Study on gas desorption law and gas loss estimation in a positive pressure reverse circulation sampling process

Abstract

The accurate estimation of gas loss is a difficult problem in direct methods for measuring coalbed gas content. When using positive pressure reverse circulation (PPRC) technology to determine gas content, estimating the gas loss of coal samples under positive pressure based on the desorption law under atmospheric pressure may lead to significant errors. To study the gas desorption law in granular coal during PPRC sampling, positive pressure (PP) and positive pressure to atmospheric pressure (PP-AP) gas desorption experiments were conducted, and the gas diffusion kinetic parameters were analysed. The results showed that when the desorption pressure changed from PP to AP, the gas desorption capacity suddenly increased. Under a PP condition, the initial gas desorption rate decreased with increasing desorption PP, which could be expressed by a power function. While the AP initial gas desorption rate increased. Compared to the AP condition, the larger mass transfer resistance Bi and the smaller mass transfer Fourier number F0 in the PP experiments indicated that PP limited the migration of gas in coal particles, resulting in a smaller gas diffusion coefficient. The t model, power model, and logarithmic model were used to analyse the gas desorption data, confirming that existing methods for estimating gas loss have significant errors, and the power model can better describe the PP desorption process. Finally, a device was proposed to avoid pressure changes during sampling and underground desorption analysis. It was verified that using this device, the estimation error in gas loss could be reduced to within 10%. These results can provide guidance for further improving PPRC sampling technology.