A comparison of experimental methods for describing shale pore features - A case study in the Bohai Bay Basin of eastern China

To compare experimental methods for describing shale pore features, we performed a series of parallel experiments on typical continental shales from the Bohai Bay Basin of eastern China, including scanning electronic microscopy (SEM), focused ion beam scanning/field emission scanning electron microscopy (FIB/FE-SEM), low-temperature nitrogen adsorption/desorption (LTNA), mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR) techniques. The results show that artificial peaks located at 3 to 5 nm, affected by the tensile strength effect (TSE), are clearly seen in the BJH pore size distributions (PSDs) calculated using the low-temperature nitrogen desorption curve of shale with narrow-neck pores (e.g., ink-bottle pores). Although the LTNA technique cannot completely reveal large pores, the adsorption/desorption curve and the PSD acquired from it can still indicate the relatively high or low extent of development of large pores in different shales. Shale within the “oil window” primarily contains inorganic pores, as organic nanopores are generally formed during the high or over-mature stage of organic matter. During the mercury intrusion, high pressures forcing liquid mercury to enter small pores (less than 40 to 70 nm) will cause pores in the samples to crack, especially for shale with slit-shaped and wedge-shaped pores. Unlike LTNA, MIP measures only pore throats and does not detect pore bodies; therefore, the effective pore size revealed by the LTNA method may be up to 300 nm instead of only 50 nm when LTNA is combined with MIP to reveal the PSD of shales with narrow-necked pores. NMR and LTAN can be complemented by each other, and the PSD results revealed by them are anastomotic, which suggests that the combination of NMR and LTAN is an ideal method for revealing the PSD of shale.