas phases become more mobile at the same phase saturations. For gas/oil IFTs
in the range of 0.03 to 2.3 mN/m, we observed an approximately 10-fold increase
in the oil and gas relative permeabilities against an approximately 100-fold
decrease in the IFT.
Variations in gas and oil relative permeabilities as a function of IFT are
of particular importance in the area of composi-tional processes such as
high-pressure gas injection, where oil and gas compositions can vary
significantly both spatially and temporally. Because gas-injection processes
routinely include three-phase flow (either because the reservoir has been
water-flooded previously or because water is injected alternately with gas to
improve overall reservoir sweep efficiency), the effect of IFT variations on
three-phase relative permeabilities must be delineated if the performance of
the gas-injection process is to be predicted accurately. The development of
mul-ticontact miscibility in a gas-injection process will create zones of low
IFT between gas and oil phases in the presence of water.
Although there have been studies of the effect of low IFT on two-phase
relative permeability,1–14 there are limited ex-perimental data published so
far analyzing the effect of low IFT on three-phase relative
permeabilities.15,16 Most authors have focused on the effect of IFT on oil and
solvent relative permeabilities.17 Experimental results show that residual oil
saturation and relative permeability are strongly affected by IFT, especially
when the IFT is lower than approximately
0.1 mN/m (corresponding to a range of capillary number of 10–2 to 10–3).
Bardon and Longeron3 observed that oil relative permeability increased linearly
as IFT was reduced from ap-proximately 12.5 mN/m to 0.04 mN/m and that for IFT
below 0.04, the oil relative permeability curves shifted more rapidly with
further reductions in IFT. Later, Asar and Handy6 showed that oil relative
permeability curves began to shift as IFT was reduced below 0.18 mN/m for a
gas/condensate system near the critical point.
Delshad et al.15 presented experimental data for low-IFT three-phase
relative permeabilities in Berea sandstone cores. They used a
brine/oil/surfactant/alcohol mixture that included a microemulsion and excess
oil and brine. The measurements were done at steady-state conditions with a
constant capillary number of 10–2 between the microemulsion and other phases.
The IFTs of microemulsion/oil and microemulsion/brine were low, whereas the IFT
between oil and brine was high. They concluded that low-IFT three-phase
relative permeabilities are functions of their own saturations only. Amin and
Smith18 re-cently have published experimental data showing that the IFTs for
each binary mixture of brine, oil, and gas phases vary as pressure increases
(Fig. 1). Fig. 1 shows that the IFT of a gas/oil pair decreases as the pressure
increases, whereas the IFTs of the gas/brine and oil/brine pairs approach each
© 2005. Society of Petroleum Engineers
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