Mechanism study of water control and oil recovery improvement by polymer gels based on nuclear magnetic resonance

Abstract

Polymer gels have been widely used to control excessive water production and improve oil recovery. NMR techniques have been gradually applied to analyze the remaining oil distributions in the core during gel treatment in recent years. However, studies about the NMR technique applied to analyze the gelation performance and microscopic water control mechanisms are still immature. In this paper, we first compared the gelation performance of polymer gels using both the Bottle Test method and core displacement method by monitoring the changes in the real-time tested relaxation time (T2) values. Then, the T2 changes of both oil- and water-phase permeabilities after gel treatment in cores were investigated separately. Finally, the disproportionate permeability reduction (DPR) mechanism of polymer gels was investigated by combining the NMR technique in the displacement experiment. Results show that the gelation time of polymer gel in both the tube and the cores can be measured- by the relationship curve between 1/T2 and reaction time. The reduction of single-phase permeability of oil after polymer gel treatment is much smaller than that tested by the water phase. T2 data also proves that the polymer gel has certain selective water plugging performance. In addition, when the oil and water coexist, polymer gel's disproportionate permeability reduction (DPR) mechanism is mainly reflected in the gradual increase of oil saturation after gel treatment. However, the flow resistance of water in the pore space gradually increases due to the continuous water absorption and swelling of gels. It can squeeze the crude oil channel during the chase water flooding period. Finally, gel injection in heterogeneous cores can plug the high-permeability layer. MRI results demonstrated that the chase water could divert and flood the low-permeability layer usually with high residual oils, thus improving oil recovery. In summary, this study's results will help further enhance the understanding of polymer gel cross-linking mechanisms in porous media and provide an experimental basis in the analysis of the DPR mechanism of polymer gels. It can also lay a theoretical foundation for the field application of polymer gels.