Mixtures containing surfactants and polymers to be used in enhanced oil recovery operations should display special features: most notably low (or ultra-low) oil/water interfacial tensions and moderate viscosity. There is a variety of methods employed to assess formulations performance for enhanced oil recovery. Some of these involved wettability measurements, such as the Amott test, but these are typically time consuming.
This communication reports on the use of low-field nuclear magnetic resonance (NMR) to probe rock wettability in the presence of different formulations. Based on the relaxations time, this technique allows a direct assessment of molecular mobility, which in turn is determined if the liquid is bounded to surfaces or free. Based on this, it is possible to monitor the exchange of oil from the porous of rock by water solution. Therefore, the efficiency of formulations can be determined.
Two rock samples, sandstone and carbonate, were employed to reproduce some reservoir conditions. These were first impregnated with oil samples and then exposed to different aqueous formulations and the fractions of oil and water were directly monitored based on the measurement of the spin-spin relaxation time of the hydrogen atoms.
Typical formulations included mixtures of non-ionic and zwitterionic surfactants and high molar mass polymers. These surfactants display little propensity to ion strength effects and cloud points associated to lower interfacial tension values that could be varied with composition. Effects such as nature of the surfactant chain (double vs. single chain, chain length) and mixture composition with respect to their cloud points were found relevant to affect oil displacement capacity.
These results confirm the validity and advantages of this technique, as well as reveal important information about surfactant mixtures with potential for use in enhanced oil recovery.