EXPERIMENTAL INVESTIGATION ON VARIATION OF GEOCHEMICAL COMPOSITIONS OF NATURAL GAS DURING DIFFUSION UNDER STRATIGRAPHIC CONDITION

In order to study the geochemical characteristics of natural gas during diffusion,an experimental simulation under stratigraphic condition (70 ℃,8 MPa) has been performed on a solid sample.During the diffusion of natural gas,the relative percentage of methane in reservoir gas gradually decreases,but the relative percentages of ethane and propane increase correspondingly.The diffusive gas shows the same compositional varying tendency as the reservoir gas,but the relative percentage of methane in diffusive gas is higher than that in reservoir gas at the same time,and the relative percentages of ethane and propane in diffusive gas are lower than those in reservoir gas.These phenomena are all caused by stratigraphic chromatographic effect.The relative percentage of normal butane in reservoir gas is higher than that in diffusive gas at the same time,and the relative percentage of isobutane in reservoir gas is lower than that in diffusive gas.This is also the result of stratigraphic chromatographic effect.The C₁/(C₂+C₃) and iC₄/nC₄ values may be used as the geochemical indexes of natural gas migration.There exist a logarithmic linear correlative relation between the relative percentage of compositions of reservoir gas and the diffusive gas percentage and a quadratic relation between the relative percentage of compositions of diffusive gas and the diffusive gas percentage.These relations may be helpful for calculating the amount of diffusive gas,evaluating the reservoir gas resource prospect,deeply understanding into the variation of compositions of natural gas during diffusion and predicting the total resources of natural gas derived from the same source rock but preserved in reservoirs with different ages.