Application of nuclear magnetic resonance technology in reservoir characterization and CO2 enhanced recovery for shale oil: A review

The reservoir characterization and development for shale oil faces significant challenges due to low porosity, low permeability, multi-scale pore space, and complex fluid composition. Nuclear magnetic resonance (NMR)technologies applied in shale oil reservoirs include one-dimensional (1D) NMR T₂ map, two-dimensional (2D) NMR T₁-T₂ map, nuclear magnetic imaging (MRI) technology, and stratified T₂ technology. The 1D T₂ map nondestructively characterizes the full-scale pore size distribution (PSD) of shale oil reservoirs and can be combined with other experimental methods to extend the functions: (1) Through combining with centrifugation and thermal treatment, the NMR T₂ cutoff value can be determined to quantitatively distinguish movable fluid, capillary bound fluid, and immovable fluid; (2) In conjunction with a confining core holder, stress sensitivity of the shale matrix and fracture systems can be characterized; (3) Through combining with spontaneous imbibition experiments, the spontaneous imbibition characteristics and wettability can be quantitatively evaluated; (4) Based on an online high-temperature and high-pressure CO₂ enhanced shale oil recovery (CO₂-ESOR) apparatus, dynamic oil recovery factors can be quantitatively calculated. Furthermore, the 2D T₁-T₂ map has unique advantages in the identification of various fluid types and in-situ content of fluids in different occurrence states in shale oil reservoirs. MRI technology has significant potential to characterize the spatial distribution of the gas-liquid interface during the CO₂-ESOR process. However, current resolution capabilities are generally inadequate for imaging shale oil reservoir samples, particularly those with low porosity and permeability. The stratified T₂ technology provides spatially resolved T₂ distributions and profiles of oil saturation in shale oil reservoirs. However, these applications still face challenges, such as developing novel probes, mitigating the impact of paramagnetic minerals on NMR measurement, and enhancing the resolution capabilities of MRI technology. The continuous advancement of NMR technology will further enhance the applications in the exploration and development of shale oil reservoirs.