Determination of Diffusion Coefficients of Ethane in Water at High Pressure and Temperature with In-Situ Raman Spectroscopy

Ethane and methane are two main components of natural gas. Accurate diffusion coefficients at high temperature for ethane and methane are the key to calculate the diffusion and fractionation of gases in deep natural gas reservoirs, especially in unfractured shale and other geological materials with low permeability. However, the diffusion coefficients for ethane at pressures and temperatures near the reservoir conditions are scarce in the literature. In this study, diffusive transfer of ethane in water at 20MPa and from 273 to 393K was observed in a high-pressure optical capillary cell via Raman spectroscopy. Diffusion coefficients were then determined by the least-square method. The relationship between diffusion coefficients [D(C₂H₆) in m²/s] and temperature (T in K) was derived with Speedy-Angell power-law equation as: D(C₂H₆)=D₀[(T/T_s)-1]^γ, where D₀=13.8055×10^-9m²/s, T_s=237.4K, γ=1.7397. The diffusion coefficients of ethane are much smaller than those of methane at the same condition. The amount of methane and ethane diffused through some thick low permeability layers are calculated and the results show that such diffusivity difference can cause a significant fractionation of methane and ethane in thick shale layer.