Petrophysical characterization of oil-bearing shales by low-field nuclear magnetic resonance (NMR)

Shales are important components of petroleum systems, and it is necessary to reveal their petrophysical properties as reservoirs. Low-field nuclear magnetic resonance (NMR) has proven to be a non-invasive and effective technique for petrophysical characterization of sandstone, carbonate and coal. However, little study has been done using this technique to systematically investigate the petrophysical properties of oil-bearing shales, which is essential for characterizing shale oil reservoirs. In this study, two sets of NMR measurements were performed to reveal the porosity, permeability, movable fluid volume and pore size distribution (PSD) of oil-bearing shales. The results show the following: (1) The NMR T2 spectrum strongly relates to the content of quartz and the amount of dissolution pores within shale. Three T2 spectrum peaks were identified by the relaxation times at 0.03-1 ms, 1-20 ms and >20 ms, corresponding to micropores (<100 nm), mesopores (100-1000 nm) and macropores (>1000 nm), respectively. (2) By combining NMR with centrifugation, both NMR porosity and movable porosity (MP) were calculated. NMR porosity is well consistent with helium porosity, and the MP values strongly relate to the development of mesopores in shales. (3) Based on the T2 spectrum, an effective permeability model was proposed, which can conveniently estimate the permeability of shales. Moreover, two classical permeability models (Coates and SDR) were also used to estimate shale permeability, and the permeability values determined by these models agrees well with helium permeability. (4) In combination with SEM images analyses, an NMR-based PSD was constructed that agrees well with the PSD determined by SEM. The results demonstrate the applicability of NMR relaxation in a systematic investigation of the petrophysical properties of shales, which has potential applications for geophysical logging in shale oil exploration.