Laboratory study of cryogenic treatment induced pore-scale structural alterations of Illinois coal and their implications on gas sorption and diffusion behaviors

Abstract

Cryogenic fracturing using liquid nitrogen is a choice for CBM reservoir stimulation. However, the development of cryogenic fracturing is still at its infancy. This work focused on induced pore structural alterations due to cryogenic treatment and their effects on gas sorption and diffusion behaviors. The changes in the pore structure of coal induced by cyclic nitrogen injections were studied by physical adsorption at low temperatures. A micromechanical model is proposed to simulate the microscopic process and predict the degree of deterioration due to low temperature treatments. As a common characteristic of modeled results and experimental results, the total volume of mesopore and macropore increased with cryogenic treatment, but the growth rate of pore volume became much smaller as freezing-thawing were repeated. Pores of different sizes experienced different degrees of deterioration. In the range of micropores, no significant alteration of pore volume occurred with the repetition of freezing and thawing. In the range of mesopores, pore volume increased with the repetition of freezing and thawing. In the range of macropores, pore volume increased after the first cycle of freezing and thawing but decreased after three cycles of freezing and thawing. Because of pore structural alteration, cryogenic treatment enhanced gas transport process as the diffusion coefficients of the freeze-thawed coal samples were increased by 18.76% and 30.18% in the adsorption and desorption process. The effect of cyclic cryogenic treatment on the pore structure of tested coal varied depending on coal properties. For the studied Illinois coal sample, repetitive applications of cryogenic treatment reduced macropore volume and increase mesopore volume. For the tested sample, the diffusion coefficient of the coal sample that underwent three cycles of freezing-thawing was lower than that of the coal sample that underwent a single cycle of freezing and thawing. The outcome of this study provides justification for a post-cryogenic fracturing effect on diffusion improvement and gas production enhancement, especially for high “sorption time” CBM reservoirs.