Preparation, optimization, and modification of urea-formaldehyde resin for used as a plugging agent in fractured and caved oil and gas reservoirs

Based on the requirements of resin-based plugging agents on initial viscosity, gel time and cost, the urea-formaldehyde resin Poly-UF was synthesized via aqueous solution polymerization. Subsequently, the Poly-UF was further modified by using melamine, polyvinyl alcohol (PVA), and oxidized starch to enhance its application performances, which were marked by Poly-UF-M, Poly-UF-PVA and Poly-UF-OS, respectively. Simultaneously, the relationship between the preparation process, structural, and performance characteristics of the different resin types was investigated using a single-factor experimental approach, specifically, the solid content, viscosity, and free formaldehyde content were evaluated to gauge its application performance. In this study, the chemical structure, thermal stability and microscopic morphology of Poly-UF-M, Poly-UF-PVA and Poly-UF-OS were studied by using FTIR, TG-DTG and SEM. As a result, it was confirmed that the Poly-UF was successfully modified, and the thermal stability was obviously improved, and the microstructure was further enhanced by melamine, PVA and oxidized starch. The outcomes revealed a notable reduction in free formaldehyde content and a significant increase in solid content for Poly-UF-M, Poly-UF-PVA, and Poly-UF-OS, specifically, the free formaldehyde content decreased by 47.15%, 36.59%, and 32.52%, respectively, while the solid content increased by 3.47%, 1.62%, and 1.39%, respectively. Moreover, the acid and alkali solubility, mechanical strength of cured Poly-UF-M, Poly-UF-PVA and Poly-UF-OS were improved attribute to excellent membrane-forming performance. The innovation of this study is that the systematic investigation and optimization of urea-formaldehyde resin are presented, providing technical insights on its utilization as a plugging agent in fractured and caved oil and gas reservoirs.