Advances of polymer microsphere and its application in porous media for enhanced oil recovery

Water flooding is widely employed as the core technology for secondary oil recovery, aimed at supplementing reservoir energy and displacing crude oil to enhance recovery efficiency. However, inherent reservoir heterogeneity (e.g., high-permeability layers, fractures, and cavernous) frequently results in a rapid rise and persistently high water cut in production wells, rendering the remaining oil difficult to be displaced. Deep profile control technology is recognized as a key method for mitigating water channeling and improving water flooding performance. Its principle is based on the blockage of preferential flow channels within high-permeability zones, thereby regulating the subsequent water injection profile. This technology has been extensively applied in reservoirs exhibiting high and ultra-high water cuts. Polymer microspheres have emerged as significant chemical agents for deep profile control systems due to their exceptional elastic deformability. Their mechanism of action is characterized as follows: the microspheres are transported deep into the reservoir formation along with the injected fluid. Leveraging their smart deformable characteristics, they dynamically adapt to pore throat structures of varying sizes. They are preferentially retained and accumulated within the preferential flow channels (characterized by lower flow resistance), where effective plugging is formed. Consequently, subsequent displacing fluids are forced to divert towards and sweep low-permeability zones that were previously non swept by water flooding and possess higher oil saturation. This unique combination of deep migration and intelligent deformable plugging effectively overcomes the limitations of traditional rigid particle plugging agents, which are often difficult to transport deep into the reservoir or prone to causing excessive near-wellbore blockage. Consequently, the sweep volume and oil displacement efficiency of the displacing fluid within heterogeneous reservoirs are significantly enhanced, ultimately leading to increased crude oil recovery. Nevertheless, despite abundant research achievements on polymer microspheres, the current knowledge landscape is characterized by fragmentation and dispersion. A systematic integration is lacking, particularly concerning the establishment of an organic link between structural design, performance regulation, mechanism of action, and practical application effectiveness. Therefore, this study is designed to systematically synthesize the knowledge on polymer microspheres for deep profile control from the following three aspects: (1) Function-Structure-Mechanism Correlation: The intrinsic correlations between chemical modification strategies for functionalized polymer microspheres and their enhanced performance are systematically revealed. (2) Synergistic Mechanisms in Heterogeneous Composite Systems: The profile control performance and synergistic enhancement mechanisms of heterogeneous composite systems based on polymer microspheres are summarized. (3) Application Effectiveness in Porous Media: The practical application effectiveness of microspheres within reservoir porous media is synthesized. Based on the systematic elaboration above, the research bottlenecks currently faced by polymer microspheres under prevailing reservoir conditions are further analyzed. Future research trajectories are also prospected, with the aim of broadening their application perspective within the field of oilfield development.