Foam Transport in Porous Media: in-Situ Capillary Pressure Measurement and Application to Enhanced Heavy Oil Recovery
Vavra, Eric Daniel
Biswal, Sibani L.; Hirasaki, George J.
Doctor of Philosophy
Aqueous foam flow in porous media has been the subject of an increasing number of studies in recent years. Foam is a dynamic colloid that can exhibit unintuitive properties when flowing in porous media; thus, foam experiments often produce unclear or conflicting results. With potentially lucrative applications ranging from enhanced oil recovery (EOR) to subterranean CO2 storage, there is great incentive to understand the fundamental physiochemical processes that accurately describe and predict the nature of flowing foam in porous media. One important aspect of foam flowing in porous media is stability. Many variables, such as quality of the foam, permeability of the medium, velocities of the phases, and type of gas, can influence foam stability. Classically, foam strength is thought to be governed by the stability of liquid lamellae that separate individual gas bubbles and by the a “limiting” capillary pressure above which foam lamellae rupture. In this thesis, a custom probe was designed and constructed for directly measuring in-situ capillary pressures of foam in porous media. Foam quality scan experiments were conducted primarily in a 143-Darcy sand pack with AOS14-16-stabilized N2 foam at ambient lab conditions and constant gas flow rates. Capillary pressure was observed to increase with increasing foam quality before plateauing over a range of qualities in the low-quality regime. Then, in contrast to the classical view, capillary pressure decreased with increasing foam quality in the high-quality regime. The measured capillary pressure decreases were correlated with in-situ observations of increasing bubble size. These general trends occurred regardless of gas velocity over the range of velocities that were tested. Increasing velocity led to increasing transition foam qualities and plateau capillary pressures. This finding implies that the foam mechanisms which are a function of velocity, such as foam generation by lamella division, were significant in determining the behavior of the foam in porous media. Additionally, several other findings improved understanding of foam flow in the sand pack. A nearly constant transition liquid velocity, separating the low- and high-quality regimes, was identified regardless of gas velocity. The rheology of the N2 foam was found to be shear thinning in the low-quality regime and described by a power law model with an exponent of -0.9. In the high-quality regime, the behavior of the coarse bubble and continuous-gas flow systems was weakly shear thinning or, at the slowest velocities, nearly Newtonian as expected for gas flow alone. Comparing gas composition with N2 or CO2 tests revealed the same transition foam quality but different apparent viscosities and capillary pressures. Trends with absolute pressure and temperature are also discussed. An application of interest in this thesis is foam EOR. Generally, foams collapse in the presence of crude oil, but foaming formulations can be chemically engineered to interact synergistically with oil. In this thesis alkali-surfactant-foam (ASF) EOR for the recovery of viscous and heavy oils was documented. For this process, careful characterization of the physiochemical interactions among aqueous, oleic, gas, and solid phases is a must. To aid in this, a novel phase-behavior viscosity map was developed to conveniently select optimal injection conditions. The map is constructed from phase behavior test results as a function of log(added salinity) vs. soap fraction and from viscosities measured by the falling sphere method. For the viscous oil that was tested, conditions resulting in low-viscosity oil-in-water (O/W) emulsions were the most favorable. The characteristic soap fraction was selected as a benchmark to relate dynamic flow behavior in micromodel experiments to static phase behavior in sealed pipettes. Microfluidic devices have proven to be useful for visualizing and confirming flow processes of foam in porous media that would otherwise be much more challenging to observe. For this reason, microfluidic devices mimicking porous media were designed for multiphase flow characterization. A detailed description for the construction of oil-resistant polymer micromodels is provided in this thesis. This micromodel platform was utilized to conduct four microflooding experiments. Foam was found to be stable across all flooding experiments. The experimental results at different characteristic soap fractions and salinities were found to be consistent with predictions made based on the phase-viscosity map. The microfluidic platform also provided new insights into the role of wettability alteration and emulsion formation. In the most hydrophilic case (FE1-), 90% of the 5,855 cP heavy oil was recovered at an apparent viscosity of 820 cP. This result was made possible by wettability alteration towards water-wet and the formation of low apparent-viscosity O/W macroemulsions. Conversely, the most hydrophobic case (FE2) resulted in a lower total oil recovery (70%) accompanied by a large increase in apparent viscosity, likely due to the formation of water-in-oil (W/O) macroemulsions, as predicted by referencing the phase-behavior viscosity map. Additionally, wettability alteration and bubble-oil pinch-off were identified as contributing mechanisms to the formation of O/W macroemulsions in the more hydrophilic flooding experiments. Foam was more effective at recovering oil in these cases presumably due to more favorable mobility control.