Experimental Research of the Characteristics of Microscopic Pore Production during CO2 Displacement in a Shale Oil Reservoir

Abstract

The evaluation method for determining the pore production range during CO2 displacement in shale cores is established based on CO2 displacement experiments, core analysis using online nuclear magnetic resonance (NMR), and pore size analysis using high-pressure mercury injection. By transforming the abscissa of the NMR T2 spectrum into pore-throat radius using the conversion formula between relaxation time and pore radius, the production behavior of crude oil in pores of different scales during CO2 displacement is analyzed. The target shale oil reservoir, located in eastern China, consists of two sets of salt beds. Crude oil is predominantly found in mesopores and smaller macropores. While water injection can saturate the salt and enhance seepage, the reactive areas of water imbibition are still smaller compared to those of CO2 injection. It is observed that the displacement efficiency of CO2 injection exhibits a negative correlation with the clay mineral content, with the swept pores mainly consisting of mesopores and small macropores. The proportion of crude oil produced from pores with diameters less than 0.1 μm during CO2 displacement is significantly higher than that during water displacement. The displacement effect of CO2 in mesopores and smaller macropores improves with increasing injection pressure. Above the miscible pressure condition, the oil displacement efficiency of CO2 is 11% higher than that of water displacement, indicating that CO2 displacement is more suitable for this shale oil reservoir. Exploiting oil from shale formations through water injection is challenging due to the presence of micrometer or nanoscale pores. However, in the target reservoir, which is a typical saline deposition, both water injection and gas injection, such as supercritical CO2 injection, are suitable for enhancing oil recovery. When compared to water injection, supercritical CO2 injection offers advantages such as higher injection capacity, wider swept volume, and better displacement effect. It is particularly effective in producing oil from pores smaller than 0.1 μm. Furthermore, increasing the CO2 injection pressure improves the displacement efficiency. In this type of shale oil reservoir, the average oil displacement efficiency of CO2 displacement is nearly 11% higher than that of water displacement. This method not only provides energy for the reservoir and enhances shale oil recovery but also contributes to carbon storage, making it an advanced approach for shale oil reservoir development.