Compositional Grading—Theory and Practice

Abstract

This paper quantifies the potential variation in composition and pressure/volume/temperature (PVT) properties with depth owing to gravity, chemical, and thermal forces. A wide range of reservoir fluid systems has been studied using all of the known published models for thermal diffusion in the nonisothermal mass-transport problem.Previous studies dealing with the combined effects of gravity and vertical thermal gradients on compositional grading have been either (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/nonisothermal models published to date. In particular, we show quantitative differences in the various models for a wide range of reservoir fluid systems. Solution algorithms and numerical stability problems are discussed for the nonisothermal models that require numerical discretization, unlike the exact analytical solution of the isothermal gradient problem.We discuss 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 (GOC) is emphasized, particularly for newly discovered reservoirs in which only gas samples are available and the reservoirs are near-saturated.Finally, we present two field case histories-one in which 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 in which the isothermal model is grossly inconsistent with measured data and convection or thermal diffusion has apparently resulted in a more-orless constant composition over a vertical column of some 5,000 ft.

Analytical solution for compositional profile driven by gravitational segregation and diffusion

“…Baro-or pressure diffusion can have a significant influence on the distribution of components in multi-component fluid mixtures especially for the thermodynamic systems that extend over a large domain or are near the critical point. Such non-uniform equilibrium profiles have been seen in natural systems such as oil reservoirs [1][2][3][4][5] and have been postulated to occur at the core-mantle boundary [6]. Baro-diffusion can be an important consideration when separating components from a mixture in a centrifuge [7], considering the impact of nitrogen diffusing into austentic stainless steel [8] and predicting the yield from thermonuclear devices [9].…”

“…Oil and gas reservoirs are important examples of naturally occurring systems where the pressure gradient driving the diffusion is a result of gravity and where the segregation is of practical significance [1][2][3][4][5]. For oil reservoirs it is important to predict the influence that baro-diffusion has on the initial hydrocarbon component distribution in order to be able to: (i) estimate the value of the hydrocarbon resource in the reservoir [5,12]; (ii) obtain an indication of the reservoir connectivity [13,14] and (iii) design suitable recovery schemes, especially if the compositional gradient results in changing phase behaviour with depth [15].…”

“…For oil reservoirs it is important to predict the influence that baro-diffusion has on the initial hydrocarbon component distribution in order to be able to: (i) estimate the value of the hydrocarbon resource in the reservoir [5,12]; (ii) obtain an indication of the reservoir connectivity [13,14] and (iii) design suitable recovery schemes, especially if the compositional gradient results in changing phase behaviour with depth [15]. Identifying poor connectivity early on in a field development ensures that wells can be optimally placed, thus maximizing the recovery.…”

“…Temperature, which also increases with depth in most reservoirs, can also drive segregation of components through temperature diffusion (the Soret effect), [17][18][19][20] sometimes counteracting the gravity driven segregation. Typically, temperature has the greatest effect when there is also a horizontal temperature gradient which may alter the compositional distribution via natural convection or when the system is near its critical point [5,17,[20][21].…”

“…Over the years a number of models [5,16,[19][20][21][22][23][24][25][26][27] and references therein] have been developed to predict the distribution of components within the oil reservoirs. Most models have focussed on predicting the vertical distribution of components, although some have also examined the 3D distribution [17,18,27].…”