Estimating capillary pressure from NMR measurements using a pore-size-dependent fluid substitution method

This report introduces a workflow to calculate capillary pressure curves from NMR transverse relaxation time ( T₂ ) distributions of partially hydrocarbon-saturated measurements. First, a pore-size-dependent fluid-substitution (PSDFS) joint inversion method is developed to correct T₂ distributions for hydrocarbon effects in partially hydrocarbon-saturated rocks. A PSDFS joint inversion on the T₂ distributions of samples at different hydrocarbon saturations is used to estimate input parameters for fluid substitution and to reconstruct the fully water-saturated T₂ distribution. Next, the T₂ distribution of the fully water saturated sample is converted to a pore-size distribution using an estimated surface relaxivity. Finally, assuming a linear relationship between pore and throat size distributions, the saturation-dependent capillary pressure curve can be estimated using a triangular tube model. The PSDFS joint inversion is validated on NMR measurements of Berea sandstone samples with different values of hydrocarbon saturation. The feasibility of our joint inversion method is confirmed by comparing the calculated fully water-saturated T₂ distribution to the T₂ distribution of the measured fully water-saturated rock sample. The capillary pressure curves are derived from fluid-substituted fully water-saturated T₂ distributions and compared to mercury injection capillary pressure (MICP) measurements. Capillary pressure curves derived with the PSDFS method agree well with MICP measurementsPetroleum and Geosystems Engineerin