Polymeric Barium and Strontium Sulfate Scale Inhibitors P(Maleic Anhydride/Acrylic Acid/Dimethyl Diallyl Ammonium Chloride/Sodium Styrene Sulfonate) with Quaternary Amine for Downhole Squeeze Application in Sandstone Reservoir at 120°C Seawater Condi

Summary In the Weizhou Oilfield, located in the South China Sea, seawater flooding containing SO42− with formation water rich in Ba2+ and Sr2+, severe barium sulfate (BaSO4), and strontium sulfate scale formation reduces reservoir production. A quaternary polymer scale inhibitor was synthesized through aqueous solution polymerization to counteract the formation of barium/strontium sulfate scale during reservoir production. This process used acrylic acid (AA), maleic anhydride (MA), dimethyl diallyl ammonium chloride (DMDAAC), and sodium styrene sulfonate (SSS) as monomers, and it utilized ammonium persulfate as the initiator and isopropanol as the chain transfer agent (CTA). The resulting terpolymer P(MA/AA/DMDAAC) and quaternary copolymer p(MA/AA/DMDAAC/SSS) scale inhibitors were characterized by Fourier transform infrared (FTIR) and 1H-nuclear magnetic resonance (NMR). The study investigated the effects of scale inhibitor dosage, temperature, and pH value on scale inhibition efficiency, the minimum inhibitor concentration (MIC) of scale inhibitors, and the adsorption-desorption performance of scale inhibitors. The optimized synthetic condition of the P(MA/AA/DMDAAC/SSS) scale inhibitor was determined by the ratio of n(MA):n(AA):n(DMDAAC) = 0.35:1:0.125. Additionally, the amount of SSS was set at 10% of the total mass of the main monomer (MA and AA), the amount of ammonium persulfate as the initiator was 6% of the total weight of the monomer, the amount of isopropanol as the CTA was 2% of the total weight of the monomer, and the polymerization time was set at 5 hours, with a polymerization temperature of 75°C. The measured scale inhibition efficiency of the P(MA/AA/DMDAAC/SSS) polymer was 94.4%, and the scale inhibitor showed good compatibility with seawater. The static MICs of the P(MA/AA/DMDAAC) and P(MA/AA/DMDAAC/SSS) scale inhibitors were 80 mg/L and 60 mg/L, respectively, in the dynamic loop test. It was observed that the performance of scale inhibitors is greatly affected by the pH value, and it was found that the two scale inhibitors performed best at a pH value of 9. This indicates that the chelating performance of dual carboxylate groups in the copolymer backbone works best with Ba2+. When the temperature reached 120°C, the P(MA/AA/DMDAAC/SSS) scale inhibition efficiency after aging for 16 hours was greater than 85%. Furthermore, dynamic adsorption and desorption performance tests show that the adsorption performance of the P(MA/AA/DMDAAC/SSS) scale inhibitor with the cationic group was found to be better than that of the polyepoxysuccinic acid (PESA) scale inhibitor with the anionic group. This suggests that including cationic quaternary ammonium groups in polymeric scale inhibitor molecules improves their ability to adsorb on anionic surface charges found in sandstone reservoirs. Finally, an analysis using environmental scanning electron microscopy (SEM) showed that using P(MA/AA/DMDAAC/SSS) scale inhibitors resulted in smaller inorganic scale crystals, which inhibited the nucleation and growth of these crystals.