An attractive alternative of sequestering CO 2 is to inject it into coalbed methane reservoirs, particularly since it has been shown to enhance the production of methane during near depletion stages. The basis for enhanced coalbed methane recovery and simultaneous sequestration of carbon dioxide in deep coals is the preferential sorption property of coal, with its affinity for carbon dioxide being significantly higher than that for methane. Yet, the sorption behavior of coal under competitive sorptive environment is not fully understood. Hence, the original objective of this research study was to carry out a laboratory study to investigate the effect of studying the sorption behavior of coal in the presence of multiple gases, primarily methane, CO 2 and nitrogen, in order to understand the mechanisms involved in displacement of methane and its movement in coal. This had to be modified slightly since the PVT property of gas mixtures is still not well understood, and any laboratory work in the area of sorption of gases requires a definite equation of state to calculate the volumes of different gases in free and adsorbed forms.This research study started with establishing gas adsorption isotherms for pure methane and CO 2 . The standard gas expansion technique based on volumetric analysis was used for the experimental work with the additional feature of incorporating a gas chromatograph for analysis of gas composition. The results were analyzed first using the Langmuir theory. As expected, the Langmuir analysis indicated that CO 2 is more than three times as sorptive as methane. This was followed by carrying out a partial desorption isotherm for methane, and then injecting CO 2 to displace methane. The results indicated that CO 2 injection at low pressure displaced all of the sorbed methane, even when the total pressure continued to be high. However, the displacement appeared to be occurring due to a combination of the preferential sorption property of coal and reduction in the partial pressure of methane.As a final step, the Extended Langmuir (EL) model was used to model the coalmethane-CO 2 binary adsorption system. The EL model was found to be very accurate in predicting adsorption of CO 2 , but not so in predicting desorption of methane. The selectivity of CO 2 over methane was calculated to be 4.3:1. This is, of course, not in very good agreement with the measured values which showed the ratio to be 3.5:1. However, the measured results are in good agreement with the field observation at one of the CO 2 injection sites.
The strain softening behavior of the rock-encased-backfill (RB) structure decides the stability of the underground backfilling stope during cyclic excavation and blasting. To investigate the cyclic failure behavior of the RB specimen, a series of triaxial cyclic loading experiments were conducted in different volume fractions of rock, confining pressures, and cyclic loading rates. Results show the strain hysteresis curve increases from sparse to dense with the increase of the confining pressure, showing a significant strain-softening characteristic. The elastic strain energy increases linearly and the dissipation energy increases exponentially with the number of cycles. The percentage of shear microcracks during each circle increased quadratically, which was very similar to the trend of dissipation energy. When VF=0.73 and the loading and unloading rate was 400 N/s, the percentage of shear microcracks in RB specimens was higher than that in other cases, especially higher under 9 MPa. This finding indicates that RB specimens are more prone to meso damage and elastic-plastic transformation under a specific condition, and the importance to conduct more studies on the coupling mechanism of the bi-geological-materials structure. Introduction Backfill is one kind of artificial and green material reserved in the underground, which is made with waste tailings after mining excavation. Its mechanical properties are similar to soil and cement, which is far away from that rock mass nearby. After the backfill is consolidated, the field stress will be redistributed in the rock-encased-backfill structure (RB).
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