Literature Interpretation | Professor Pan Zhejun's team from Northeast Petroleum University and Associate Professor Du Yi's team from Northwest University《Application of nuclear magnetic resonance technology in reservoir characterization and CO2 enhanced recovery for shale oil: A review》：Application Overview of Nuclear Magnetic Resonance Technology in Shale Oil Reservoir Characterization and CO₂-Enhanced Oil Recovery (EOR)

Research Background

Due to the low porosity, low permeability, multi-scale pore spaces, and complex fluid components, characterizing and developing shale oil reservoirs present significant challenges. Nuclear Magnetic Resonance (NMR) technology has a distinct advantage in shale oil reservoir characterization and development research due to its high resolution, non-destructive and rapid capabilities. This paper summarizes and looks ahead to the progress of NMR technology in shale oil exploration and development over the past decade.

Abstract

The NMR technologies applied in shale oil exploration and development laboratory studies mainly include one-dimensional NMR T₂ maps, two-dimensional NMR T₁-T₂ maps, MRI technology, and layered T₂ technology. T₂ maps non-destructively characterize the full pore size distribution of shale oil reservoirs, and can be combined with other experimental methods to expand their functionality:
(1) Combine with centrifugation and thermal treatment to determine the NMR T₂ cutoff value, enabling quantitative separation of movable fluids, capillary-bound fluids, and immovable fluids (bound state);
(2) Combine with core holders to characterize the stress sensitivity of the shale matrix and fracture system;
(3) Combine with imbibition experiments to quantitatively evaluate imbibition patterns and wettability;
(4) Use an online high-temperature high-pressure CO₂ displacement setup to quantitatively calculate dynamic recovery rates. Additionally, two-dimensional T₁-T₂ maps have unique advantages in identifying various fluid types and in-situ fluid content in shale oil layers under different reservoir conditions. During CO₂ enhanced recovery in shale oil reservoirs, MRI technology has immense potential for characterizing the gas-liquid interface spatial distribution. Layered T₂ technology provides spatially resolved T₂ distributions of samples during CO₂ flooding processes and gas-liquid interface distributions.

Application Details and Examples

(1) NMR T₂ spectral technology can non-destructively characterize the full pore size distribution of shale oil reservoirs, but there are technical gaps compared to low-temperature nitrogen adsorption and high-pressure mercury intrusion methods in analyzing pore geometry and specific surface area.

(2) By combining T₂ spectra with centrifugation and thermal treatment, an NMR T₂ cutoff calibration method can be established to achieve the three-phase quantitative separation of immovable fluids (bound state), capillary-bound fluids, and movable fluids (free state).

Figure 1: Calculation method for two T₂ cutoff values of shale samples (Xu et al. 2022)

(3) Using a loadable confining pressure core holder, T₂ spectra can dynamically monitor changes in core porosity under different effective stresses, quantitatively characterizing reservoir stress sensitivity.

(4) Combined with imbibition kinetics experiments, T₂ spectra can simultaneously capture the saturation changes during the imbibition process and establish a wettability evaluation model.

Figure 2: Schematic diagram of NMR spontaneous imbibition experiment process (Wang et al. 2022)

(5) An online T₂ monitoring system integrated into the high-temperature high-pressure CO₂ displacement device enables in-situ non-destructive monitoring of fluid migration during the displacement process, significantly improving the reliability of dynamic recovery data.

Figure 3: Spatial distribution of oil saturation before (a) and after (b) CO₂ huff-n-puff experiment (Tang et al. 2023)

(6) Two-dimensional T₁-T₂ spectra have unique advantages in the quantitative identification of fluid types in shale reservoirs and distinguishing between solid kerogen, viscous bitumen, and fluids in different states (adsorbed and free).

Figure 4: Typical shale oil reservoir component identification T₁-T₂ maps (Top: Li et al., 2018; Bottom: Zhang et al., 2020)

(7) Layered T₂ technology, through spatially resolved T₂ distribution analysis, can reconstruct the oil saturation profile during displacement and assess the sweep efficiency based on CO₂ front expansion characteristics.

Figure 5: Spatial distribution characteristics of shale oil during CO₂ huff-n-puff from rounds 1 to 5 (Luo et al. 2022)

Summary and Outlook:

(1) Two-dimensional T₁-T₂ spectra demonstrate significant technical advantages in identifying multicomponent fluid states (adsorbed/free) and quantitative analysis of in-situ fluid content in shale oil reservoirs.

(2) Two-dimensional T₁-T₂ spectra show engineering application potential in monitoring fluid migration during CO₂ flooding, but their existing two-dimensional sequence measurement takes too long, limiting their rapid field response capability. Optimization of pulse sequences and hardware upgrades are needed to achieve breakthroughs in real-time dynamic monitoring.

(3) Magnetic Resonance Imaging (MRI) technology can dynamically capture gas-liquid interface migration characteristics during the displacement process. However, the current millimeter-level spatial resolution does not meet the observation requirements for ultra-low permeability shale micro-nano pore throat systems. With breakthroughs in imaging technology, it is hoped that online characterization of multiphase flow migration profiles in shale oil reservoirs can be achieved, thereby expanding the application boundaries of NMR technology in unconventional oil and gas development.

References

Xu, J., Ge, Y., He, Y., Pu, R., Liu, L., Du, K., Li, H., He, X., Duan, L., 2022. Quantification of pore size and movable fluid distribution in Zhangjiatan shale of the Ordos Basin, China. Geofluids 2022, 1–16.

Wang, H., Yang, L., Wang, S., Guo, D., Li, M., Zhao, Z., Dong, H., Zheng, Z., 2022. Study on imbibition characteristics of glutenite with different boundary conditions based on NMR experiments. Energy & Fuels, 36: 14101-14112.

Tang W, Sheng J, Jiang T. Further discussion of CO₂ huff-n-puff mechanisms in tight oil reservoirs based on NMR monitored fluids spatial distributions[J]. Petroleum Science, 2023, 20:350-361.

Li, J., Huang, W., Lu, S., Wang, M., Chen, G., Tian, W., Guo, Z., 2018. Nuclear magnetic resonance T₁–T₂ map division method for hydrogen-bearing components in continental shale. Energy & Fuels, 32: 9043–9054.

Zhang, P., Lu, S., Li, J., Chang, X., 2020. 1D and 2D nuclear magnetic resonance (NMR) relaxation behaviors of protons in clay, kerogen and oil-bearing shale rocks. Marine Petroleum Geology, 114: 104210.

Luo, Y., Zheng, T., Xiao, H., Liu, X., Zhang, H., Wu, Z., Zhao, X., Xia, D., 2022. Identification of distinctions of immiscible CO2 huff and puff performance in Chang-7 tight sandstone oil reservoir by applying NMR, microscope and reservoir simulation. Journal of Petroleum Science and Engineering, 209: 109719.