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 convecti on are also discussed.The second part of the paper is concerned with prac-tical 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 convection may have an influence. These may be classified into two categories: (a) convective motions already existing in a reservoir and having an influence on the handling of the recovery methods; (b) 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 {1,6,11,25). 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 mollify 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 case in which compressibility effects are negligible as com-pared ti) the thermal expansion effect and where the porous layer is homogeneous and bounded by parallel planes. The problems considered in this paper are (1) con-vection in horizontal layers, (2) convection in sloped layers and (3) mixed (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 prob-lems in a quantitative manner is not always easy. This difficulty is due to the complexity of real boundary conditions, the non-uniformity of a real porous structure and also in some cases, the influence of com-pressibility effects. The practical examples given in this paper tend to show that, in spite...

Heat as a Ground Water Tracer

“…Blanchard and Sharp (1985) presented graphs (reproduced in Sharp et al 1988) that show the critical permeability necessary for the onset of free convection for selected layer thickness and geothermal gradients. Following Aziz et al (1973), they also presented evidence in the form of areal patterns of ground water temperature to support the existence of convection cells in thick sandstone units in the Gulf Coast basin under normal geothermal gradients.…”

Effect of diffusing layer thickness on the density-driven natural convection of miscible fluids in porous media: Modeling of mass transport

“…Natural convection in porous media (Nield and Bejan 2006) has applications in diverse fields including carbon dioxide capture and storage (CCS) (Green and Ennis-King 2013;Huppert Neufeld 2014;Lindeberg and Wessel-Berg 1997;Riaz and Cinar 2014;Islam et al, 2014), petroleum engineering (Aziz et al 1973), geothermal energy recovery (Barvier 2002;Henley and Ellis 1983;Oldenburg and Pruess 1998), and groundwater systems. (Herbert et al 1988;Huyakorn et al 1987;Oostrom et al 1992;Simmons et al 2001; Van et al 2009).…”

Analytic Investigation of Convection During Conduction Heating of a Heavy-Oil Reservoir

“…In conclusion, at early times or for very thick systems, although the vicinity of the heat source is dominated by convection, overall heat transfer is predominantly conduction. Experimental and complex numerical studies reported similar conclusions, even in dipping systems (Aziz et al, 1973;Holst and Aziz, 1972). Aziz et al (1968) carried out 2D simulations for free convection in a bottom-heated model, rigorously described with the conservation equations and an equation-of-state.…”

“…When the vertical component of the temperature gradient and gravity are in the same direction, as implied in Fig. 1, induced thermal convective motion has been shown to increase the vertical heat transfer (Aziz et al, 1973;Aziz et al, 1968), resulting in a higher Nusselt number (Aziz et al, 1973).…”

“…Field observations, from petroleum and non-petroleum reservoirs, affirm this phenomenon (Aziz et al, 1973). However, most of the methods for studying the phenomenon are based on coupled conservation equations and equation-of-state, yielding a set of nonlinear partial differential equations (PDE).…”