Investigation of the unblocking and permeability enhancing properties of low-permeability coal by using pulsed nitrogen injection technology

The pulsed nitrogen injection for methane displacement (PNI-MD) technology is a novel approach aimed at reducing greenhouse gas emissions and enhancing the extraction efficiency and utilization of methane from low-permeability coal seams. By conducting experiments with PNI-MD, the influence of nitrogen injection duration on the methane displacement effect and the coal deformation characteristics is analyzed. The gas flow state and the mechanism of blockage-unclogging and permeability-increasing are investigated, and the field application mode of PNI-MD technology in low-permeability coal seams is constructed. The results show that with the prolongation of nitrogen injection time, the methane displacement efficiency increases while the methane displacement ratio decreases. During the nitrogen injection stage (Stage A), the high-pressure nitrogen gradually affects the gas flow state within the coal and at the outlet end. The gas flow state within the coal in Cycle 1 is divided into five stages. High-pressure pulsed nitrogen generates periodic tensile and compressive stresses with strong alternating pressure, which promotes repeated changes in the mechanical properties of coal, and the cumulative effect of damage leads to fatigue deterioration of the coal. The blockage-unclogging and permeability-increasing effects throughout the entire process of PNI-MD are divided into four stages. Under varying nitrogen injection durations, the increase in coal permeability after the completion of PNI-MD ranges from 0.50% to 3.63%. The PNI-MD technology effectively leverages the dual role of multi-gas displacement for desorption and energy injection for changing coal properties to achieve the objectives of blockage-unclogging and permeability-increasing. The dominant mechanisms during the Stage A include carrying methane with charging energy (CCWCE), promoting methane desorption by sharing pressure (PCDBSP), enlarging pores with injecting energy (EPWIE), and expanding pores with injecting energy (EPWIE1). The dominant mechanisms during the stop-injection stage (Stage B) include promoting methane diffusion by dilution its concentration (PCDBDIC) and enlarging pores with promoting methane desorption (EPWPCD). The damaging pore structure/spread fracture network by pulse gas flow to increase the methane flow (DPSSFNBPGF) throughout the entire process of PNI-MD. According to the properties of methane occurrence in low-permeability coal seams in China, the construction technology and equipment for underground PNI-MD technology are summarized. A flexible, simple and multi-selectable method for optimizing the parameters of the PNI-MD process is proposed, which is applicable for varying methane occurrence characteristics in different coal seams, thereby contributing to the precise extraction of methane from low-permeability coal seams, as well as carbon peak and carbon neutrality in China.