Nuclear Magnetic Resonance (NMR) is a powerful tool for pore structure analysis and fluid identification in logging applications and core laboratory analysis. For example, T2 cut-off times are set to determine the effective porosity from total porosity in conventional sandstones and carbonates. The conventional knowledge is not directly applicable for unconventional reservoirs because of insufficient understanding of the NMR response for fluids in shale rocks. Therefore, it is desirable to investigate the NMR response of fluids in shale rocks to gain a better knowledge of the shale properties.
Fourteen shale core plugs were examined using NMR and retort methods. T2, T1 spectra and T1- T2 plots and their physical properties were obtained for all the samples in the ‘as received’ state. Two-step retort experiments were performed on crushed samples, the weight changes recorded and the fluid produced during retort collected. After each retort step, NMR measuritalicents were performed and the results were compared with the original spectra.
The core samples were classified into two groups, Type I and Type II, after analyzing the NMR, density and porosity data. The NMR spectra of Type I samples had one major peak for T2 data in the fast relaxation regime, while Type II samples showed two or multiple peaks in T2 and T1 spectra. The T1/ T2 ratio was as high as 20 for the Type I samples, which can be an indication of a larger amount of organic matter in rocks. Using NMR spectra obtained before and after retort, effective porosity of the shale samples was obtained by determining the T2 cut-off times for the two types of samples.