The calculated retention time was then defined as the sum of the time spend in part 1 (tR1) and the time spend in part 2 (tR2). So the improved model was built considering methane as a retained compound with a retention factor which was also dependent on the zone in the GC instrument due to the temperature difference in these two zones. AC1, BC1, AC2, BC2, and permeability B0 were obtained using Excel’s solver minimizing the sum of the squares of the residues between experimental and calculated values (Eqs. (6), (1) and (9))