Research program on fractured petroleum reservoirs. Second quarterly, April 1--June 30, 1995

Abstract

Very large compositional variation both areally and vertically has been observed in some hydrocarbon reservoirs. Several mechanisms are believed to contribute to such variations: gravitational segregation, molecular diffusion, thermal diffusion, and thermal convection. At isothermal conditions only gravitational segregation and molecular diffusion contribute to vertical compositional grading. The Gibbs segregation concept can properly account for this process. Under nonisothermal conditions, which is often the case, the process is thermodynamically irreversible and therefore Gibbs criteria of equilibrium cannot be invoked. The current literature often combines the Gibbs segregation concept and the natural convection process to formulate the interaction of convection and gravity segregation for multicomponent systems at nonisothermal conditions. The Dary law is also used without the modification of the velocity weighing for multicomponent systems. Such a formulation may not describe the process properly. This report formulates compositional variation in hydrocarbon reservoirs at nonisothermal condition. Results for the special case of gravity and thermal diffusion are also presented.