Pore Structure Characterization of a Shale Sample Using SEM Images

Abstract

Abstract We analyze a compiled stack of Scanning Electron Microscopy (SEM) images of an Eagle Ford shale sample to assess pore structure and complex pore connectivity. In recent years, shale and tight oil formations have played an important role as energy resource. However, uncertainties associated with pore structure and pore connectivity as an important parameter controlling the well productivity remain to be resolved; that is why, high-resolution image processing has been an emerging research tool. In this study, 600 contiguous high-resolution SEM images of a 6 μm Eagle Ford sample is analyzed using an image processing software. We obtain 2D and 3D porosity and pore size distribution. In addition, we map out the connected pores to obtain the connected porosity from 3D volumetric fraction. Next, we analyze total and connected porosity as a function of sample size in both forward and backward sequences. Finally, results obtained from the shale sample is compared with a sandstone sample to determine differences in their pore structure properties. We conclude that the connected porosity is 6.1% for the shale sample, smaller than that of the sandstone (19.6%). The attributed pore size distribution ranges from 0.01μm to approximately 0.1μm, while the sandstone’s pore radius size ranges from 1 μm up to 12.7 mm. Through evaluation of sandstone sample, it turns out that 3D total porosity of the sandstone sample is almost the same as the connected porosity. Moreover, although the connected porosity of the stacked images gradually and slightly decreases as the sample size increases. However, for the shale sample, total porosity remains almost constant as a function of sample size while the connected porosity significantly decreases from 13.3% to 6.1%. More interestingly, if the images are analyzed in the backward sequences, then both porosity and the connected porosity increase. Overall, the results suggest that for the studied shale sample, Representative Elementary Volume (REV) describing pore connectivity is around 4 μm. The results from this study will provide a new insight into complex pore structure and help better understand the production performance of shale reservoirs. In addition, the outcome of this paper would have some implications on research work related to fluid flow in shale.