CO2 Huff-n-Puff and Storage in Vaca Muerta Shale

Abstract This study examines the co-application of CO2 huff-n-puff (HnP) technology for enhanced oil recovery (EOR) and sequestration in Vaca Muerta shale rocks. The rock samples undergo a primary production stage to establish a depleted rock sample before conducting CO2 HnP and storage experiments. This stage is performed using the associated produced hydrocarbon gas under representative reservoir conditions. The primary objective is to identify the key oil-recovery mechanisms during the HnP stage and quantify the CO2 storage capacity in a depleted shale matrix after both primary production and CO2 HnP stages. CO2 HnP is conducted at 5,000 psig with a soaking time of 16 hours, while the CO2 storage experiment is performed at a slightly lower pressure of 4,420 psig with an extended soaking duration of 420 hours to maximize CO2 diffusion into the depleted shale matrix. The findings reveal that CO₂ HnP, despite its shorter soaking interval, enhances incremental oil recovery from the depleted Vaca Muerta shale matrix, though oil recovery during the primary production stage is significantly higher. Solution-gas drive followed by oil vaporization are identified as the dominant oil-recovery mechanisms in both the primary production and CO2 HnP stages. The CO2 storage experiment achieves 48% greater CO2 diffusion compared to the CO2 HnP experiment, primarily due to the extended soaking period and reduced oil in place. The CO₂ storage capacity of the shale core is estimated at 96.1% of the pore volume in the CO2 storage experiment, compared to 65.2% of the pore volume in the CO2 HnP test, where oil production follows CO2 soaking.