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Simple Summary: The characteristics of high-pore-volume (high-PV) waterflooding and the mechanisms of enhanced oil recovery (EOR) are unclear. an experiment was conducted using nuclear magnetic resonance (NMR) technology-assisted waterflooding to monitor oil migration and pore structure changes during the displacement process. We quantified the ultimate oil displacement efficiency waterflooding and microscopic oil recovery at the pore scale. Additionally, the variations in petrophysical properties seen during this process were analyzed. The experiment revealed that the high-PV displacement process had a high EOR effect, even in the ultrahigh water cut stage. During this period, oil production came mainly from the mesopores. Furthermore, flooding changed the rock structure, increasing the pore volume. However, these changes were inconsistent at both the pore scale and spatial scale. At the pore scale, the volume of macropores decreased, while that of the micropores and mesopores increased. At the spatial scale, the porosity and the average pore size at the front end increased, while those at the back end decreased due to particle migration during flooding. Our findings suggested that pore structure variations at the front ends of experimental cores were representative of those in reservoirs, indicating that high-PV waterflooding could increase the permeabilities of reservoirs. A single paragraph of about 200 words maximum. For research articles, abstracts should give a pertinent overview of the work. We strongly encourage authors to use the following style of structured abstracts, but without headings: (1) Background: Place the question addressed in a broad context and highlight the purpose of the study; (2) Methods: briefly describe the main methods or treatments applied; (3) Results: summarize the article's main findings; (4) Conclusions: indicate the main conclusions or interpretations. The abstract should be an objective representation of the article and it must not contain results that are not presented and substantiated in the main text and should not exaggerate the main conclusions. [ABSTRACT FROM AUTHOR]

A single paragraph of about 200 words maximum. For research articles, abstracts should give a pertinent overview of the work. We strongly encourage authors to use the following style of structured abstracts, but without headings: (1) Background: Place the question addressed in a broad context and highlight the purpose of the study; (2) Methods: briefly describe the main methods or treatments applied; (3) Results: summarize the article’s main findings; (4) Conclusions: indicate the main conclusions or interpretations. The abstract should be an objective representation of the article and it must not contain results that are not presented and substantiated in the main text and should not exaggerate the main conclusions.