Study on Oil Extraction Characteristics in Micropores of a Typical Terrestrial Shale Reservoir in China by CO₂ Injection and Surfactant Imbibition

Abstract

The heterogeneity in terrestrial shale reservoirs is higher, and pore-fracture structures are more complex in China, which directly affects the oil extraction of the micropores of shale. Therefore, the oil extraction performances in different sizes of micropores using different methods such as CO2 flooding, CO2 huff and puff, and surfactant imbibition are quantitatively studied by in situ nuclear magnetic resonance in this paper. The changes in the properties and phase behaviors of oil–CO2 systems are described with the PVT experiments and the Peng–Robinson equation of state (PR-EOS). The results indicate that the CO2 injection can effectively reduce the viscosity of oil and swell oil and also enhance the saturation pressure of oil, thus increasing the energy of dissolved gas for oil displacement. During CO2 immiscible flooding and huff and puff, the oil is mainly extracted from the big pores above 0.1 and 0.11 μm in the matrix. However, the oil recovery is enhanced by 3.3% with the CO2 immiscible huff and puff due to the ability to restrain premature gas breakthrough and the better mass transfer between CO2 and oil. As a comparison, the CO2 miscible flooding and the CO2 miscible huff and puff can reduce the low limit of the pore radius of oil production and enhance the oil recovery in all scales of pores of shale. However, the oil recovery by surfactant imbibition is lowest and the movable oil is mainly extracted from the bigger pores over 0.18 μm. The oil recovery is 35.7%, 47.9%, 51.2%, 71.2%, and 72.4% with surfactant imbibition, CO2 immiscible flooding, CO2 immiscible huff and puff, CO2 miscible flooding, and CO2 miscible huff and puff, respectively.