Transmission and Scanning Electron Microscopy Investigation of Pore Connectivity of Gas Shales on the Nanoscale

Abstract Shale reservoirs represent a significant and increasing portion of natural gas production within North America. In order to better understand gas deliverability of these reservoirs, the flow paths and their connectivity within shales need to be understood at a fundamental level. The pores within gas shales are on the nanoscale and below optical resolution. This necessitates methods such as electron microscopy to image them. Recently, the introduction of Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) technology to the petroleum industry has enabled the imaging of these nanoscale pore structures in three dimensions (3D) for the first time. Imaging of 125 μm3 shale volumes has been performed on different gas shales and the pore systems within these volumes have been reconstructed. Throughout our investigations it has been difficult to obtain pore connectivity across the reconstructed shale volumes. High pressure mercury injection capillary pressure (MICP) measurements on gas shales suggest that the connections between many of these pores are less than 2 nm. Because the size of the smallest pores and pore throats detected within gas shales approaches the resolution limits of SEM, higher resolution techniques must be employed. Transmission electron microscopy (TEM) can image the internal structure of thin shales specimens and enables much higher resolution imaging than SEM. Utilizing this imaging technique in conjunction with other methods we discuss our investigations of pores and their connections that are below the SEM imaging resolution used in previous studies. Initial imaging suggests that isolated pores previously imaged with the SEM exhibit connectivity. It is the connections of pore spaces that control the gas flow in shales and such insights will have far reaching implications in better understanding this flow.