Blending of certain types of surfactants is known to promote synergism as studied by bulk measurements. This study analyzes if such synergistic interactions are beneficial for foam rheology in porous media. Foam experiments were conducted systematically in porous media, at different ratios of zwitterionic and anionic surfactants, both in the presence and absence of crude oil. Interfacial studies were conducted to explain the behavior of surfactant mixtures with respect to foam rheology in porous media.
The zwitterionic surfactants used in this study were C12 straight chain betaine- Lauryl betaine (LB), C12 straight chain sultaine- Lauryl sultaine (LS), C18 tailed amido betaine (Rhodia A), C 18-22 tailed amido sultaine (Rhodia B), C 18-22 tailed amido betaine - with more C 22 (Rhodia C) and C 18-22 tailed amido betaine -with more C18 (Rhodia D). LB and LS surfactants had a viscosity close to that of water ~1 cP at room temperature. On the other hand 0.5 wt% of Rhodia A, Rhodia B, Rhodia C and Rhodia D were viscoelastic and shear thinning fluids due to the presence of wormlike micelles. Rheological studies which were conducted at room temperature revealed that salinity had a prominent effect on Rhodia A. On increasing salinity from ~ 4% to 12%, the relaxation time of Rhodia A increased by three orders of magnitude, thereby causing the weakly viscoelastic surfactant solution to change to a strongly viscoelastic solution. On the other hand salinity had a negligible effect on Rhodia B, Rhodia C and Rhodia D.
When 1 wt% surfactant solutions of Rhodia A, B, C or D were mixed with ~ 35% synthetic crude oil (mass basis), all surfactant solutions lost viscosity and viscoelasticity except Rhodia C. Crude oil had an adverse effect on Rhodia A perhaps due to the conversion of wormlike micelles to spherical micelles. Rhodia B and D had lower elastic and viscous moduli most likely due to the shortening of the wormlike micelles. Additional tests were done to study the flow of these complex fluids in a 100 Darcy silica sand pack. Rhodia A, B and D showed no elongational effects during flow in porous media. Their shear thinning apparent viscosities in porous media were very close to the rheometric data in shear flow. Rhodia C exhibited yield stress behavior and hence could not be injected in a porous medium.
Zwitterionic surfactants Rhodia A/LB/LS were blended with anionic Alpha Olefin Sulfonate AOS 14-16 (AOS) surfactant at specific ratios - one with high and one with low bulk mass ratio of zwitterionic to anionic. Rhodia A which was weakly viscoelastic by itself, when blended with AOS in the ratio 9:1 respectively (by mass) produced a strongly viscoelastic solution. Nitrogen foam experiments were conducted in 100 Darcy silica sand at 25° C for Rhodia A and AOS blends and, in Bentheimer sandstone cores at 45° C, for LB/AOS blends and LS/AOS blends. Zwitterionic surfactants of this type have been reported to be “foam boosters” for bulk foams when added to anionic surfactant.
Rhodia A betaine was a weak foamer both in the presence and absence of oil. However when blended with AOS (9:1 ratio), its foam strength significantly improved in the absence of oil. In the presence of oil the viscoelastic surfactant helped generate strong foam in fewer pore volumes (PVs- a dimensional unit of time) but took a longer time than AOS to propagate through the sand pack.
In the case of LB/AOS and LS/AOS surfactant systems, the zwitterionic (LB, LS) foam by itself was weak, but AOS and the blends of zwitterionic and AOS had strong foam with comparable foam rheology. The regular solution theory approach of Rubingh combined with Rosen’s application to water-air film interfaces and its adaption to oil-water interfaces was applied to understand this behavior, especially the high foam strength observed when the poor-foaming zwitterionics were added to the strong foamer AOS. It was found that the zwitterionic-anionic blends exhibited synergistic interactions. The Gibbs surface excess calculations suggested that the synergistic interactions promoted tighter packing at the interface thereby helping the poorly foaming zwitterionic surfactant to exhibit strong foam rheology in porous media. Interestingly, AOS surfactant by itself had tight packing at the interface. The trends observed in porous media were well explained by the Gibbs surface excess calculations. However, the synergism did not lead to improvement in foam performance in porous media beyond that seen for AOS alone.
Additionally foam strength in the presence of water flood residual oil was weak for the pure zwitterionic surfactants, but the blends with higher mole fraction of AOS and pure AOS had comparable foam performance. Again AOS by itself was able to achieve good mobility control in displacing residual oil. The addition of zwitterionic surfactant had apparently not boosted the foam performance of AOS in porous media in the presence of oil as well.
Interfacial shear rheology for the LB/AOS and LS/AOS systems were performed and it showed that none of the surfactants possessed interfacial shear viscosity. Qualitative film drainage studies were conducted and it was observed that a small addition of LB to AOS helped in creating very stable black film and substantially increased the longevity of the film more than AOS itself. However all these thin film studies failed to offer any explanation to porous media foam studies but perhaps help develop an understanding on bulk foam studies.
In the case of Rhodia A:AOS 9:1 viscoelastic blend, an injection strategy can be proposed where in a small slug of A:AOS 9:1 blend can be injected which can aid in quicker foam generation followed by a large AOS slug which can help in faster propagation and hence more efficient oil recovery.
Anionic AOS 14-16 surfactant did not need a foam booster contrary to the opinion in literature that a betaine surfactant (coco amido propyl betaine) is needed to boost the foam strength of an anionic surfactant (AOS 16-18) in the presence and absence of crude oil in porous media.