Abstract
Compositional gradient can be described as changes in the composition of components both vertically and horizontally in a hydrocarbon reservoir. In the present work, two-dimensional compositional gradient in multi-component gas and oil mixtures is modeled. A thermodynamic model is developed based on molecular diffusion coefficients in mass diffusive flux. A combination of Sigmund and Bird correlations is considered to estimate molecular diffusion coefficients for gas mixtures. Implementing this comprehensive developed model on a gas condensate sample shows interesting differences not only in trends of component compositions but also in gradient magnitude. A real set of data from a supergiant gas condensate field is used to validate the model. It is perceived that the developed model reduces relative absolute errors to about 50%. In the next step, a comprehensive study was conducted to understand the cross effects of molecular diffusion and natural convection in gas condensate and volatile oil samples. Gas and oil samples are selected to investigate if natural convection has the same effects in different samples. It is observed that increase in natural convection causes to reduce horizontal and vertical composition gradients. This effect is more significant in gas reservoir, as methane composition varies by more than 5.2 mol% diagonally in gas condensate sample, whereas this value is about 0.45 mol% in volatile oil sample. Lower density and higher bulk velocity in gas sample cause more disturbances in flow streams of gas mixture. Evaluation of the developed model shows that the model is reliable for reservoir studies and management programs.
Highlights
Lateral and vertical variation in the composition of reservoir fluid will be expectable due to non-uniformity of pressure and temperature through a reservoir
One can conclude that natural convection brings down the Effect of natural convection on 2D compositional gradient in volatile oil reservoir
It is investigated that using appropriate correlations for estimating molecular diffusion coefficients remarkably reduces calculation errors
Summary
Lateral and vertical variation in the composition of reservoir fluid will be expectable due to non-uniformity of pressure and temperature through a reservoir. Increase in Temperature value of diffusion coefficient could have a considerable effect on the prediction of the compositional gradient and distribution of components in gas reservoir. In part 1, to illustrate the importance of using appropriate correlations and the effect of molecular diffusion coefficients in predicting compositional gradient, Ghorayeb model is developed using a set of correlations proposed for gas mixtures.
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