Effects of Compositional Variations on CO2 Foam Under Miscible Conditions

Abstract

Foam can
potentially solve the associated problems with gas injection by reducing the
mobility of the injected gas leading to a more stable displacement front. It is
known that under immiscible conditions, the presence of oil can be detrimental
for foam stability through several mechanisms. Under miscible conditions, there
is no separate oil or gas phase; instead, CO2 and oil mix in different
proportions forming a phase with varying composition at the proximity of the
displacement front. There are then two fundamental questions, which arise from
addition of surfactant to the system: (1) what is the nature of the “mixed
phase” in the presence of the surfactant, and (2) how do the properties of this
mixture change with compositional variations? This study reports the results of
core-flood experiments conducted using CO2 and decane (nC10) as the model oil
under miscible conditions. Surfactant and a mixture of CO2-decane were
co-injected with variations of CO2 molar fractions, mixture volume fractions
and total flow rates. We found that separate injection of CO2 or oil with the
surfactant solution into the cores creates in-situ fluids that exhibit both
low-quality (increasing viscosity with decreasing fraction of surfactant) and
high-quality (decreasing viscosity with decreasing fraction of surfactant)
regimes. However, upon simultaneous injection of CO2 and oil with the
surfactant solution and depending on the molar fraction of CO2 in CO2- decane
mixture (xCO2), three distinct regimes were observed. In Regime 1 (xCO2>0.8)
the apparent viscosity of the in-situ fluid was the highest and increased with
increasing xCO2. In Regime 2 (xCO2<2) the apparent viscosity increased with
decreasing xCO2. In Regime 3 (0.2< xCO2<0.8) the apparent viscosity of
the fluid remained relatively low and insensitive to the value of xCO2.
Shear-thinning rheology was observed in all three regimes: supercritical CO2
foam (xCO2 =1), decane emulsion (xCO2 = 0), as well as CO2-decane-surfactant
floods. Moreover, in Regime 1 and Regime 2, there is a transition at shear
rates from 10 s-1 to 100 s-1, where the apparent viscosity increases by one
order of magnitude. In Regime 3, however, this transition is not observed.
Finally, we found that the current implicit-texture foam model cannot simulate
our experimental data.