Synthesis and performance study of copolymer thickener for acid diversion

In response to the inadequate gel breaking performance of crosslinked acids during acid diversion in conventional oil and gas extraction, this paper presents the synthesis of a non-crosslinked copolymer thickener, which was synthesized from betaine monomer, acid-resistant monomer, cationic hydrophobic monomer, and acrylamide by aqueous solution polymerization. The synthesis conditions were optimized via the single-factor optimization method, and the molecular structure was characterized. Subsequently, the acid solubility, thickening ability, rheological properties of spent acid, and viscosity reduction performance of the thickener were systematically investigated. The results indicate that the synthesized thickener possesses excellent acid solubility and can dissolve in acid solutions rapidly. During the acid-rock reaction, the apparent viscosity of 0.6 wt% thickening acid increased from 18 mPa·s to 189 mPa·s, indicating that the thickener possesses excellent self-thickening capacity and can continuously thicken with the progression of the acid-rock reaction. Furthermore, the spent acid solution of the thickener shows favorable viscoelasticity, which is dominated by elasticity. Due to the non-crosslinking thickening mechanism of the thickener, no gel breaker is required, and water can directly reduce the viscosity of the spent acid to below 10 mPa·s. Moreover, additional studies have shown that adding an extremely small amount (0.3-0.4 wt%) of Tween 20 can reduce the viscosity of spent acid rapidly and effectively, significantly reducing the water consumption required for viscosity reduction solely by water. These findings provide a theoretical reference for the synthesis and application of non-crosslinking copolymer-based thickeners in oilfield acidizing operations.