Abstract:

At present, China’s shale gas is gradually began to change from exploration breakthrough to development and production. The rate of decline of shale gas production mainly depends on the desorption of the adsorbed gas in shale. Therefore, it is of great significance to study the rule of methane desorption in shale for the development of shale gas. In order to study the isothermal desorption characteristics of methane in shale from Longmaxi Formation in South Sichuan Basin, the high-pressure isothermal adsorption and desorption experiments have been carried out with the shale of this formation as the research object. On this basis, the optimized model of methane desorption and the calculated desorption efficiency curve of methane in shale has been used to analyze the desorption characteristics of shale from Longmaxi Formation. The results show that the fitting effects of five models have little difference with low experimental pressure（less than 12 MPa）. While under high pressure, Weibull model and D-A model can describe the methane desorption process better than other models. Based on the experimental test results, the model fitting effect is as follows: Weibull model>D-A model>Desorption model>Langmuir model>Freundlich model. With the decrease of pressure, the shale methane desorption efficiency obtained by Weibull model with the best fitting accuracy tends to increase slowly at first and then rapidly. According to the characteristic pressure points, the methane desorption process is divided into four stages. The methane desorption amount in the rapid desorption stage accounted for 40 % of the total desorption amount, which contributes the most to shale gas production capacity.

Key words: shale gas, desorption model, desorption rule, model optimization, desorption efficiency