Multicomponent reservoir

Phase behavior of multicomponent reservoir fluids is similar. Reservoir gases, which are predominately methane, have relatively small phase diagrams with critical temperatures not much higher than the orSiranempnatiire of nietfiahe. The critical point is"fairdown the left slope of the envelope. 

Figure8-1. Schematic plot of a linearly arranged multicomponent (k = 1,2multiphase (a,)3,. ..) system with a prefixed chemical potential difference across it. = buffer reservoirs.

On its way downwards, the liquid phase is of course depleted with respect to its more volatile component(s) and enriched in its heavier component(s). At the decisive locus, however, where both phases have their final contact (i.e., the top of the column), the composition of the liquid is obviously stationary. For a desired composition of the gas mixture, the appropriate values for the liquid phase composition and the saturator temperature must be found. This is best done in two successive steps, viz. by phase equilibrium calculations followed by experimental refinement of the calculated values. The multicomponent saturator showed an excellent performance, both in a unit for atmospheric pressure [18] and in a high-pressure apparatus [19, 20] So far, the discussion of methods for generating well defined feed mixtures in flow-type units has been restricted to gaseous streams. As a rule, liquid feed streams are much easier to prepare, simply by premixing the reactants in a reservoir and conveying this mixture to the reactor by means of a pump with a pulsation-free characteristic. 

Pia. 37. Multicomponent pressure-temperature diagram iUustrating the phase behavior of typical petroleum reservoirs. 

McKay, W. N., Experiments Concerning Diffusion of Multicomponent Systems at Reservoir Conditions, J. Can. Petroleum Technol., April-June, 25-32 (1971). 

Consider a container filled with a one-phase multicomponent mixture of composition x the container is immersed in a reservoir that imposes its temperature T and pressure P on the mixture. The container is fitted with a single inlet by which more material can be reversibly injected, as shown schematically in Figure 3.9. The process considered here is addition to the container of a small amount of pure component 1. The reversible work associated with this process is given by (3.7.14) for an isobaric injection of material through one inlet with no outlets, (3.7.14) reduces to... 

Consider a multicomponent system contained in a closed vessel and maintained at constant T and P, as represented schematically in Figure 7.5. We seek the conditions under which the system can exist as two phases in equilibrium. Since the external reservoir imposes its temperature and pressure on both phases, no driving forces exist... 

Next we will present the use of the cubic equations in predicting (1) the volumetric properties of pure components, (2) the phase behavior of multicomponent mixtures, and (3) the phase behavior of reservoir fluid systems. 

The combination of successive substitution and Newton s method is a good choice and has the desirable features of both. In this approach, the successive substitution comprises the first few iterations and later, when a switching criterion is met, Newton s method is used. To our knowledge, some commercial reservoir simulation models have adopted the combined successive substitution-New ton approach after the experience with various methods of solving nonlinear flash calculation including Powell s method (1970). The application of a reduction method to phase equilibrium calculations has also been proposed (Michelsen, 1986 Hendriks, and Van Bergen, 1992). In this approach, the dimensionality of phase equilibrium problems for multicomponent mixtures can be drastically reduced. The application of reduction methods and its implementation in reservoir compositional models is under evaluation. 

Adsorption equilibria are normally considered in connection with processes occurring in porous media filters, catalysts, adsorbents, chromatographs, and rock of petroleum reservoirs. In macroporous and mesoporous media, adsorption is normally accompanied by another surface phenomenon, the capillary condensation. These two types of surface phenomena are closely connected because they are both produced by surface forces. On the other hand, these phenomena are relatively independent and may, to some extent, be discussed separately [3]. Moreover, the description of the coexistence of the adsorbed films and capillary condensate in the same capillary is a nontrivial problem. We present capillary condensation and adsorption separately, although their eommon roots are discussed in Section II. The (more or less) comprehensive description of the thermodynamics of multicomponent capillary condensation... 

In this section, we consider the problems relevant to equiUbriiim of the two multicomponent phases separated by a curved interface. This is the classical and the most well-studied case of the thermodynamic equilibrium involving surface effects. Such equilibrium is present in macro-porous and mesoporous media, like the porous rocks of petroleum reservoirs, where it accompanies adsorption. In the pores of smaller sizes, the forces produced by the solid surfaces may modify the properties of the bulk (Uquid and gas) phases. However, the present study is also important to the pores of smaller sizes, as it makes it possible to separate the effects connected with the gas-liquid surface tension (and, of course, the contact angle) from additional contributions of the solid walls. The corrections related to the last type of interactions have been considered in, for example. Refs. [13-15]. For brevity, we will apply the term capillary equilibrium to the narrow case being described, but it must be remembered, however, that a wider understanding of the capillary equilibrium is available. 

For a multicomponent mixture, the results may be qualitatively different from a binary case. Let us proceed from a mixture characteristic of a gas-condensate reservoir, whose composition is presented in Table 1. This mixture exhibits retrograde condensation with a dew-point pressure of 200 bar at T = 323 K. Figure 10 shows the nonmonotonous dependence of the surface tension on the distance to the binodal curve, as the molar fractions of Cl and C7 vary. Figure 11 indicates that under varying Cl and CIO, the variation of composition is possible only within a 0.2% region, although the Kelvin radii cover the whole range of macropores and mesopores. These examples show that capillary condensation may produce a rich variety of unusual physical effects in multicomponent mixtures, which are not observed in mixtures with a low mrniber of components. 

These assumptions are violated at high pressures when the vapor phase becomes nonideal and in the cases where the multicomponent nature of the mixture cannot be neglected. The extreme case of non-Kelvin behavior is the behavior of hydrocarbon mixtures in od-gas-condensate reservoirs. Although the Kelvin equation is apphed to tests of the porous media of the reservoirs [74,75], it cannot be used for modeling of equilibria in such reservoirs, not only because of the high pressure and the multicomponent composition of the mixture but also because this mixture exhibits retrograde behavior when the liquid phase precipitates as the pressure decreases. Such a behavior cannot, in principle, be described in terms of a single-component model. 

The Score model (10) now updated in the ATHOS model was used to simulate the polymer injection at field scale. It is a 3 dimensional 3 phases multicomponent model, with all the features of a large reservoir model. A very elaborate treatment of the physics of polymer allows a detailed description of the flow... 

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