A new nondestructive method for shale porosity evaluation based on nuclear magnetic resonance: Shale-ΦT1/T2

ABSTRACT Porosity serves as a critical parameter in shale oil reservoir evaluation and resource quantification. Limitations in evaluation principles and shale heterogeneity lead to varying effectiveness among different methods. Moreover, solvent extraction may cause fluid retention, toxicity risks, and data distortion, compromising the accuracy of shale oil resource assessment. This study proposes the Shale-ΦT1/T2 (a nuclear magnetic resonance (NMR) method combining longitudinal relaxation and transverse relaxation for shale porosity) porosity evaluation methodology for conventional core shales as an innovative approach, using sealed core samples from the fourth member of the Shahejie Formation, Dongying sag as the study object. A NMR-based NMR-ΦT1/T2 model was established to characterize sealed core shale fluid porosity (Φtotal1), residual fluid porosity (Φ1) in conventional cores after surface exposure, and saturation-fluid porosity (Φ2) in pressurized-saturated shales. Total porosity (Φtotal2) is obtained by summing the two-stage results (Φ1 + Φ2). Results demonstrate high consistency between sealed core porosity (Φtotal1) and Shale-ΦT1/T2-derived porosity (Φtotal2), with error coefficients systematically decreasing as porosity increases. This method avoids core extraction and solvent-induced pore distortion, significantly improving detection efficiency. As an eco-friendly solution addressing fidelity challenges in shale porosity assessment, it can be rapidly adopted as a primary method for porosity determination in shale oil reserve evaluation and resource quantification.