Dynamic Evolution of Coal Reservoir Parameters in CBM Extraction by Parallel Boreholes Along Coal Seam

Coalbed methane (CBM) is increasingly receiving attention as a relatively clean-burning energy source, and many experimental studies have been conducted to enhance CBM production. The specimen dimensions used previously have generally been too small to install boreholes or sensors, and small-scale samples usually cannot avoid the seepage field error induced by the boundary problem. In this study, a novel piece of large-scale, multi-field coupling equipment was designed to closely approximate the real environment for CBM extraction by parallel boreholes along coal seam. The dimensions of the specimen were 1050?×?400?×?400 mm³. In the study, 4 boreholes, 40 gas pressure sensors, and 14 temperature sensors were installed inside the specimen in total. According to the experimental findings, the dynamic evolution of the gas flow rate, gas pressure, and temperature shows similar variation trends, dropping sharply at first and then leveling off. Distance from the gas pressure sensors to the boreholes is the key factor influencing gas pressure (i.e., the shorter the distance, the faster the gas pressure drop). Gas pressure isobars in the Z3 plane, which is perpendicular to the borehole, show a circular distribution with the four boreholes at the center. The permeability declines more quickly closer to the borehole during the initial stage of CBM extraction and rebounds positively correlated with its proximity to the borehole later. At the end of the experiment, the cumulative flow rate of the four branch boreholes is 634.9, 609.3, 611.0, and 594.7 L, which accounts for 79.3% of the total quantity injected (3088.2 L). The second-order exponential decay function fits the cumulative flow rate well and may be used to predict the gas production in the future.