A novel NMR-capillary pressure method for quantifying pore connectivity and its impact on permeability evolution

Abstract

Pore connectivity within rocks is critical for predicting permeability and plays a significant role in evaluating geological reservoirs and engineering applications. However, quantifying pore connectivity remains challenging due to the multiscale pore structures. This study proposes a novel method combining nuclear magnetic resonance (NMR) and capillary pressure (P_c) to quantitatively characterize pore connectivity. Initially, a joint function was established using the relaxation spectra (T₂) of movable water obtained at various capillary pressures. Subsequently, a novel NMR method considering fluid trapping and migration was proposed to quantitatively characterize the pore connectivity of sandstone samples. Finally, a positive correlation between pore connectivity and permeability confirmed the validity of the proposed NMR method. The experimental results indicate that permeability evolution is more strongly correlated with pore connectivity than with porosity. The contribution of pore connectivity to permeability exhibits different stages, influenced by the proportion of pores across multiple scales. The RZ sample, with a pore connectivity of 0.02, suggests that the high percentage of residual water saturation in meso- and micropores is the primary limiting factor. The sorting coefficients of the six samples are ranked as DY < YB < WH < ZG < JN < RZ, reflecting a more homogeneous pore size distribution and demonstrating the best pore connectivity in the DY sample.