Effects of Various Depressurization Paths on Desorption Deformation and Gas Production of Medium-Rank Coal in Qinshui Basin

Abstract

A combination of physical and numerical simulations is employed to compare the differences in desorption deformation and desorption volumes of coal samples under varying depressurization paths, aiming to understand their impact on coalbed methane (CBM) extraction. In this work, two medium-rank coal samples from the central-eastern region of the Qinshui Basin were chosen for the desorption–strain experiments. The experiment facilitated real-time observation of desorption gas volumes and coal matrix deformation under various depressurization paths. Finite element analysis was utilized to model and analyze the evolution of pore pressure during depressurization and desorption. The research outcomes indicate a dependency of desorption gas volumes on the chosen depressurization path. With the slow depressurization path, the desorption gas volume over 12 h was 8% higher than that achieved with the rapid depressurization path. When the pressure difference across the pores fell below the pressure difference required for gas migration, the gas cannot overcome the resistance, leading to residual gas being trapped in the pores. With the slow depressurization path, the coal matrix exhibited notably lower residual pore pressure and remaining gas volume compared to the rapid depressurization path. The differences in desorption volumes under various depressurization paths were mainly driven by the pore structure and matrix strain. Rapid depressurization led to pore contraction, which decreased pore size and connectivity, increasing resistance to gas migration and decreasing absorption rates. Conversely, the slow depressurization path led to a more gradual pore contraction and minimal strain, supporting the continuous production of CBM.