Study Of Differential Swelling Of Coal/Gas System Via In Situ Optical Measurement

The energy structure is irrational at present in China. For one thing, taking coal as main energy leads to severe air pollution and greenhouse effect; in the next place, a kind of quality energy, coal-bed methane, has not been developed and used efficiently.The technology of ECBM, on the one side, it can not only get the purpose of less emission of greenhouse gas and keep its long-term sequestration in deep coal seams, but also can enhance the coal-bed methane recovery. Artfully, resolving two major problems of china, the environmental pollution and energy development; On the other side, swelling of coal matrix by the injection of CO₂ reduce the permeability of coal reservoirs, eventually, lead to lower recovery, and then weaken the feasibility of coal-bed methane recovery and CO₂ sequestration.In this paper, we quantitatively studied the swelling of coal matrix: the experiment was performed by using different gases, namely CO₂, CH4 and N2, and different rank of coals. Hope the results are helpful for the development of coal bed methane and CO₂ sequestration in deep coal seams. An apparatus capable of directly observing, by an optical method, swelling of coal in CO₂ and other g ases was described. The system was used to measure CO₂, CH4, N2-induced swelling in three different rank coals at temperatures 40℃and pressures up to 12MPa. The results were fitted by a modified Dubinin–Radushkevich model. Results showed:（1） The swelling relationship between all the three coals and the adsorbed gases were CO₂>CH4>N2;（2） The swelling of coals of different rank were different, Jincheng coal （Ro=2.89%） was the largest, Huozhou coal （Ro=0.85%） was the smallest;（3） All three coals showed similar behaviour, with swelling increasing as a function of pressure up to about 10 MPa, after which no further increase in swelling was observed;（4） Significant anisotropy was observed; swelling in the plane perpendicular to the bedding plane was always substantially higher than in the parallel plane.Correlation results showed that:Swelling could be well represented by a modified Dubinin– Radushkevich model, in which:（1） Jincheng coal: The correlation results of swelling in N2 was the best, the average absolute relative deviation was only 1.48%, the maximum relative deviation was 3.04%; swelling of coal in CO₂ was the worst, the average absolute relative deviation was 2.86%, the maximum relative deviation was 9.17%, swelling of coal in CH4 was the midst;（2） Changzhi coal: The correlation results of swelling in CH4 and N2 were negligible difference, the average absolute relative deviation were 2.29% and 2.6%, the maximum relative deviation were 5.95% and 5.63%; swelling of coal in CO₂ was the worst, the average absolute relative deviation was 2.80%, the maximum relative deviation was 9.27%;（3） Huozhou coal: The correlation results of swelling in CH4 and N2 were roughly the same, the average absolute relative deviation were 2.13%、and 2.30%, the maximum relative deviation were 4.47% and 3.66%; swelling of coal in CO₂ was the worst, the average absolute relative deviation was 3.62%, the maximum relative deviation was 13%;...