The experimental study on multiple mechanisms of enhanced CBM recovery by autogenous nitrogen fracturing fluid

Abstract

Hydraulic fracturing is a widely adopted technique for enhancing the permeability of coalbed methane (CBM) reservoirs, thus improving the recovery of CBM. Although previous studies have extensively explored the role of fracturing fluids in creating fractures and improving permeability, there remains a notable deficiency in the literature regarding the impact of these fluids on enhancing gas desorption and diffusion. This research employs an experimental investigation into a novel autogenous nitrogen (N2) fracturing fluid, demonstrating its significant potential for increasing CBM reservoir permeability and enhancing the desorption and diffusion of CBM. Through optimization experiments, the study initially determines the optimal proportions of sodium nitrite (NaNO2), ammonium chloride (NH4Cl), and an acidic activator to maximize the generation of N2. Kinetic experiments and subsequent analyses reveal that the autogenous N2 fracturing fluid releases a substantial amount of N2 and heat, with the reaction rate highly influenced by initial pressure and temperature conditions. Fracturing experiments on coal samples demonstrate that the release of N2 increases the fluid pressure within the pore spaces, initiating micro-fractures and enhancing permeability. Additionally, the increased pore pressure mitigates the water block effect and improves the gas diffusion. The heat and gas released during the reaction significantly facilitate the desorption of CBM within coals. This fracturing fluid offers a comprehensive approach for enhancing CBM recovery.