Study on Change Rules of Factors Affecting Gas Loss during Coalbed Air Reverse Circulation Sampling

Abstract

The gas loss in sampling is the root of coalbed gas content measurement error. The pressure and particle size have a significant impact on the gas loss. Using the self-developed coal particle pneumatic pipeline transportation experimental system, this study investigated the pressure and particle size changes in the sampling pipeline. It is found that the sampling process can be divided into four stages: no flow field stage, sample outburst stage, stable conveying stage, and tail purging stage. The extreme pressure in the sampling pipeline appears at the sample outburst stage; and the pressure in the pipeline has levelled off after sharp decrease in the stable conveying stage. It is also found that the extreme pressure increases first and then decreases with the increase of particle size. The duration of outburst stage is negatively correlated with particle size, and that of stable conveying stage is positively correlated with particle size. In addition, the results show that the loss rate of 1–3 mm particles is the smallest after the test but that particles less than 1 mm increase by about two times and particles greater than 3 mm decrease by more than three times. The study also shows that the particle size distribution of coal samples is a single peak with left skew distribution, and the gas reverse circulation sampling test does not change the location of the peak but makes it higher and sharper. The single size coal sample is more likely to collide than the mixture. This study can help to advance the understanding of impact factors on gas loss during reverse circulation sampling.

Mechanism of Gas Desorption in Coal Particles under Different Positive Pressures: Experiments and Numerical Model

“…This method can not only be used for hard rock drilling and taking while drilling but also has the advantages of fast sampling, high precision, cleanness, and no pollution [13]. However, in the sampling process, the coal sample is in a positive pressure environment, and during the transportation of the coal samples, the coal samples collide, resulting in a reduction in the particle size [14].…”

“…However, in the sampling process, the coal sample is in a positive pressure environment, and during the transportation of the coal samples, the coal samples collide, resulting in a reduction in the particle size [14].…”

Experimental Study on Gas Loss Derived from Positive Pressure

“…The sampling method has a significant impact on the loss determination results . The larger the particle size is, the smaller the loss is . The traditional loss calculation method is based on the law of gas desorption under normal pressure, which is contradictory to the positive pressure sampling environment in engineering applications, and hence, this method is prone to leading to excessive error in both theory and practical applications .…”

“… 8 The traditional loss calculation method is based on the law of gas desorption under normal pressure, 9 which is contradictory to the positive pressure sampling environment in engineering applications, and hence, this method is prone to leading to excessive error in both theory and practical applications. 8 When sampling in a positive pressure environment, we drill to the predetermined sampling location, then use the drill bit to grind the coal into pulverized coal in situ, and the coal sample is quickly discharged from the drill bit through the drill pipe under the action of compressed air, with finally the discharged pulverized coal being collected into the coal sample tank. This method has the advantages of a faster sampling speed, more accurate sampling location, and shorter measurement time.…”