Experimental Evaluation of Recoverable Oil Dynamic and CO ₂ Storage Efficiency in Pore Scale by the NMR CO ₂ Huff-and-Puff Injection Process in Tight Oil Reservoirs

This study investigates the impact of various factors on the carbon dioxide (CO₂) huff-and-puff-enhanced oil recovery process in tight oil reservoirs, particularly the V oil group in the Gao 5 fault block. Utilizing the online NMR core flooding technology, experiments were conducted under reservoir conditions (55 MPa pressure and 127 °C temperature) to analyze the effects of injection timing, pressure, and soaking time on displacement efficiency, oil production dynamics at the pore scale, and CO₂ storage efficiency. Breaking new ground in understanding the interplay between operational parameters and reservoir response, the research reveals that a precisely calibrated pressure coefficient of 0.7 maximizes recovery across multiple cycles, while deviations from 0.8 trigger an unexpected 8.02% decline in recovery, uncovering a previously unrecognized threshold in pressure optimization. The work fundamentally advances knowledge of pore-scale fluid behavior by demonstrating how excessive pressure differentials induce irreversible pore structure damage, with striking evidence that micropores (>1 μm) serve as dual-function hotspotscontributing over 48.57% of total oil production while unexpectedly emerging as dominant CO₂ sequestration sites. Challenging conventional wisdom, the study identifies a critical time-dependent mechanism where delayed injection timing selectively impairs oil mobilization from micro- to nanoscale pores (0.1-1 μm), creating novel opportunities for the synchronized optimization of both recovery efficiency and carbon storage. These transformative insights redefine the engineering paradigm for tight reservoir development by establishing quantitative relationships between injection protocols, pore-scale fluid redistribution, and the coupled performance of hydrocarbon production and CO₂ storage.