Experimental Research On The Reaction Of Coal Reservoir And Gases

Differential scanning calorimetry （DSC） experiments were carried out in the atmosphere of He, N₂ and CO₂ with coal samples from Huozhou, Changzhi and Jincheng area. The curves in the He atmosphere are consist with the base curves, indicating no adsorption occurs, hence no enthalpy appears. The curves in the N₂ atmosphere are smooth and upper compared with the curves in the He atmosphere, which indicated that physical adsorption occurs, charactered with low enthalpy. However, in the CO₂ atmosphere, exothermic peaks and endothermic peaks with different shapes occur in the curves. It is proposed that the lowest unoccupied molecular orbital of CO2 accepted electron from coal molecular, and formed an electron donor-acceptor complex （i.e., an "EDA complex"）, in other words, chemical reaction occurs, hence exothermic peaks. The formation of EDA complex lead to lower system energy and higher stabilization, which weakened the interconnection to other groups in coal molecular, so easily to be broken, and the endothermic peaks occur.The swelling of coals in CO₂, CH₄ and N₂ were measured by an optical method. It was founded that the maximum swelling values appear around 10MPa, and swelling in the plane perpendicular to the bedding plane was always substantially higher than that in the parallel plane, which was due to the preferential orientation of cross-link in coal’s macromolecular network. Sorption-induced maximum swelling increased one by one for CO₂, CH₄ and N₂. It is proposed that only physical interaction occurs for CH₄ and N₂, however, both physical interaction and chemical interaction occur for CO₂, which result in more serious destroy for the interconnection among coal network, hence larger swelling. Though both CH₄ and N₂ are nonpolar, the C-H bonding in CH₄ is polar, leading to stronger interaction with coal and larger swelling.Isotherm adsorption experiments for CO₂ were conducted with dry and as received samples. It indicated that the adsorption amount are larger for dry samples, and the difference increased positively with coal rank. It is because that the water molecular in micropore of high-rank coal is difficult to be removed and has more impact on adsorption. With the correction by considering coal swelling and adsorption phase, the real adsorption amount were obtained, which are 18.42～21.02% higher than that of the measured values.Based on the comparison of the field experience and laboratory studies for CO₂ injection, it was proposed that the transporting model of CO₂ in coal reservoir is similar to multi-wave, i.e., lots of intensed zones of CO₂ with higher pressure. Towards the production well, the wave disappeared, duing to the CO₂ dissolving in water and readsorbing on coal, and the pressure transferred to the force that driving water towards the production well. As the process continues, the transporting of gas became free, so the injection gas will enhance in the late phase.