Comparative Study on the Macroscopic Exploitation and Microscopic Mobilization Characteristics of CO<sub>2</sub> and N<sub>2</sub> HnP to Enhance Shale Oil Recovery
Feng Liu, Yong Kang, Mengda Zhang, Yi Hu, Yong Huang, Lian Li, Haizeng Pan
Abstract
Gas huff and puff (HnP) is one of the means to improve shale oil recovery, and the commonly used gases are CO 2 and N 2 . However, the current research on CO 2 and N 2 HnP enhancing shale oil recovery is not sufficient. This study compares the macroscopic exploitation and microscopic mobilization characteristics of CO 2 and N 2 HnP by the HnP experiments with a full-diameter core, gas chromatography–mass spectrometry (GC-MS) tests, and online nuclear magnetic resonance (NMR) tests. Moreover, the impact of cycle number on the oil recovery efficiency of HnP is analyzed. The results show that CO 2 HnP achieves a 7.23% higher final oil recovery factor (ORF) compared to N 2 HnP and CO 2 has a stronger injection capacity. Moreover, CO 2 has a better extraction ability, and the CO 2 HnP process can realize the storage of CO 2 . Furthermore, the lower limit of pore mobilization for CO 2 HnP is significantly lower, and the final ORF from micropores during CO 2 HnP is 20.98% higher than that during N 2 HnP. In summary, CO 2 HnP has a stronger recovery effect compared to N 2 HnP. As the cycle number grows, the incremental ORF decreases, while the growth rate of cumulative ORF slows down, and the produced gas–oil ratio (GOR) rises rapidly. Meanwhile, the gas injection volume increases and the gas–oil exchange ratio decreases. These indicate that the oil recovery effect and economic efficiency of HnP deteriorate with an increase in the cycle number.