Published: May 12, 2011 r2011 American Chemical Society 2730 dx.doi.org/10.1021/ef200282q | Energy Fuels 2011, 25, 2730–2740 ARTICLE pubs.acs.org/EF Investigation into the Methane Displacement Behavior by Cyclic, Pure Carbon Dioxide Injection in Dry, Powdered, Bituminous Indian Coals Santanu Bhowmik and Pratik Dutta* Department of Mining Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, West Bengal, India ABSTRACT: Understanding competitive sorption effects for the carbon dioxideÀmethane system in coal is essential for the implementation of enhanced methane production with concomitant CO 2 sequestration in coal. The paper discusses the methane displacement behavior of a set of eight dry, powdered Indian bituminous coal samples when subjected to cyclic, pure CO 2 injection, employing a “huff and puff” scheme. The coal samples were partially saturated with methane, and CO 2 was then injected at a fixed pressure. This was followed by gas drainage to reduce pressure to the pre-injection level, and about 12À15 such cycles of CO 2 injection and gas drainage were carried out. In general, the process successfully displaced the adsorbed methane. The adsorption ratio of CO 2 /methane was found to be higher than pure gas sorption capacities at the same pressure. Carbon dioxide was preferentially adsorbed into the coals, and during gas drainage, preferential desorption of methane was observed for all coals. The coals also exhibited different methane displacement behavior. For three coal samples, it was possible to recover 1 mol of methane by injecting less than 1 mol of CO 2 . For the other coal samples, 1.5À2 mol of CO 2 was required for 1 mol of methane. However, no relationship could be established between the methane release characteristics of the coals and their petrographic composition. ’ INTRODUCTION Coalbed methane (CBM) reservoir is considered as a dual- porosity and single-permeability structure, where more than 95% methane is stored, in an adsorbed state, in the porous spaces of the coal matrix and the remaining gas is stored as free gas in the cleat system. 1 The total gas-in-place is governed by the depth, reservoir pressure, temperature, coal type, and coal rank. At a particular pressure, the reservoir may be undersaturated and can adsorb more gas at higher pressures. 2 Gas production from the CBM reservoir starts when the reservoir pressure falls below the critical desorption pressure after dewatering of the reservoir, and a significant amount of gas (up to ∼50% of the total gas-in-place) may still remain in the reservoir after the abandonment pressure is reached. 3À5 This limitation of gas production from the conventional CBM reservoir can be improved by N 2 or CO 2 injection, the process known as enhanced coalbed methane (ECBM) recovery. N 2 injection enhances methane recovery by lowering the partial pressure of methane in the cleats, which, in turn, results in further desorption of methane from the coal matrix. However, N 2 breakthrough at the production well occurs rapidly too, which creates problems by mixing with methane at the production well, requiring additional costs of gas processing for the removal of N 2 from the produced gas stream. 3 In CO 2 ECBM, CO 2 is injected into the coal reservoir, where it becomes preferentially adsorbed into the coal matrix with simultaneous desorption of methane into the free state for recovery. 3,6 CO 2 ECBM not only recovers additional methane but also provides a site for storage of a high volume of anthropogenic CO 2 into coal, which, otherwise, will be released to the atmosphere. It has been observed that, at a particular temperature and pressure, CO 2 has a much higher adsorption affinity to coal than methane and can become preferentially adsorbed into the coal. 4,5,7À13 The strong adsorption characteristics of CO 2 can result in rapid and complete displacement of adsorbed methane. The adsorption ratio of pure CO 2 /methane is generally 2:1, although varying ratios of as high as 10:1 to less than 2:1 have also been observed depending upon the coal composition and coal rank. 7À10,12 Therefore, in terms of pure gas sorption capacity, to displace 1 mol of methane, 2À10 mol of CO 2 has to be injected into the coal, with a higher displaced methane/injected CO 2 ratio and a lower operational cost of injection. 6 On the other hand, the process also offers the added benefit of sequestering the green- house gas into unminable coal for a geologically significant time period. 14 There are only a few field-based studies on CO 2 ECBM/CO 2 sequestration, and the number of laboratory studies is also limited. The world’s first large-scale ECBM pilot-scale study was conducted at the Allison Unit of the San Juan Basin in New Mexico, to investigate the feasibility of CO 2 sequestration in deep, unminable coal seams. 14,15 The Allison Unit was operated by Burlington Resources, in which CO 2 injection continued for 6 months after which CO 2 breakthrough was observed in the production well. This field test confirmed the feasibility of CO 2 sequestration and enhanced gas recovery, but the permeability of the reservoir reduced by 2 orders of magnitude because of coal swelling with CO 2 adsorption. In Europe, the first field experi- ment for CO 2 storage was conducted in EC, RECOPOL Project in the Upper Silesian Coal Basin of Poland. CO 2 injection started in 2004; however, continuous injection could not be achieved because of the reduction in permeability by coal swelling, and after 5 months, a slow rise in the CO 2 concentration in the production well was observed. However, the study indicated that, at higher injection pressure, negative effects for the reduction in the permeability because of coal swelling could be overcome. 16 Received: February 23, 2011 Revised: May 12, 2011