Determination of Apparent Pore Size Distributions of Organic Matter and Inorganic Matter in Shale Rocks Based on Water and N-2 Adsorption

Knowledge of a shale pore structure is essential and critical to estimating gas storage and predicting gas production. Shale is a heterogeneous rock in terms of its composition and consists of organic matter (OM) and inorganic matter (IOM). Due to their difference in surface wettability and affinity to methane, the quantitative determination of apparent pore size distributions (APSDs) of OM and IOM as well as their contributions to the Barrett-Joyner-Halenda (BJH) pore volume is a very important task, but it is still a puzzling issue, especially considering the occurrence of water under the in situ condition. In this work, combining the water adsorption and low-pressure N-2 adsorption-desorption experiments, we obtained the APSDs of dry and moist shale and clay samples, where the latter is assumed to the representative of IOM. Then, given the water distribution related to the surface wettability, a novel approach is proposed to quantitatively determine the APSDs of OM and IOM as well as their contributions to the BJH pore volume of shale under dry and in situ conditions. Our experimental results demonstrated that small nanopores (<5 nm) presented under a dry condition may be absent on the clay APSD curves under a moist condition due to the capillary condensation effect but are still shown on the shale APSD curves due to the existence of hydrophobic OM nanopores. Under a dry condition, the APSDs of OM and IOM for our samples show that OM is rich in small nanopores (i.e., 3-50 nm) while IOM pores show a wider range of size (i.e., 3-200 nm), and the contribution of OM-hosted volume to BJH pore volume is 26.7% and 20%. While under a moist condition (RH = 98%), IOM pore volumes of 26% and 20% are occupied by water molecules, and the OM-hosted proportion increases to 36% and 26%, respectively. This study proposed a significant approach to deeply characterize shale pore structure, which provides a more solid and reliable foundation for the shale gas storage estimation and well performance prediction.