Experimental study on enhanced coal seam gas extraction by uniform pressure/pulse pressure N2 injection

Gas extraction is the fundamental way to prevent and control gas as well as to obtain clean energy. Based on the analysis of the mechanism of conventional negative-pressure gas extraction technology and N2 injection enhanced gas extraction technology, the technology of pulse pressure N2 injection enhanced gas extraction is proposed. The experiments of CH4 displacement by uniform pressure/pulse pressure N2 injection are carried out. The migration pattern of gas components during the whole process of CH4 displacement by N2 injection is quantitatively characterized. The influence pattern and sensitivity of N2 injection pressure on the effect of CH4 displacement are analyzed. The economic benefits of N2 injection by uniform pressure/pulse pressure to displace CH4 are investigated. The results indicate that conventional negative-pressure gas extraction technology has disadvantages such as insufficient seepage power and gas extraction will soon enter the exhaustion stage. While N2 injection enhanced gas extraction technology not only has the flow increasing effect of gas itself factors but also has the flow increasing effect of injected energy for changing coal properties. During the process of displacing CH4 by uniform pressure/pulse pressure N2 injection, the CH4 volume fraction decreases and the N2 volume fraction increases as the N2 injection pressure increases. Increasing the injection pressure of two N2 injection methods can effectively shorten the T50% (the time when the CH4 volume fraction is equal to the N2 volume fraction is equal to 50%), but with a reduced degree of impact. Increasing injection pressure not only increases the amount of CH4 precipitation but also increases the amount of N2 precipitation, which is not conducive to the later utilization of the precipitated CH4. With the increase of N2 injection pressure, the sensitivity of CH4 displacement efficiency by uniform pressure N2 injection decreases, the sensitivity of CH4 displacement efficiency by pulse pressure N2 injection remains the same, and the sensitivity of CH4 displacement ratio by uniform pressure/pulse pressure N2 injection decreases. The CH4 prevent and control effect, N2 injection cost, and CH4 resource utilization of pulse pressure N2 injection are better than that of uniform pressure N2 injection. The technology of enhanced gas extraction by pulse pressure N2 injection not only improves the CH4 prevent and control effect but also reduces N2 injection cost. At the same time, it promotes the efficient utilization of CH4 resources. The research results have great significance and value for improving the level of gas disaster prevention and resource development.