Impact of CO₂ Cooling on Fluid Mobility in Shale: A T2 Tomography Study

CO₂ cooling damage significantly impairs reservoir recovery near the wellbore in a spatially non-uniformity way. Nuclear Magnetic Resonance (NMR) T2 tomography, an advanced technique for studying non- uniform fluid mobilization, was employed in this study. We first optimize the T2 tomography sequence to address challenges of low SNR and loss of microporous signals through short echo time (TE) scanning, achieving superior microporous imaging compared to conventional methods. Subsequent cyclic CO₂ huff-n-puff experiments on shale plugs, with controlled puff-rate adjustments, were conducted to investigate how CO₂ cooling influences oil mobilization in pores. Results demonstrated that: (1) The non-uniformity of cooling manifests as stronger temperature reduction closer to the bottom-end, leading to an overall 36% production decline during the first HnP cycle. (2) Faster puff-rates intensify cooling in plug's bottom-end, elevating residual oil saturation. (3) While rapid gas release enhances initial marginal recoveries, it later triggers severe pore blockage as cooled oil droplets from smaller pores migrate and accumulate in larger pores (T2 >1 ms), severely impairing long-term recovery. (4) Slower puff-rates enhance total recovery, as mild cooling exerts a weaker inhibitory effect on late-stage oil mobilization. This research provides critical insights for mitigating CO₂ cooling damage and optimizing gas injection strategies in shale reservoir development. Keywords: shale, CO₂ cooling effect, NMR, tomography