Evaluation of lost gas in the borehole drilling stage: Implication for the direct method of coalbed methane content determination

Abstract

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.