Abstract
This paper quantifies the potential variation in composition and PVT properties with depth due to gravity, chemical, and thermal forces. A wide range of reservoir fluid systems have been studied using all of the known published models for thermal diffusion in the non-isothermal mass transport problem.
Previous studies dealing with the combined effect of gravity and vertical thermal gradients on compositional grading have either been (1) of a theoretical nature, without examples from reservoir fluid systems, or (2) proposing one particular thermal diffusion model, usually for a specific reservoir, without comparing the results with other thermal diffusion models.
We give a short review of gravity/non-isothermal models published to date. In particular, we show quantitative differences in the various models for a wide range of reservoir fluids systems. Solution algorithms and numerical stability problems are discussed for the non-isothermal models which require numerical discretization, unlike the exact analytical solution of the isothermal gradient problem.
A discussion is given of the problems related to fluid initialization in reservoir models of complex fluid systems. This involves the synthesis of measured sample data and theoretical models. Specific recommendations are given for interpolation and extrapolation of vertical compositional gradients. The importance of dewpoint on the estimation of a gas-oil contact is emphasized, particularly for newly-discovered reservoirs where only a gas sample is available and the reservoir is near saturated.
Finally, we present two field case histories – one where the isothermal gravity/chemical equilibrium model describes measured compositional gradients in a reservoir grading continuously from a rich gas condensate to a volatile oil; and another example where the isothermal model is grossly inconsistent with measured data, and convection has apparently resulted in a more-or-less constant composition over a vertical column of nearly 5000 ft.
