Determination and controlling of gas channel in CO2 immiscible flooding

• A new method is proposed to determine the gas channeling time.
• The oil–gas period becomes the key point in improving CO2 flooding recovery.
• An optimal displacement pressure can improve CO2 recovery greatly.

In order to study the effect of gas channel on CO2 flooding in porous medium, the gas flow velocity is divided into two parts: gas breakthrough stage and gas channeling stage. The breakthrough velocity has an exponential relationship with concentration in the frontal zone of gas area, while the channeling velocity has a linear relationship with injection pressure drop. A new method is proposed to determine the gas channeling time by using the trend line of gas–oil ratio in gas breakthrough and gas channeling stage. The production characteristics in CO2 flooding show that recovery greatly improves after the gas breakthrough at the outlet of core sample, and most of the oil displaces before gas channeling. Thus, the extension of the stage between gas breakthrough and gas channeling becomes a key factor to improve CO2 recovery efficiency. Improving the injection pressure drop increases the dissolved gas diffusion, which will improve the oil displacement efficiency of simultaneous oil and gas production stage, but it also increases gas channeling velocity, which will increase the gas/oil ratio sharply and result in ineffective gas injection. An optimal displacement pressure can control the diffusion rate, the channeling rate and improve the recovery of CO2 flooding effectively. Keeping a constant pressure drop, increasing of injection pressure can not only increase the diffusion rate, but also reduce the gas channeling velocity. The achievement of a lower gas channeling velocity is advisable for extending the stage of gas–liquid production, and improving the immiscible CO2 recovery significantly.