Gas desorption and diffusion characteristics in different rank coals under different pressure-drop conditions

Abstract

To compare gas desorption and diffusion characteristics under different pressure-drop conditions, different rank coals were collected to conduct desorption and diffusion experiments under the direct pressure-drop condition (DPDC) and the progressive pressure-drop condition (PPDC). Moreover, mercury injection capillary pressure (MICP) analysis was performed to determine pore structure characteristics. When the pressure-drop condition is changed from DPDC to PPDC, the initial desorption rate decreases, while the desorption volume and diffusion coefficient increase, suggesting the damage on desorption capacity and diffusion capacity resulted from DPDC. The maximum changes of these three parameters occur in the medium-volatile bituminous coal, which decrease from 0.255 to 0.063 cm3/(g·min), increase from 2.96 to 18.82 cm3/g and increase from 0.4732 × 10−10 to 3.8239 × 10−10 cm2/s, respectively. Micropores and transition pores predominate in all samples. With increasing coal rank, the pore volume generally shows a decreasing trend, whereas the fractal dimension shows an increasing trend. The difference in diffusion coefficient between DPDC and PPDC of different rank coals is predominated by micropores, which arises from the various diffusion regimes at different pressure ranges in micropores. Specifically, the rapid pressure-drop rate in DPDC causes the atmospheric pressure condition, and Knudsen diffusion and Transitional diffusion play dominant roles in micropores, resulting in the lower diffusion coefficient. The difference in desorption volume between DPDC and PPDC of different rank coals is predominated by transition pores, which results from the water block effect in transition pores. In DPDC, large bubbles easily form in transition pore throats and show large deformation when they pass through transition pore throats, which increases capillary resistance and intensifies the water block effect. However, in PPDC, the size of gas bubbles in the same transition pore throats is smaller than that in DPDC, which weakens the water block effect and enhances desorption capacity. This research can provide strategies for optimizing the pressure-drop rate during CBM production. PPDC is more suitable for CBM exploration. The drainage intensity should be reduced, or secondary reformation measures should be taken to reduce the proportions of micropores and transition pores in DPDC to reduce the damage of desorption capacity and diffusion capacity.