Micropore Structure Changes in Response to H2O2 Treatment of Coals with Different Ranks: Implications for Oxidant Stimulation Enhancing CBM Recovery

Abstract

The investigation of micropore structure changes by H2O2-coal reactions can provide an understanding of macroscopic effects of oxidant stimulation on coalbed methane (CBM) diffusion and seepage behavior. In this study, we prepared H2O2 to oxidize coals with three different ranks, and scanning electron microscopy (SEM), high-pressure mercury intrusion (HPMI), low-temperature N2 adsorption (LT-N2A), low field nuclear magnetic resonance (LF-NMR) tests were conducted to characterize variations in micropore structures of the coals treated with H2O2. The results showed that H2O2 treatment can change, to varying degrees, the pore structure characteristics of coals with different rank. The SEM results showed that coal surface structures were destroyed by oxidation and dissolution, and pores and fissures became more developed after H2O2 treatment. The HPMI and LT-N2A tests revealed that the porosity, total pore volume, maximum pore throat diameter and average pore diameter of coals with different rank generally increased after H2O2 treatment. The HPMI-derived fractal dimensions of seepage pores decreased after oxidation, indicating that the heterogeneity of pore structure was weakened and the connectivity was improved. The fractal dimensions of adsorption pores derived from LT-N2A generally showed a downward trend, indicating that oxidation can reduce the roughness and complexity of adsorption pores, and improve the pore connectivity. The results of LF-NMR tests also corroborated the above findings. The mechanism of coal permeability enhancement by H2O2 treatment is reflected mainly in the oxidation and dissolution of organics and inorganic minerals. Thus, H2O2 treatment can lead to more significant oxidation effects on low- and medium-rank coals. This study suggests that H2O2 can be used for fracturing addition fluid in low- to medium-rank coals to improve coal permeability, thereby remove plugging and enhancing CBM recovery effectively.