Impact of water on miscibility characteristics of the CO2/n-hexadecane system using the pendant drop shape analysis method

Miscible CO2 flooding has shown bright prospects for improving oil recovery from unconventional reservoirs as well as for storing greenhouse gas in underground formations. Although the favorable water presence effect has been reported in the near miscible CO2 flooding practices, there is still a lack of target research works on the impact of water on the miscibility characteristics of the oil/gas systems. Therefore in this paper, the effect of water presence on the Minimum Miscibility Pressure (MMP) of the CO2/n-Hexadecane (n-C16H34) system is experimentally investigated based on the Oil Droplet Volume Measurement (ODVM) method. By pre-saturating CO2 with water in the high-pressure high-temperature cell, the water component is introduced at the CO2/oil interface. Measurement results show the water presence could result in lower MMPs of the CO2/oil system. Under five temperature levels of 40 °C, 51 °C, 61 °C, 72 °C, and 82 °C, the water presence decreases the MMPs of the CO2/n-C16H34 system from 8.2 to 7.8 MPa, from 9.6 to 8.8 MPa, from 11.6 to 10.2 MPa, from 13.0 MPa to 12.2 MPa and from 14.6 MPa to 13.2 MPa respectively. Thermodynamic calculations provide consistent results as the experimental observations, indicating the main mechanism behind the lower MMPs of the water presence CO2/oil system could be the decreased CO2 molar fraction in the water presence system. This research is expected to provide an innovative viewpoint to understand the water presence effect in the CO2 flooding processes.