The Mechanism Research Of Methane Displacement By Carbon Dioxide

Adsorption, desorption and displacement of shale is studied by many scholars and the corresponding results were obtained, these researches have a great practical value for the development of shale gas in the future. However, the samples used in the experiments about adsorption, desorption and displacement of shale were granular shale, which damage the original structure of shale seriously, so the samples were unable to reflect the real reservoir conditions truly. Supported by the National Natural Science Foundation of China(Grant No. 51374257) and the New Century Excellent Talent Foundation from MOE of China(Grant No. NCET-09-0844), in order to reflect the property of adsorption, desorption and displacement of shale in the real reservoir conditions truly, the experiments in this paper were conducted with a diameter of 50 mm and high 100 mm shale on which applied certain stress. Compared with granular shale, the interior of shale sample kept original geological structure can simulate the real reservoir conditions. The researches were carried out about part of influence factors of adsorption, desorption and displacement, the main research results as follow:①The experiment samples are cylindrical, bulk shale core, the interior of shale samples keep original geological structure, a certain axial pressure and confining pressure are applied on the cores and the way of constant temperature is water bath in the test to simulate the corresponding shale reservoir environment and ensure the test can more truly reflect the adsorption, desorption, displacement properties of shale reservoir.②Based on a large number of indoor experiments of adsorption process of shale core, a detailed analysis was done about migration, adsorption process of methane and carbon dioxide in shale core, and the adsorption process was considered as a integrated process of penetration, diffusion, adsorption, desorption; the reasons were explained about the decline of adsorption rate with the passage of time. Mainly, the early stage of the adsorption is given priority to infiltration, adsorption, and late is given priority to diffusion, adsorption, the infiltration effect of gas was much faster than diffusion effect in shale reservoir; the “step”, “negative adsorption” phenomenon in the adsorption process curves were analyzed in detail, and this phenomenon is caused by the internal pore structure changes with the adsorption quantity increases.③With the methane isothermal adsorption curves comparison of different temperature, different shale core, found that these curves have the same change trend, the adsorption quantity increases with pressure, and the sharp rise phenomenon of in the adsorption quantity appear on the corresponding pressure point, and the pressure point increases with temperature; By the comparison and analysis of adsorption, desorption curves of the shale cores, found that the desorption curves of core lag the adsorption curves, and this phenomenon are analyzed in detail from the three aspects of the forms of occurrence of methane in shale reservoir, molecular energy and molecular adsorption form.④In order to explore the influence of temperature on the adsorption, the comparison and analysis of isothermal adsorption curves was made with the same shale sample under different temperatures, a poor correlation between temperature and the adsorption quantity was found, and a detailed analysis of the phenomenon was made, the main reason leads to the adsorption quantity change was internal structure change of shale. It indicated that shale core adsorption properties were influenced by many factors, compared with granular, adsorption properties were more instability.⑤Based on the comparison and analysis of the isothermal adsorption curves of different shale samples at the same temperature, the influence of shale reservoir anisotropy on adsorption was explored and It found that adsorption was different on different samples; With the comparison and analysis of different gas(CH4 and CO2) adsorption experiment on the same samples, at the same time, the CO2 adsorption capacity was more than CH4, and CH4 adsorption equilibrium pressure is greater than CO2, It indicated that the adsorption of CO2 was stronger than CH4 in shale core.⑥The shale core adsorption experiment results will be analyzed and compared with the adsorption results of coal particles, shale particles and small specimens at the same temperature, and the reasons of difference on adsorption quantity and adsorption curve form are analyzed in detail from the aspects of rock structure, and It shows that specimen size influenced adsorption capacity largely. With the increase of specimen size, the nonuniformity of shale reservoir will be more obvious, and it can simulate the real reservoir conditions better.⑦the adsorption desorption experiment data was fitted with Langmuir model, Freundlich formula, BET model, D-R model, L-F model and D-A model to explore the adsorption mechanism of methane in shale core, the fitting results show that the average correlation coefficient of Freundlich formula is 0.95635; the average correlation coefficient of D-R model is 0.96559; the average correlation coefficient of D-A model is 0.97963, the fitting effect of above three kinds of model is good, however the physical significance of parameters in Freundlich formula is not clear and can’t represent adsorption mechanism, thus methane adsorption mechanism in the shale core is mainly pore filling or volume filling.⑧Based on the displacement experiment data of shale core, the displacement mechanism was analyzed and elaborated in detail, It was considered that displacement process was comprehensive process of penetration, diffusion and competitive adsorption; The displacement results were compared in the different temperatures and displacement modes, and It found that temperature, displacement mode influenced the displacement effect, and the displacement modes had a stronger effect; Analyzed and compared with the displacement results under different volume stress, the volume stress showed almost no effect on displacement; Discuss the calculation of replacement ratio, and expound the new calculation method which can make the parameter more accurate to represent CO2 displacement ability.