Oil saturation quantitative evaluation in lacustrine shale: Novel insights from NMR T1-T2 and displacement experiments

Abstract

Oil saturation is important for shale reservior to identify favorable sections and determine the geological sweet spots. Current oil saturation evaluation methods, including experiments and empirical formulas, are not suitable for shale reservoir because of the complex mineral, fluid components and pore structure characteristics. A novel method was proposed to accurately calculate the shale oil saturation through the morphological dynamic variation characteristics of the 1D NMR T2 spectra and the signal intensity of the different hydrocarbon-bearing components of the 2D NMR T1-T2 map of shale samples with different oil and water saturation The results (1) indicated that the NMR T2 spectrum revealed that the pore structure of shale samples is complex and highly heterogeneous, and the primary factor that determines the volume of displaced oil in the displacement experiment with slight differences in wettability is the shale’s pore size distribution. (2) shown that the NMR T1-T2 map serves as a useful tool in distinguishing different hydrocarbon-bearing components of shale samples, including kerogen, adsorbed oil, free oil, structural, adsorbed water, and free water. Typically, kerogen is found in the top left region of the T1-T2 map with 10 < T1 < 100 ms & 0.02 < T2 < 0.2 ms. Based on this, T2 threshold for free oil and adsorbed oil are 2 and 0.2 ms, and the corresponding threshold of pore radius are 40 and 4 nm according to the NMR theory. (3) found that the morphological characteristics of 1D NMR T2 spectra vary with the variation of shale oil saturation. Specifically, T2 geometric mean (T2gm) and interval porosity corresponding to the first peak (Ap1) are positively and negatively correlated with oil saturation respectively. Then, oil saturation calculation method is established using the above two parameters and the Root Mean Square Error (RMSE) between the measured and calculated results is 5.85 %, which reflecting the accuracy and validity. In general, this method allows the shale oil saturation to be accurately calculated and provides basis for the determination of favorable sections and evaluation of resource of shale oil reservoir. Moreover, it also offers a new idea for the oil saturation prediction by NMR logging.