Morphological complexity and azimuthal disorder of evolving pore space in low-maturity oil shale during in-situ thermal upgrading and impacts on permeability

Abstract

In-situ thermal upgrading is used to tune the pore system in low-maturity oil shales. We introduce fractal dimension (D), form factor (ff) and stochastic entropy (H) to quantify the heating-induced evolution of pore morphological complexity and azimuthal disorder and develop a model to estimate the impact on seepage capacity via permeability. Experiments are conducted under recreated in-situ temperatures and consider anisotropic properties—both parallel and perpendicular to bedding. Results indicate that azimuthal distribution of pores in the bedding-parallel direction are dispersed, while those in the bedding-perpendicular direction are concentrated. D values indicate that higher temperatures reduce the uniformity of the pore size distribution (PSD) in the bedding-parallel direction but narrow the PSD in the bedding-perpendicular direction. The greater ff (> 0.7) values in the bedding-parallel direction account for a large proportion, while the dominated in the bedding-perpendicular direction locates within 0.2–0.7, for all temperatures. The H value of the bedding-parallel sample remains stable at ∼0.925 during heating, but gradually increases from 0.808 at 25 °C to 0.879 at 500 °C for the bedding-perpendicular sample. Congruent with a mechanistic model, the permeability at 500 °C is elevated ∼1.83 times (bedding-parallel) and ∼6.08 times (bedding-perpendicular) relative to that at 25 °C—confirming the effectiveness of thermal treatment in potentially enhancing production from low-maturity oil shales.