Pore Structure Evolution in Deepwater Sandstone Reservoirs During Long-Term High-Rate Water Injection: A Fractal Analysis

Deepwater sandstone reservoirs are important hydrocarbon-bearing media and are prone to pore structure reconfiguration and seepage path evolution under long-term high-rate development conditions, which can significantly affect reservoir performance. However, their dynamic response mechanisms remain insufficiently understood. To address this issue, this study investigates the responses of deepwater sandstone reservoirs with different initial pore throat structures to prolonged high-rate water injection. Reservoir samples are first classified into three classes based on pore throat characteristics and fractal parameters derived from mercury intrusion capillary pressure (MICP) data. Representative cores are then subjected to long-term water injection, with permeability and pore structure evolution monitored up to 1000 PV. In situ nuclear magnetic resonance (NMR), together with scanning electron microscopy (SEM) and X-ray diffraction (XRD) observations before and after injection, is used to relate changes in flow capacity to multiscale pore redistribution and mineralogical–microstructural alteration. Pore sizes are quantified from the T2 spectra, and fractal dimensions are calculated to characterize multiscale structural complexity. The results show that (1) the fractal parameters correspond closely to the pore throat structure and petrophysical properties. From Class I to Class III, the macropore throat fractal dimension generally increases, whereas the median pore throat radius, porosity, and permeability decrease. (2) During long-term high-rate water injection, permeability continuously decreases by 15.8–26.3%, accompanied by mineral redistribution, localized dissolution, and pore throat filling associated with fine-particle migration and retention. (3) Over the same injection period, the micropore fraction increases by 4.1–6.0%, whereas the macropore fraction decreases by 3.2–10.9%. The micropore fractal dimension increases markedly by 63.8–212.0%, whereas the changes in the mesopore and macropore domains remain limited to no more than 2.3%. Overall, the weakening of macropores, together with increasing heterogeneity in the micropore domain, progressively constrains effective flow pathways, resulting in sustained permeability decline and differentiated responses among reservoir classes.