Study on desorption and diffusion dynamics of coal reservoir through step-by-step depressurization simulation-an experimental simulation study based on LF-NMR technology

摘要

Pressure dropping control is one of the key technologies for coalbed methane surface development. Low-field nuclear magnetic resonance (LF-NMR) technology was adopted to study methane diffusion and pore dynamics of various pressure drop procedures. Calibration, isothermal desorption, pressure step-down desorption, and pore dynamic experiments were conducted with high-rank coal samples. The results include the following four aspects. 1) Calibration experiments of methane content and LF-NMR T-2 spectrum in confined cylindrical coal samples show that adsorbed, porous medium-confined and bulk methane can be distinguished and quantitatively calculated using LF-NMR technology. 2) According to the desorption equation fitting using LF-NMR technology, ultimate desorption volume at different stages of depressurization is calculated and the overall desorption process has been divided into inefficiency, slow, rapid, and sensitive stage. 3) LF-NMR desorption experiments with constant confine pressure show that cumulative desorption volume of two and three step-down pressure reductions are higher than that of one-step and uniform pressure reductions. Synchronisation of rapid or sensitive desorption stage and large effective diffusion coefficient (D-e) are keys to achieve a high cumulative gas production. 4) D-e varies significantly along with the variation of equilibrium gas pressure at the inefficiency and slow desorption stage, which is mainly affected by the surface coverage. Stress compression and matrix shrinkage coupling influence the pore structure at rapid and sensitive stage, thereby on D-e. The different desorption results using various depressurization schemes are the reason that pore deformation behaviour causing dynamic variation of the diffusion process. The above results can provide a basis for optimising the drainage work of coalbed methane.