Comparative Study on the Macroscopic Exploitation and Microscopic Mobilization Characteristics of CO₂ and N₂ HnP to Enhance Shale Oil Recovery

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.

SC-CO2 and Brine Exposure Altering the Mineralogy, Microstructure, and Micro- and Macromechanical Properties of Shale

“…Relevant investigations have shown that supercritical carbon dioxide (SC-CO 2 ) can significantly enhance the recovery of shale reservoirs. − When the pressure and temperature of CO 2 reach a certain condition (generally P ≥ 7.38 MPa, T ≥ 31.1 °C), it will enter the supercritical phase. Thanks to its high density, minimal viscosity, and excellent diffusivity, supercritical carbon dioxide (SC-CO 2 ) easily mixes with crude oil and can effectively lower its viscosity. − This makes it easier for more shale oil to flow to the wellbore and efficiently enhances the recovery. , Meanwhile, CO 2 used can also be sequestered in shale reservoirs in the form of dissolution, trapping, and mineralization. − …”