Quantification of Gas Transport Behavior During Coalbed Methane Extraction in A Coal Seam Considering a Dual-Porosity/Single-Permeability Model

Due to the inherent deposition environment and extraction challenges in a coal seam, quantification of its gas transport behavior serves as the prerequisite to predict and evaluate accurately dynamic coalbed methane (CBM) recovery. In this work, a dual-porosity/single-permeability (DPSP) model is proposed for evaluating the gaseous methane flow performance by considering both the free gas density gradient and pressure gradient. More specifically, methane extraction dynamics was monitored via on-site boreholes with different spacings, and the two key gas flow parameters (i.e., diffusion coefficient and permeability coefficient) were determined and validated by minimizing the deviations between the field measurements and the simulated production profiles. Sensitivity analysis was conducted to examine the effect of both gas flow rate in a borehole and gas mass transfer flux from coal matrix to fractures on the production performance. The gas flow rate was found to be very sensitive to the permeability coefficient at the early extraction stage and then dominated mainly by the diffusion coefficient during the later extraction stage. The gas mass transfer flux from the matrix subsystem to the fracture subsystem exhibited distinct peak features due to the hysteresis of gas desorption in coal matrix. To extract methane in a coal seam, gas pressure in the matrix subsystem decreases much less than that in the fracture subsystem. Compared with the traditional single-porosity/single-permeability model, the newly proposed DPSP model allowed us to correct the overestimated CBM production and identify the underlying gas flow mechanisms. By introducing a critical extraction radius (CER) during a CBM extraction process, a theoretical equation associated with the CER, original methane content in a coal seam and methane extraction time was eventually formulated so that the CER and borehole spacing in some coal seam areas with different methane contents can be determined in a convenient and accurate manner.