Reservoir Analysis Based on Compositional Gradients


Olubusola Thomas







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Variations in fluid composition as a function of location within a reservoir have been observed in oil and gas reservoirs around the world. This is usually referred to as compositional variation. Over the many years that is takes a reservoir to form, processes such as oil migration, fluid convection and diffusion play important roles in the development of compositional gradients 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 components and heat within the reservoir is characterized using various metrics such as the Nusselt and Rayleigh numbers, as well as separation and density factors. These metrics are able to show, in a concise manner, the effect of changes in the reservoir and fluid properties on compositional variations.

It was observed in this study, that for multicomponent fluids in reservoir with compositional gradients, 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. 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 a 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 was negligible. Other observations include a consistency in the results obtained irrespective of the grid size used, for example, the three-dimensional grids produced the same average pressure, temperature, composition and velocity profiles as the two-dimensional grids for certain reservoir configurations.

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