Influence of produced natural gas on CO2-crude oil systems and the cyclic CO2 injection process

• Systematic study on the effect of natural gas on the crude oil-CO2 interactions.
• Laboratory test on the EOR performance by the cyclic injection of CO2, natural gas and the mixture of CO2 and natural gas.
• Revealing the influence of produced natural gas on the cyclic CO2 injection process.

In cyclic CO2 injection practice, CO2 has always been recycled to improve CO2 utilization and reduce cost. To study the influence of produced natural gas on CO2-crude oil systems and the cyclic CO2 injection process, the extraction efficiency and solubility of a mixture of CO2 and natural gas were investigated by extraction and solubility experiments. Interfacial tension (IFT) was measured by using the axisymmetric drop shape analysis technique, and cyclic injection experiments were conducted to study the performance of gas injection with various compositions. The results showed that both the CO2 extraction capability and solubility in the crude oil can be reduced when adding the produced natural gas in the CO2 stream. The reduction of extraction efficiency and solubility increases with increasing pressure. The interfacial tension (IFT) between crude oil and CO2 linearly decreases with increasing pressure over two distinct ranges. The IFT generally increases as more natural gas is added to CO2. In the low pressure range, the interfacial tension increase is caused by the reduced solubility of CO2, while in the high pressure range, the weakened extraction efficiency of CO2 is responsible for the increase. Increased interfacial tension results in a rise of 71.1% of the minimum miscibility pressure and an increase of 92.4% of the first contact miscibility pressure. Recovery of the cyclic CO2 injection in the coreflooding is, therefore, lowered by 6.2% in the immiscible case and by 10.7% in the miscible case. The higher the injection pressure is, the more the recovery will be decreased.