Fluid States and Movable Fluid Distribution from Conventional Reservoirs to Unconventional Reservoirs Based on Nuclear Magnetic Resonance Characterization

Summary Driven by the depletion of conventional reservoirs and the escalating global energy demand, the focus has shifted toward unconventional reservoirs. These reservoirs exhibit intricate fluid flow behaviors due to their lower permeability and smaller pore sizes compared with conventional reservoirs. Traditional characterization techniques, such as nitrogen adsorption and mercury injection capillary pressure (MICP), are significantly limited in accurately characterizing fluid flow in these complex reservoirs. Nuclear magnetic resonance (NMR) emerges as a nondestructive and highly precise method, providing valuable insights into fluid-rock interactions, pore-size distribution, and fluid identification at the pore scale. This study investigates the behavior of typical oleic and aqueous fluids, including both single-component and multicomponent fluids, in both conventional and unconventional reservoirs. The 1D and 2D NMR properties of these fluids are characterized in both bulk state and porous media. The impacts of fluid composition and pore size on T1, T2, and T1–T2 spectra are examined to distinguish NMR behaviors in nanoscale and microscale pores. In the bulk fluid state, 1D NMR reveals that T1 and T2 spectral peaks decrease with increasing fluid viscosity. Meanwhile, T1–T2 spectra show that fluids with different viscosities are distributed within a T1/T2 ratio range of 0.5–2, shifting leftward and downward as viscosity increases. In porous media, increased fluid viscosity causes a leftward shift in T1 and T2 spectra, consistent with bulk fluid behavior. In high-permeability cores (HPCs), T1 and T2 spectra of single- and multicomponent fluids exhibit unimodal distributions, aligning with pore size distribution. In contrast, in low-permeability cores (LPCs), T1 spectra of all fluids remain unimodal, while T2 spectra of multicomponent fluids become bimodal due to diffusion relaxation effects. This is attributed to differences in diffusion coefficients and enhanced internal gradient fields in nanoscale pores. Thus, T1 spectra are more suitable for characterizing multicomponent fluid distribution in low-permeability reservoirs with nanoscale pores. Core centrifugation tests combined with NMR scans are used to assess fluid mobility changes influenced by viscosity and centrifugal pressure. For fluid mobility in different porous media, cutoff values of T1 and T2 and movable fluid saturations are determined through centrifugation tests. The deviation of T2 spectra from the real pore structure does not affect the calculation of movable fluid saturation. However, T1 spectra provide a more accurate representation of fluid distribution during centrifugation, despite longer measurement times. This study validates the adaptability of 1D NMR for characterizing the pore structure and fluid distribution of unconventional reservoirs. It provides a reliable reference for multifluid identification in real underground conditions. The study enhances the understanding of fluid behavior and pore-scale interactions in complex reservoirs and offers a robust framework for optimizing NMR applications in unconventional reservoirs.