From NMR signals to fluid volumes: accurate quantification of shale fluids via a multi-component/nanopore-coupled framework

Abstract

Nuclear Magnetic Resonance (NMR) technology serves as a critical tool for fluid evaluation in shale reservoirs; however, its accuracy relies on reliable conversion of relaxation signals to fluid volume. Current research predominantly focuses on identifying hydrogen-containing components while neglecting systematic calibration of quantitative conversion relationships. This study establishes a free-state/nano-pore coupled calibration method, constructing a comprehensive calibration framework using multiple hydrogen-nucleus-density fluids (crude oil + formation water + crude oil components). Validation was achieved through fluid dynamic loss experiments on sealed core samples and distillation–extraction tests. Key findings include: (1) Free-state calibration reveals that the crude oil-formation water combination better replicates in-situ characteristics. The calibration coefficient k-value increases from 3,590.9 to 6,027.1 (a 68 % rise) with thermal maturity (R₀: 0.7 %–1.67 %), yet decreases from 4,350.8 to 3,816.5 (a 12.3 % reduction) with increasing salinity, showing statistically high consistency (R² = 0.89) with the crude oil component calibration model; (2) Under shale nano-pore confinement, the calibration conversion coefficient (k) for crude oil decreases slightly (6.5 %), while that for formation water increases modestly (5.29 %); (3) Quantitative results from this calibration system demonstrate high consistency between NMR and distillation–extraction evaluations. This research overcomes prior limitations in free-state fluid calibration, elucidates quantitative regulatory patterns of thermal maturity and salinity on k-values, quantifies nano-pore confinement effects on conversion coefficients, and ensures accuracy via a dedicated calibration coefficient verification model. The framework advances shale fluid quantification from “empirical estimation” to in-situ precision quantification, providing technical support for target zone optimization.