Abstract
By far, in most reservoirs, even in most homogeneous ones, fluid properties are not the same all through the reservoir extent; the simplest reason is that the gravity makes a heavier fluid toward deeper zones. However, this is not always the case, because there are other reasons that may oppose this factor like a temperature gradient, capillary pressure, reservoir compartmentalization, reservoir filling, density overturn, and genesis processes. There are numerous reports of such phenomena in the literature.
This variation of composition can highly affect reserve estimation, production and EOR strategies and even a completion scheme (remember how a gas-oil ratio (GOR) and miscibility can change with depth as the fluid composition changes). As a result, more and more efforts have been devoted to including these effects in reservoir simulation through the initialization of a reservoir. Furthermore, some authors have tried to utilize this phenomenon to detect reservoir compartments.
In this study, we consider the effects of gravity and thermal diffusion on the change of reservoir fluid composition and predict the location of possible GOC (gas-oil contact) in a reservoir. Additionally we develop a simple model to predict the change of the plus fraction molecular weight (MW) in a nonisothermal reservoir using continuous thermodynamics and the theory of irreversible processes. In two case studies, we validate our calculation procedure for the general compositional grading, GOC detection and the plus fraction MW change in the reservoir versus the data in the literature. The computational results show that the model developed works satisfactorily. Furthermore we will run several sensitivity analyses to show what factors affect the compositional grading modeling output and when the reservoir engineer should be concerned about severe change of fluid properties caused by compositional gradient.
