Abstract: In order to further explore the seepage mechanism of fluid between matrix and fracture in CO₂ huff-puff process in fractured tight reservoirs, a self-designed high-pressure non-magnetic core holder was used to perform pulsed CO₂ huff-puff core experiment. Based on the principle of nuclear magnetic resonance T₂ spectrum testing, the core was scanned online in the process of CO₂ huff and puff to study the micro production characteristics of crude oil in the pores of matrix and the seepage characteristics of fluid between matrix and fracture during CO₂ huff and puff. he results of the study show that after CO₂ entered the fracture, the crude oil was displaced along the fracture, but the crude oil in the matrix could not be produced. At the early stage of well shut-in, the crude oil saturated with CO₂ in the matrix macropores (20.5 ms ＜ T₂ ≤ 716.0 ms) entered the fractures under the action of expansion. At the later stage of the shut-in, the differential diffusion of crude oil concentration caused by CO₂ extraction and the capillary force of pores in different sizes gradually became the the dominant force, and prolongation of shut-in time could effectively increase the production degree of crude oil in the matrix. The recovery efficiency of crude oil was main contributed by the large pores, while the production degree of small pores was poor. With the increase in the pulsed gas injection cycle, the cyclic matrix recovery percent was continuously reduced. The CO₂ huff-puff cycle should be reduced and the shut-in time should be prolonged during the implementation at the mine. In addition, the lower limit of depletion pressure should be set close tot he saturation pressure to improve the overall recovery efficiency during huff and puff. The study results provide reference and basis for enhancing the recovery efficiency of fractured tight oil reservoirs.

Key words: tight oil reservoir, nuclear magnetic resonance, CO₂ huff-puff, matrix-fracture, micro-pore, seepage characteristics