NMR saturation index and its application to fluid saturation estimation

Key words:  laboratory nuclear magnetic resonance  pseudo Archie 's equation  two-phase fluid  nuclear magnetic resonance relaxation index  nuclear magnetic resonance saturation index  water saturation model

Abstract:

Based on laboratory core nuclear magnetic resonance (NMR) measurements of two-phase fluids such as oil-water, gas-water and gas hydrate-water,the influence of fluid saturation on the transversal relaxation time (T₂) spectrum was analyzed. The definition of the NMR relaxation index was proposed from the perspective of the application of NMR logging data. Similar to the well-known Archie 's equation, the empirical relationship between the water saturation and the NMR relaxation index is established, and the characteristics and influencing factors of NMR saturation index are discussed. The results show that the water saturation is exponential with the NMR relaxation index and can be fitted by the exponential function. In additional, the NMR saturation coefficient is distributed around 1, but the NMR saturation index has a wide distribution range, which is influenced by many factors such as grain size, fluid property, as well as pore structure. The predicted water saturation generally agrees with the experimental data, revealing the accuracy of the proposed model. It is suggested that laboratory NMR experiments with different fluid saturations should be carried out according to specific geological units for a better recognition of the reservoir 's NMR logging response, and for the empirical NMR interpretation models.