Experimental study on the negative pressure loss generated by the gas influx process around a long borehole

Abstract

In the process of gas extraction in long boreholes, gas influx in the radial direction causes a non-uniform variable mass flow field due to bore wall friction. This flow field can lead to a negative pressure decay along the hole length, thus affecting the gas extraction effect. With this effect in mind, a gas variation mass test was designed for pore-containing specimens with different pore wall roughnesses. It is also based on the variable mass flow pressure drop theory and analyzes the pressure variation, hole wall roughness, and gas influx rate inside the borehole. The result indicates that radial influx of gas along the borehole wall can interfere with the main flow in the borehole. The increase of the radial influx flow rate of the gas produces an obvious swirling phenomenon, which gradually makes the fluid mixing flow situation more and more complicated the more it differs from the mainstream velocity. The mainstream velocity and the pore wall influx velocity are the main factors causing mixing loss. The study results are important for understanding the long borehole gas sink process and the design of reasonable borehole parameters.