Research on Quantitative Analysis for Nanopore Structure Characteristics of Shale Based on NMR and NMR Cryoporometry

Shale gas is a key natural gas resource of very great significance in China. On the basis of the differences in pore structure characteristics between marine and continental shale in China, the Weiyuan marine shale (sample #1), Jiao Shiba marine shale (sample #2), and Yaoqu continental shale (samples #5 and #6) were selected to study the pore structure characteristics and controlling factors using cold field-emission scanning electron microscopy and nuclear magnetic resonance (NMR). NMR cryoporometry (NMRC) was employed to determine the nanoscale pore structure. This method can be extended to micron measurements combining NMR relaxation analysis to detect in detail the pore structure of shales on different aperture scales. The smaller the test temperature gradient is, the finer is the pore distribution result. The test results show decreasing porosity in going from sample #5 to samples #2, #6, and #1 to #4. NMRC, low-field NMR (LFNMR), mercury intrusion porosimetry (MIP), and gas adsorption (GA) methods show good agreement for the pore distribution in their respective scopes of application. The NMR methods result in a much better estimate for the total pore volume than the more common MIP and GA methods. Hence, the pore structure of the reservoir shale can be evaluated more accurately by combining NMRC and LFNMR with GA and MIP. Thus, the nanopores of continental shale (sample #5, Yaoqu shale) are clearly better developed and will more likely have a higher commercial exploitation value than marine shale.