一种适用于深部煤岩的甲烷损失气量计算方法

A method for calculating lost methane gas in deep coal rocks

针对煤岩在井筒提升过程及解吸罐中的甲烷解吸扩散规律问题,前人主要采用定常数外边界条件进行构建模型,这将与深部煤岩提升过程中甲烷实际扩散特征差异较大。由于煤岩在井筒实际提升过程中其外表面压力随时间呈非线性变化,考虑该变化特征,并基于质量守恒定律、连续性原理,构建了甲烷定常数扩散系数的解吸扩散数学模型,通过数学物理方程求解方法求得其解析解。在此基础上,利用最优化方法,对解吸罐中的实测解吸气量进行拟合,进而推测出煤岩在井筒中的损失气量。结果表明:①所建立数学模型更符合深部煤岩提升过程中甲烷的实际扩散特征;②与直接法(USBM法)相比,所提方法的计算结果更加准确。以某一具体的煤层岩样为例,用直接法求得煤岩在井筒提升过程中的甲烷损失气量为2712 cm^(3),文中求解方法得到的甲烷损失气量为2951 cm^(3)。这与直接法使用的前提条件(t0设定)导致的实际损失气量时间减少相吻合。

To investigate the methane desorption and diffusion in coal rocks during lifting and in desorption tank,previous study often made modeling under constant outer boundary conditions.The modeled results are quite different from the actual diffusion behaviors during the lifting from deep coal rocks.In the process of lifting,the pressure on the outer surface of coal rocks presents nonlinear variation with time.So,based on mass-conservation law and continuity principle,a new mathematical model on desorption and diffusion for constant diffusion coefficient was built,and an analytical solution was obtained by solving mathematical and physical equations.Then,the optimization method was used to fit the measured gas content desorbed in the desorption tank,and the lost gas in coal rocks at wellbore was estimated.Results show that(1)this model is more consistent with the actual diffusion characteristics during lifting from deep coal rocks;and(2)the proposed method provides more accurate results than those from the direct method of the United States Bureau of Mines(USBM method).For a selected coal sample,the lost methane gas is 2,712 cm^(3) estimated from USBM method,whereas 2,951 cm^(3) from the built method in this paper,which may be caused by the reduction in actual gas loss time inferred from the precondition(t0)of USBM method.