Influencing factors of supercritical carbon dioxide huff and puff to enhance shale oil recovery

At present， the CO2 huff and puff method is mainly used in international oilfields to improve the recovery of shale reservoirs. However， challenges arise during the development stage of high-cycle huff and puff， including poor boundary physical properties， limited regional conventional huff and puff effectiveness， and usability difficulties. Supercritical carbon dioxide （SC-CO2） has the characteristics of low viscosity， high density， and strong solubility. In order to reveal the huff and puff effect of SC-CO2， the CO2 huff and puff test was carried out on the core after SC-CO2 immersion treatment. The influence of the huff and puff period， the shut-in time， and the injection pressure on the enhanced oil recovery （EOR） of SC-CO2 was analyzed. The shale before and after the huff and puff process was scanned using low-field nuclear magnetic resonance technology to analyze the distribution and extraction of oil from the shale matrix pores. The results show that the cumulative recovery of the shale samples after SC-CO2 immersion treatment increases with the increase in huff and puff cycles， while the single-cycle recovery decreases with the increase in huff and puff cycles. The core permeability after immersion increases by 1-2 orders of magnitude， and the porosity increases by 3-4 times； the shale surface becomes more CO2-absorbent. The extraction capacity of SC-CO2 can be increased by extending the shut-in from 6 hours to 24 hours， resulting in an approximately 10% increase in the recovery. The cumulative recovery and the single-cycle recovery of shale increase with the rise in injection pressure. The recovery can increase by 8%-17% When the pressure increases from 6 MPa to 15 MPa. The saturated oil in shale primarily exists in micropores and mesopores in the shale matrix， and the maximum amount of oil is produced within the first cycle of huff and puff.