Gas Loss Compensation Method for Prolonged Exposure Samples from Deep Boreholes: Experiment, Theory, and Numerical...

Coal seam gas content is a crucial parameter for ensuring coal mine safety and accurately assessing coalbed methane reserves. With the increasing demand for advancing gas control in mining faces, the depth of boreholes for gas content determination continues to extend, making it difficult to guarantee short sampling times. Consequently, existing gas loss compensation methods struggle to apply to prolonged exposure loss estimation. To address the limitations of current methods, which rely heavily on initial desorption data and are inadequate for long-duration loss estimation, a transient gas diffusion equation incorporating gas adsorption effects was established. The relationship between the dimensionless time number T and the dimensionless desorption quantity number Y was analyzed. Furthermore, novel loss compensation methods based on the tangent at the initial measurement point and the tangent at a dynamic point were proposed. The results indicate that the developed diffusion model can effectively characterize long-term gas diffusion behavior. The T-Y relationship conforms to a power-law function, but its exponent varies with the selected time interval; fixing the exponent leads to significant errors. Directly extrapolating the loss amount by fitting field desorption data to a power function introduces substantial inaccuracies. Therefore, the tangent at the initial point of the field desorption curve was used to substitute for the desorption curve during the exposure period for loss estimation (i.e., the Initial Measurement Point Tangent Method, CQSB). Comparative analysis revealed that the calculation error increases with longer coal sample exposure time, making this method suitable only for scenarios with short exposure times (less than 2 min). An improved method was proposed, which involves using the fitted power function to predict the slope at a dynamic point within the exposure period and then utilizing the tangent at this point to calculate the gas loss (i.e., the Dynamic Point Tangent Method, DQSB). This method enables accurate determination of gas loss under prolonged exposure conditions (3-30 min). This research outcome provides a new approach for estimating gas loss during deep hole drilling and sampling processes using either open or sealed core barrels. However, how to theoretically determine the location of the dynamic point without relying on experimental data, and whether the method proposed in this paper is applicable to lump coal samples containing fractures, require further research and experimental verification.