Characterization of Nanoporous Systems in Gas Shales by Low Field NMR Cryoporometry

Shale gas and oil is an increasingly important source of unconventional energy. Shale gas and oil reservoirs differ from their conventional counterparts mainly in the nanoporous structures of the former, which play a critical role not only for the resource estimation but also for the shale gas/oil extraction and development. However, the traditional methods for characterizing rock porosities, such as gas sorption (the Brunauer–Emmett–Teller technique, BET), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), cannot satisfactorily and adequately measure and characterize the nanoporous structures. Nuclear magnetic resonance (NMR) spectroscopy is known for its sensitivity to local environments at the atomic level and, therefore, can provide an alternative method for the investigation of nanoporous structures in gas shales. This study has refined the low field NMR cryoporometry (NMRC) method and applied it along with other methods such as NMR relaxometry (NMRR) to measure and characterize the nanoporous structures (i.e., the pore size distribution, PSD) of selected shale samples from the Sinian-Cambrian-Ordovician strata at the Low Yangzi Plateau, China. Our NMRC measurements of a controlled porous glass (CPG) and shale samples show that the organic compound octamethylcyclotetrasiloxane (OMCTS) is a superior NMR probe liquid in terms of improved spectral resolution and signal/noise (S/N) ratio. Comparisons of the NMRC shale PSD results with those from NMRR and gas sorption show that NMRC is an independent and effective method for determining the distribution of nanosized pores in gas shales. Moreover, important parameters such as porosity can also be estimated from the low field NMR cryoporometry.