A Salt-Induced Tackifying Polymer for Enhancing Oil Recovery in High-Salt Reservoirs: Synthesis, Evaluation, and Mechanism

Abstract

Polymer flooding is effective in reducing the mobility ratio between the water phase and oil phase, which has been widely adopted to enhance oil recovery (EOR) around the world. However, the effectiveness of polymer flooding would be diminished when applied in high-salt/ultra-high-salt reservoirs, such as Tsiengui Oilfield (50℃, 1010-3760md, salinity of 28.6×104mg/L) and Obangue Oilfield (51℃, 500-1500md, salinity of 20×104mg/L). The traditional polymers utilized in enhancing oil recovery for these reservoirs are facing challenges, due to poor salt resistance and rapid viscosity reduction in high-salt environments. To overcome the high-salt challenges, we propose a zwitterion salt-responsive polymer (PAMNS) with increased viscosity as salinity increases, which is obtained by polymerization of acrylamide and zwitterion monomer methylacrylamide propyl-N, N-dimethylbutylsulfonate (NS). The zwitterion monomer is synthesized from 1,4-butanesultone and dimethylamino propyl methylacrylamide. The polymerization process was optimized, and the resulting molecular structure was characterized. Furthermore, the rheology properties, salt-responsivity, flow characteristics, and EOR efficiency were evaluated. According to experimental results, the synthesized polymer possesses a desirable salt-induced tackifying characteristic, and the supramolecular interaction between polymers increases with the elevated salinity. Thus, the final viscosity increases to 2.4 times with the salinity up to 30×104mg/L sodium chloride. The properties of salt-induced tackifying are positively correlated with the valence of the ions, whereby higher valence ions generally exhibit stronger viscosity-enhancing effects. In addition, the on-off function of the supramolecular interaction endows PAMNS solution a “rupture-recombination” flow pattern in the pore-throat structure, indicating the good shear resistance. Moreover, it has been demonstrated that PAMNS exhibits excellent enhanced oil recovery effects in high-salt reservoirs.

Keywords: High-salt reservoirs, Zwitterionic monomer, Salt-induced tackifying polymer, Microflow characteristic, Enhanced oil recovery