Temporary Plugging Performance of Degradable Preformed Particle Gels (DPPG) in Heterogeneous Reservoir Cores Based on the Low-Field Nuclear Magnetic Resonance Technology

Selective and degradable temporary plugging materials are regarded as an effective means to improve conformance while maintaining low matrix damage. In this study, the temporary plugging and self-degradation behaviors of DPPG were investigated using three-layer heterogeneous cores to simulate realistic vertical heterogeneity. Unlike traditional methods, online monitoring by low-field nuclear magnetic resonance (LF-NMR) was employed to nondestructively track the in situ distribution of fluids and particles within multiscale pores. This approach is essential for elucidating the selective plugging mechanisms in high-permeability layers and quantitatively evaluating the resulting matrix damage in low-permeability zones under dynamic conditions. The injection, plugging, and degradation behaviors of DPPG in heterogeneous cores are analyzed. Results showed that the swelling of DPPG became faster, and the final swelling ratio increased when the salinity of the solution decreased. At a reservoir temperature of 73 °C, high salinity water (100,000–200,000 mg/L) caused complete degradation within about 150 h. Low salinity (5000 mg/L) delayed degradation, which took about 5 d to complete. At an injection rate of 0.25 mL/min, the peak injection pressure difference of DPPG in the heterogeneous core reached about 0.642 MPa, indicating effective plugging of high-permeability layers. After degradation, the secondary water flooding pressure decreased to about 0.035 MPa, showing excellent temporary plugging performance. NMR T 2 spectra indicated that temporary plugging particles in large and medium pores could completely self-degrade, while those retained in smaller pores were more difficult to degrade and required longer degradation time. It was found that DPPG achieved effective temporary plugging in high-permeability layers within strongly heterogeneous reservoirs and realized controllable self-degradation under suitable conditions, while minimizing damage to low-permeability layers. These findings guided the optimization of injection parameters and waiting time in oilfield operations.