A Method for Evaluating the Methane Content of the Nanoscale Pores in Deep Coalbeds

Since the sampling depth is large in deep coalbed methane wells, during the lifting process of coalbed cores, the core surface pressure drops nonlinearly with time, which is contradictory to the premise of the conventional United States Bureau of Mines (USBM) method and the Smith-Williams method. In this paper, a desorption-diffusion model was established to quantitatively characterize the actual escape process of methane gas from nanoscale pores in coal cores in both the wellbore and desorption tank by considering the nonlinear relationship between the core surface pressure and time. Based on the optimization method, the measured volume of the desorbed gas in the desorption tank was fitted, and then, the amount of lost gas in the wellbore was inferred. The calculation result of the USBM method was smaller than that of the method used in this paper. In the calculation model of lost gas volume proposed in this paper, the lost gas time was corrected, and the non-uniform decreasing characteristics of the core surface pressure were considered. Therefore, the lost gas obtained by this model was more accurate than that obtained by the conventional method.