Pore structures and fractal characteristics of nano-pores in shale of Lucaogou formation from Junggar Basin during water pressure-controlled artificial pyrolysis

Abstract

To analyse the pore structures and fractal characteristics of nano-pores during OM evolution, a series artificial hydrous pyrolysis experiments were conducted on shale samples at 250℃∼500℃ under 50 MPa water pressure. Low-pressure N2 adsorption was performed to investigate the pore structure and fractal dimension of pores on pyrolysis residues. Based on the model by Frenkel-Halsey-Hill, the fractal dimensions of D (relative pressure P/Po from 0 to 1), D1 (before P/Po = 0.5) and D2 (after P/Po = 0.5) were calculated. With the simulation temperature increasing, the pyrolysis products presented three stages that corresponded with the lower maturity (< 350℃), maturity (350℃∼400℃) and higher maturity (400℃∼500℃) stages. For pore structures, Type IV isotherms and Type H3 hysteresis loops occurred in the shale samples, and the adsorbed nitrogen quantity presented an increasing tendency. By analysing the relationship between the pore volume, surface area, and average diameter with OM evolution, pores also occurred during a stage of no change and increased and increased sharply, all of which corresponded to the thermal evolution stages. In the fractal dimension, the relationships between D, D1, D2 and the simulation temperature showed that shale heterogeneity was associated with the OM evolution stage. The rand-size relatio hip between D1 and D2 at different thermocatalytic stages suggested that the main influencing factors on the adsorption of shale gas were the micro-, meso-pores and the quantities of micropores. Therefore, through the research on pore structure and the fractal dimension of pyrolysis residues, we could determine the developmental characteristics of pores and the changing situation of shale heterogeneity based on the migration, preservation, and accumulation of generated hydrocarbons during OM evolution. This research provides a theoretical foundation for the exploration and exploitation of shale gas.