Experimental investigation of CO2 huff-n-puff for improving heavy oil recovery and CO2 underground storage

Geological CO₂ storage during CO₂-enhanced oil recovery (CO₂-EOR) is a promising approach to simultaneously increase hydrocarbon production and mitigate carbon emissions. In this study, the potential of CO₂ huff-n-puff for heavy oil cold production and its associated CO₂ sequestration efficiency is experimentally evaluated. Five CO₂ huff-n-puff cycles were conducted at different injection pressures to determine the heavy oil recovery factor and CO₂ storage efficiency, and comparative tests were performed using N₂ and CH₄ huff-n-puff and viscosity-reducer-assisted CO₂ huff-n-puff. The results demonstrate that both heavy oil recovery factor and CO₂ storage efficiency increase with injection pressure and cycle number. At an injection pressure of 20 MPa, the cumulative oil recovery factor and CO₂ storage efficiency are 39.23% and 28.97%, representing increases of 14.61% and 16.76%, respectively, relative to 8 MPa. CO₂ exhibits the highest dissolved gas–oil ratio in heavy oil among the three gases tested at 16 MPa, and the resulting heavy oil recovery factor after CO₂ huff-n-puff are 3.59 and 1.86 times those obtained with N₂ and CH₄, respectively. The cumulative oil recovery factor and total oil exchange ratio are 10.12% and 1.76 t/t for N₂, 36.41% and 1.17 t/t for CO₂, and 19.57% and 4.96 t/t for CH₄. The heavy oil recovery factor is increased by approximately 3.24%–6.45%. These findings provide quantitative guidance for optimizing injection pressure and gas selection in CO₂ huff-n-puff schemes, thereby supporting the design and implementation of CCUS-oriented geological storage in heavy oil reservoirs.