The Influence of Natural Convection In Gas, Oil And Water Reservoirs

Abstract Natural convection in a fluid layer is a phenomenon which has been the topic of numerous studies. A similar phenomenon has been observed in porous media. In the first part of this paper we review some recent theoretical and experimental studies which allow a better understanding of some quantitative aspects of natural convection in porous media. This review is confined to the case in which compressibility effects are negligible as compared to thermal expansion effects and where the porous layer is homogeneous. Some results are presented on natural convection in a horizontal layer with different types of boundary conditions and in a sloped layer bounded by isothermal planes. Some aspects of mixed convection are also discussed. The second part of the paper is concerned with practical cases. As a major example we give the description of natural convection occurring in the Lacq gas reservoir. We also present some examples of oil production operations where thermal connection may have an influence. These may be classified into two categories:convective motions already existing in a reservoir and having an influence on the handling of the recovery methods;convective motion induced by the recovery technique itself (thermal methods). Finally we discuss the case of thermal convection in aquifers, with special reference to the understanding of geothermal areas. INTRODUCTION NATURAL (or free) thermal convection in a fluid layer is a physical phenomenon which' has been the topic of numerous studies(l, 6, 11, 28). When in a fluid phase the vertical component of the temperature gradient has the same direction as gravity, the upper layer of the fluid is heavier than the lower and some motions, called convection motions, may exist. These motions tend to reduce the adverse density gradient. The existence of convective motions in areas of geothermal activity has been known for a long time. The existence of the solid matrix of the porous medium does not modify in any important manner the qualitative aspects of the phenomenon. This analogy between the fluid layer and the porous medium has been justified by a number of recent studies which have also provided considerable quantitative information (7, 12, 19. 22, 24. 26). These studies pertain to the ease in which compressibility effects are negligible as compared to the thermal expansion effect and where the porous layer is homogeneous and bounded by parallel planes. The problems considered in this paper areconvection in horizontal layers,convection in sloped Jayers andmixed (forced and free) convection. It is possible to gain a good understanding of the phenomenon for almost all practical cases; however, the direct application of the results to practical problems in a quantitative manner is not always easy. This difficulty is due to the complexity of real boundary conditions, the nonuniformity of a real porous structure and also, in some cases, the influence of compressibility effects.