A Novel Method for Calculating Diffusion Coefficient of Shale Gas Reservoirs: A Case Study of Longmaxi Formation in Weiyuan Area, Sichuan Basin, China

Abstract

The entire process of shale gas generation, migration, and accumulation involves the diffusion of shale gas, and it is impossible to disregard the harm that gas diffusion does to gas reservoirs. The research object for this paper is the Longmaxi Formation shale gas reservoir in the Weiyuan area of the Sichuan Basin. Based on Fick’s diffusion law, an innovative mathematical model of shale gas diffusion is established, and it is clarified that the diffusion amount mainly depends on the free gas content and the diffusion coefficient. Based on the theory of fluid dynamics, the calculation equation of formation paleo-pressure is innovatively deduced. The equation fully considers gas migration, temperature and pressure changes, and the pressure control effect of organic matter gas generation, and restores the evolution history of free gas content in the reservoir. The evolution process of temperature, pressure and stratigraphic physical properties in the study area is the first to calculate the diffusion evolution history and cumulative diffusion amount of shale gas reservoirs in the study area, the reliability of the calculation results is verified by geochemical parameters. Studies have revealed that the existing Longmaxi Formation shale in the Weiyuan area of Sichuan Basin varies from 14.10 to 16.50 × 104 m3/m2 per unit area, with an average diffusion loss of 0.30 × 1012 m3 gas in the positive part and 0.30 × 1012 m3 in the negative part. The total lost gas accounts for around 1.72 times the present recappable reserves and is estimated to be 0.43 × 1012 m3 in volume. It is clear that the migration and accumulation of natural gas are significantly influenced by the research of diffusion loss.