Cooling artifacts in ex-situ pore characterization of thermally stimulated low-maturity shale: an in-situ SEM imaging reassessment

Abstract

The reliability of ex-situ methodologies in characterizing pore networks within thermally stimulated low-maturity oil shale remains debated due to potential cooling-induced artifacts. This work integrates in-situ scanning electron microscopy (SEM) with high-precision thermal control to systematically evaluate pore evolution during heating (25 °C→500 °C) and subsequent cooling (500 °C→25 °C). Results reveal that thermal upgrading at 500 °C enhances pore development (11% and 111.5% increase in total pore area for two samples). However, the cooling process further amplifies pore network complexity, inducing matrix shrinkage that generates additional macropores (up to 72% pore count increase), thereby distorting ex-situ measurements. While small pores (< 0.05 μm²) dominate across thermal stages and pore aspect ratios remain stable—supporting ex-situ validity for morphological trends—the cooling artifact systematically overestimates macropore abundance, a critical parameter for flow capacity assessment. These findings challenge conventional ex-situ techniques (e.g., gas adsorption, NMR) in replicating in-situ reservoir conditions during thermal recovery. The work proposes integrating in-situ imaging (e.g., heating-stage SEM) as a standard for high-temperature studies, developing cooling-effect calibration protocols for legacy data, and redesigning porosimetry systems with thermal controls. By resolving discrepancies between laboratory measurements and subsurface realities, this research advances predictive models for shale oil mobility and informs optimization of in-situ conversion technologies, ultimately supporting sustainable exploitation of low-maturity shale oil resources.

Supplementary Information: The online version contains supplementary material available at 10.1038/s41598-025-34434-0.

Keywords: In-situ imaging; Low-maturity oil shale; Pore network characterization; Thermal cooling effects.

Fig. 1

Brief introduction for (a) SEM setup with heating substrate and (b) temperature settings.

Fig. 2

SEM captures and the process of image digitization processing.

Fig. 3

Total pore area and count derived from SEM imaging at varying temperatures. (a), Sample A; (b), Sample B.

Fig. 4

Statistics about the perimeter and area of pores at (a) origin at 25 ℃ and (b) heated to 500 ℃ for Sample A. Note that the color transition from red to blue indicates an increase in both the perimeter and the area of the pore.

Fig. 5

Statistics about the perimeter and area of pores at (a) origin at 25 ℃ and (b) heated to 500 ℃ for Sample B. Note that the color transition from red to blue indicates an increase in both the perimeter and the area of the pore.

Fig. 6

Normal distribution of aspect ratio (AR) values for (a) Sample A and (b) Sample B at variable temperatures.

Fig. 7

Imaging evidence demonstrating pore system variation induced by temperature decrease (exemplified by Sample A). (a), heated to 500℃ and (b) cooling to 25℃.

Fig. 8

Statistics about the perimeter and area of pores for (a) Sample A and (b) Sample B at condition of cooling to 25 ℃. Note that the color transition from red to blue indicates an increase in both the perimeter and the area of the pore.