Amphoteric nano- and microgels with acrylamide backbone for potential application in oil recovery

Amphoteric nano- and microgels based on acrylamide (AAm), 3-acrylamidopropyltrimethylammonium chloride (APTAC) and 2-acrylamido-2-propanesulfonate sodium salt (AMPS) were prepared via inverse emulsion polymerization in the presence of crosslinking agent-N,N'-methylenebisacrylamide (MBAA). Several polyampholyte nano- and microgel samples AAm-APTAC-AMPS with compositions (70:15:15, 80:10:10, and 90:5:5 mol%) were obtained and they were abbreviated as AAm(70)-APTAC(15)-AMPS(15), AAm(80)-APTAC(1)0-AMPS(10) AAm(90)-APTAC(5)-AMPS(5) (where the lower indexes indicate the molar concentration of monomers). The nano- and microgels were characterized by Fourier-transform infrared spectroscopy, dynamic light scattering (DLS), transmission electron microscope (TEM), thermogravimetric analysis and interfacial tension measurements. The most attention was paid to the AAm(80)-APTAC(10)-AMPS(10) microgel because its linear analog showed the best swelling capacity and the viscosifying effect for potential application in oil recovery. The influence of monomer concentration, surfactants, volume ratio of the water/organic phases, and the hydrophilic-lyophilic balance (HLB) on the average hydrodynamic size of the AAm(80)-APTAC(10)-AMPS(10) nano- and microgels was studied. The size distribution of nano- and microgels derived from DLS data and TEM images was compared as a function of monomer concentration at constant surfactant concentration (6 wt%), HLB = 5.5 and mixture of water:oil = 60:40 vol%. Swelling of microgel particles in saline water due to the antipolyelectrolyte effect was observed. The applicability of amphoteric microgels AAm(80)-APTAC(10)-AMPS(10) for oil recovery was tested on cores simulating the conditions of a model oil reservoir. The obtained results indicated that after the injection of around three pore volumes of the 2500 ppm microgel suspension into the sandstone core the permeability to brine decreased from 1.5 to 0.78 mD. For the first half of core sample the residual resistance factor was higher than the resistance factor, whereas for the second part of core sample these parameters were almost equal. Additional experiments with longer cores are needed to evaluate in-depth propagation of microgels and permeability reduction in porous media.