Reservoir analysis based on compositional gradients

Variations in fluid composition as a function of location have been observed in oil and gas reservoirs. The phenomenon is usually referred to as compositional variation. Over the many years that it takes a reservoir to form, processes such as oil migration, fluid convection and diffusion play important roles in the development of compositional variations particularly in reservoirs with horizontal and vertical temperature gradients. This study used the general mass and energy balance equations to simulate fluid movement within a nonproducing reservoir with thermal gradients. The configuration of fluid component and heat within the reservoir was characterized using various metrics such as the Nusselt and Rayleigh numbers, as well as separation and density factors.; It was observed that for multicomponent fluids in reservoirs with compositional variations, the movement of fluid components compensates for the drastic change in thermal gradients that is typically noticed in single-component, single-phase fluids, such as water, heated from below. The results obtained with the inclusion of an energy balance imply that the components align themselves primarily to honor the pressure and temperature gradients. Changes in different reservoir properties impact the compositional variation in different ways, for example, a change in the reservoir thickness results in a moderate change in the distribution of components, while a change in reservoir thermal gradients results in drastic shift in the distribution of components. Reservoir heterogeneity was found to play an important role in the variation of components, as well as the transfer of heat within the reservoir. It was also observed that because convection and diffusion in nonproducing reservoirs are slow processes and the diffusion terms are small in magnitude, the effect of variable diffusion coefficients is negligible.