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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 beenof a theoretical nature, without examples from reservoir fluid systems, orproposing 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. Introduction Composition variation with depth can result for several reasons:Gravity segregates the heaviest components towards the bottom and lighter components like methane towards the top33,34,39.Thermal diffusion (generally) segregates the lightest components towards the bottom - i.e. towards higher temperatures - and heavier components towards the top (towards lower temperatures)3,39.Thermally-induced convection creating "mixed" fluid systems with more-or-less constant compositions, often associated with very-high permeability or fractured reservoirs10,18,31.Migration and "equilibrium" distribution of hydrocarbons is not yet complete, as the times required for diffusion over distances of kilometers may be many 10's of millions of years.Dynamic flux of an aquifer passing by and contacting only part of a laterally-extensive reservoir may create a sink for the continuos depletion of lighter components such as methane.Asphaltene precipitation during migration may lead to a distribution of varying oil types in the high- and low-permeability layers in a reservoir36.Asphaltene precipitation in the lower parts of a reservoir ("tar mats") caused by non-ideal thermodynamics and gravitational forces14,32.Biodegredation varying laterally and with depth may cause significant variation in, for example, H2S content and API gravity.Regional (10–100's km) methane concentrations that may lead to neighboring fields having varying degrees of gas saturation, e.g. neighboring fault blocks which vary from saturated gas-oil systems to strongly-undersaturated oils.Multiple source rocks migrating differentially into different layers and geological units.

Summary Minimum miscibility conditions of pressure and enrichment (MMP/MME) have been computed with an equation of state (EOS) for several reservoir-fluid systems exhibiting compositional gradients with depth owing to gravity/chemical equilibrium. MMP/MME conditions are calculated with a multicell algorithm developed by Aaron Zick, where the condensing/vaporizing (C/V) mechanism of developed miscibility is used as the true measure of minimum miscibility conditions when it exists. The Zick algorithm is verified by detailed one-dimensional (1D) slimtube simulations with elimination of numerical dispersion. The miscibility conditions based on the traditional vaporizing-gas-drive (VGD) mechanism are also given for the sake of comparison, where it is typically found that this mechanism overpredicts conditions of miscibility. Significant variations in MMP and MME with depth exist for reservoirs with typical compositional gradients, particularly for near-critical oil reservoirs and gas-condensate reservoirs where the C/V mechanism exists. An important practical implication of these results is that miscible displacement in gas-condensate reservoirs can be achieved far below the initial dewpoint pressure. The requirement is that the injection gas (slug) be enriched somewhat beyond a typical separator gas composition and that the C/V miscibility mechanism exist. This behavior results in many more gas-condensate reservoirs being viable candidates for miscible gas cycling than previously assumed, and at cycling conditions with lower cost requirements (i.e., lower pressures) and greater operational flexibility (e.g., cycling only during summer months). Introduction Considerable work on miscible gas injection in oil and, to a lesser extent, gas-condensate reservoirs can be found in the literature.1,2 The phenomena of compositional variation with depth owing to gravity and thermal effects has also been studied in detail the past 20 years.3,4 However, almost nothing in the literature can be found on the variation of miscibility conditions with depth in reservoirs with compositional gradients. It is difficult to picture the variation of MMP with depth for a reservoir with varying composition and temperature. This study shows that a simple variation does not exist, but that certain features of MMP variation are characteristic for most reservoirs. For example, the simplest variation in MMP with depth is for a lean injection gas like nitrogen, where minimum miscibility conditions are developed by a purely VGD mechanism. Here the MMP is always greater than or equal to the saturation pressure. In the oil zone, MMP may be (and usually is) greater than the bubblepoint pressure, while in the gas zone the MMP is always equal to the dewpoint. The MMP variation with depth can be considerably more complicated when the injection gas contains sufficient quantities of light-intermediate components (C2 through C5) or CO2. Here, developed miscibility is usually by the condensing/vaporizing mechanism, but it may be purely vaporizing in some depth intervals of the reservoir. When the C/V mechanism exists, MMP may be (and often is) less than the saturation pressure, even for gas-condensate systems. This study quantifies the variation of MMP with depth for several reservoir-fluid systems, and we try to understand the reasons for seemingly complicated MMP variation. Perhaps the most important result of our study has been to show that miscible gas injection in gas-condensate reservoirs can exist far below the dewpoint. Economic application of enriched gas injection in partially depleted gas-condensate reservoirs may be achieved by slug injection, similar to miscible slug-injection projects in oil reservoirs.5 Calculating Minimum Miscibility Pressure Miscibility between a reservoir fluid and an injection gas usually develops through a dynamic process of mixing, with component exchange controlled by phase equilibria (K-values) and local compositional variation along the path of displacement. The exact process of mixing is not really important to the development of miscibility - i.e., the relative mobilities (permeabilities) of flowing phases are unimportant. However, to obtain the correct MMP it is important to follow a physically realistic path of developed miscibility and not assume a priori how the path to miscibility occurs. The ability of an EOS to predict minimum miscibility conditions and compositional grading is very dependent on the accurate representation of complex phase behavior and, in particular, accurate K-value predictions.4,6,7 Single-Cell Algorithms. Before 1986, it was assumed that developed miscibility followed one of two paths: Forward contact, or VGD, where the injection gas becomes enriched in C2+ by multiple contacts with original oil and, at the gas front, eventually develops miscibility with the original oil; or backward contact, or condensing gas drive (CGD), where the injection gas continuously enriches the reservoir oil in C2-C5 at the point of injection until the injection gas and enriched reservoir oil become miscible. Either process can be modeled with a single-cell calculation algorithm,8,9 where the critical tie-line is located by appropriate multiple contacts of injection gas and reservoir oil. For gas condensates, the vaporizing mechanism has always been assumed to exist in miscible gas-cycling projects and the VGD MMP is readily shown to equal the original dewpoint pressure. For reservoir oils, it is usually assumed that the VGD mechanism exists for lean injection gases, while the CGD has been assumed to describe miscible displacement for enriched gas injection. Using a single-cell calculation algorithm, the calculated VGD MMP is almost always lower than or equal to the CGD MMP, unless the gas is highly enriched. C/V Mechanism. Zick6 showed that a mixed mechanism involving both vaporization and condensation describes the actual development of minimum miscibility conditions for many systems. He showed that the location of miscibility (i.e., near-100% recovery efficiency) was not at the displacement front (VGD) or the point of injection (CGD), but in between. He also showed that the true minimum conditions of miscibility could be significantly lower than predicted by the VGD and CGD mechanisms. These findings have been verified by numerous publications during the past 10 years.7,10–12 Based on Zick's findings and his description of the mixed C/V mechanism, it is clear that the true MMP (or MME) can be calculated only if the path of developed miscibility is modeled properly. Several authors have suggested methods to calculate the C/V MMP.

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.