Evolution of pore systems in low-maturity oil shales during thermal upgrading —Quantified by dynamic SEM and machine learning

Abstract

In-situ upgrading by heating is feasible for low-maturity shale oil, where the pore space dynamically evolves. We characterize this response for a heated substrate concurrently imaged by SEM. We systematically follow the evolution of pore quantity, size (length, width and cross-sectional area), orientation, shape (aspect ratio, roundness and solidity) and their anisotropy —interpreted by machine learning. Results indicate that heating generates new pores in both organic matter and inorganic minerals. However, the newly formed pores are smaller than the original pores and thus reduce average lengths and widths of the bedding-parallel pore system. Conversely, the average pore lengths and widths are increased in the bedding-perpendicular direction. Besides, heating increases the cross-sectional area of pores in low-maturity oil shales, where this growth tendency fluctuates at < 300 °C but becomes steady at > 300 °C. In addition, the orientation and shape of the newly-formed heating-induced pores follow the habit of the original pores and follow the initial probability distributions of pore orientation and shape. Herein, limited anisotropy is detected in pore direction and shape, indicating similar modes of evolution both bedding-parallel and bedding-normal. We propose a straightforward but robust model to describe evolution of pore system in low-maturity oil shales during heating.