The current direct method of coalbed methane (CBM) content determination may underestimate the CBM content due to the negligence of lost gas in the borehole drilling stage of coal sampling, thus causing inaccurate evaluation of CBM reserves. In this work, considering the gas flow in coal body around the borehole before the coal sample is drilled down and exposed to air, the advanced lost gas in the borehole drilling stage of coal sampling is quantitatively calculated. First, a dual-porosity model coupling coal deformation and gas flow in the coal matrix and fractures is proposed to describe the dynamical behavior of gas flow in the borehole drilling stage. The field logging data in previous references are used to verify the dual-porosity model. Then, the variations of advanced lost gas as well as gas pressure at the sampling location with the drilling depth and drilling time are numerically simulated and discussed in detail. Furthermore, the effects of model parameters such as permeability, drilling velocity and borehole diameter on the advanced lost gas are investigated. The results show that the advanced lost gas accounts for about 10% of the original CBM content. The relationship between the advanced lost gas and the drilling time can be well fitted by the exponential function. It is revealed that over 90% of advanced lost gas is lost within the last 5% of the drilling time. The borehole diameter cannot affect the drilling depth where the gas begins to lose, but largely affects the amount of the advanced lost gas. Finally, an improved method of CBM content determination is conceptualized, which regards the CBM content as the sum of the advanced lost gas in the borehole drilling stage, the lost gas in the sampling stage, the measured gas and the residual gas. The results contribute to improving the accuracy of CBM content determination.

目前煤层气直接法(CBM)含量测定可能由于煤样钻孔钻井阶段漏失气的疏忽而低估了CBM含量，从而导致CBM储量评价不准确。本工作考虑煤样下钻并暴露在空气中之前钻孔周围煤体的气体流动，定量计算煤样钻孔钻孔阶段的超前漏失瓦斯。首先，提出了一种耦合煤基体和裂缝中的煤变形和气体流动的双孔隙度模型来描述钻孔阶段气体流动的动力学行为。以前参考文献中的现场测井数据用于验证双孔隙度模型。然后，数值模拟并详细讨论了先进漏失气体以及采样位置气体压力随钻进深度和钻进时间的变化规律。此外，还研究了渗透率、钻井速度和井眼直径等模型参数对先进漏失气的影响。结果表明，先进损失气约占原始煤层气含量的10%。先进漏失气与钻井时间的关系可以用指数函数很好地拟合。研究表明，超过 90% 的先进漏失气在钻井时间的最后 5% 内损失。钻孔直径不能影响瓦斯开始流失的钻孔深度，但在很大程度上影响提前流失的瓦斯量。最后，概念化了一种改进的 CBM 含量确定方法，残余气体。该结果有助于提高煤层气含量测定的准确性。