Understanding the interactions of NaClO oxidant with coal for intensified hydraulic fracturing effectiveness

Abstract

The efficacy of hydraulic fracturing coalbeds to enhance methane production suffers from a reduction of the effective permeability due to formation damage. To address this practical challenge, this paper explores the feasibility of applying the oxidative treatment to coals in the post-hydraulic fracturing period. Experimental methodologies are utilized to reveal the fundamental interactions of coal with the oxidative in the context of a combined treatment of hydraulic fracturing and NaClO solution injection. Anthracite and bituminous coals are experimented to account for the effect of coal rank. A comprehensive package of techniques, i.e., ultrasonic wave velocity and hardness (for mechanical properties), scanning electron microscopy (for microstructure and morphology), contact angle (for wettability), and FTIR spectroscopy (for functional groups) were employed to comprehensively investigate the impacts of fracturing fluid and NaClO solution on the chemical and physical properties of the coal samples. The results show that the fracturing fluid exerts varying degrees of harm to the permeability of the two coals, while the effects of oxidative treatment are rank specific in dissolving the blockage and smoothening the fracture surface. It is further unveiled that NaClO solution promotes aromatic structure decomposition at the coal surface to form oxygen-containing functional groups, and its effects on coal permeability evolve in multiple stages. The findings of this study shed light upon implementing oxidative treatment to aid hydraulic fracturing performance in CBM for clean coal production in a carbon-constrained world.