An approach to calculate bound water saturation by NMR logging spectral coefficient method

NMR logging, as one of the primary methods for calculating bound water saturation of reservoir, can be used to directly characterize properties of reservoir fluid and distinguish the volume of clay bound water and capillary bound water, respectively. And then, the bound water saturation can be obtained based on the total volume of bound water including clay bound water and capillary bound water. Both the T 2 cut-off value and spectral irreducible water saturation can explain and ascertain the bound water saturation. Since the T 2 cut-off value may have distinct variations in different layers and different blocks, the T 2 cut-off value would cause obvious errors if it is not determined properly. In the original spectral coefficient model, all spectral coefficients depend on the data from rock core analysis without considering the occurrence mechanism of the bound water. Thus, this model would create many errors in the evaluation of the bound water saturation. In this paper, we proposed an improved spectral coefficient model instead of the original model in order to determine the bound water saturation. All the spectra coefficients in the new model are modified according to the laboratory core data to cut down the fitting error. We also verified the applicability of the improved model by the actual core data. By eliminating the influence of the bound water effectively, our new model showed potential to improve the accuracy of the estimated bound water saturation from NMR logging remarkably, which is regarded as a good reference to explain the bound water saturation for complicated reservoirs.