New insights into natural gas diffusion coefficient in rocks

Based on the analysis of measuring methods and results of natural gas diffusion coefficient in rocks and the measurement of methane diffusion coefficient in the Upper Paleozoic rocks of the Ordos basin, this paper clarifies the meaning of natural gas diffusion coefficient in rocks and discusses its application in gas diffusion simulation and some influencing factors. There are three methods for measuring natural gas diffusion coefficient in rocks, including free gas method, water-dissolved method, and time delay method. The meanings of diffusive concentration and the influencing factors are different for the natural gas diffusion coefficients measured by these three methods. The diffusion coefficient measured by the free gas method is 1 to 2 order of magnitude less than that measured by the water-dissolved method, and 2 to 3 order of magnitude less than that measured by the time delay method. The meanings of diffusive concentration in the diffusive volume calculation must be in line with that in the diffusion coefficient measurement during the process of diffusive volume calculation. Natural gas diffusion coefficient in rocks has a linear relationship with porosity and a dynamic relationship with temperature, and it logarithmically decreases with pressure.