Compositional grading by depth in gas and oil fields with temperature gradient

Compositional grading (CG) is to determine changes by depth of the pressure and concentration of the hydrocarbon components (the mixture composition) in equilibrium conditions. This problem is related to the construction of PVT models of formation hydrocarbon mixtures required for subsequent hydrodynamic calculations of underground gas and oil fields. The influence of both gravitation and the vertical temperature gradient is investigated for a number of well-known models of thermal diffusion with zero mass transfer (Haase, Kempers, Firoozabadi et al.). Some disadvantages of the models are shown. A conservative algorithm for compositional grading by depth is constructed corresponding to these models. An overview of methods for calculating the enthalpy of components (absolute and residual) contained in formulas for thermo diffusion is given. Some examples of the numerical calculations are presented.