An equation-of-state (EOS) fluid characterization has been developed to model three-hydrocarbon-phase equilibria. The fluid characterization was developed as part of the enriched-gas EOR research program for the Kuparuk hydrocarbon miscible flood on the North Slope of Alaska. Laboratory experiments with the Kuparuk fluid system have quantitatively identified the third hydrocarbon phase as a second equilibrium liquid, high in asphaltene content and heavier than the original oil. A three-phase EOS fluid characterization was developed to match both two-and three-phase experimental data using a modified Peng-Robinson EOS. Experimental observations identified a relationship between the development of the third phase and the oil's pentane-insoluble fraction. The characterization uses a total of 16 components, with the heaviest two representing the pentane-soluble and pentane-insoluble portions of the C36+ fraction. The properties of these heavy pseudocomponents control the development and behavior of the heavy phase. The EOS fluid characterization was used to study three-phase, enriched-gas-drive displacement mechanisms. For the Kuparuk fluids, a condensing/vaporizing displacement mechanism is predicted in a three-phase environment Reservoir-condition micromodel displacements were conducted to observe the phase behavior and displacement mechanisms during Kuparuk enriched-gas displacements. Micromodel observations are consistent with a near-miscible condensing/vaporizing mechanism and qualitatively agree with EOS predictions. Introduction A hydrocarbon miscible flood is being conducted in the Kuparuk River Field on the North Slope of Alaska. The EOR project covers an area of 5120 acres(2070 ha) at Drill Sites 1Y and 2Z. The flood is conducted in the A and C Sands of the Kuparuk River formation, a stratified and highly faulted sandstone reservoir. The enriched gas injected at the project is a blend of lean produced gas with scrubber and low-temperature separator liquids (enriching fluids)produced onsite. Project design calls for the injection of a 30% hydrocarbon pore-volume slug of enriched gas injected alternately with water at a 1:1volume ratio for mobility control. Fluid properties at Kuparuk vary somewhat across the areal extent of the reservoir. Oil gravities range from 18 to 25 degrees API (0.90 to 0.95 g/cm3)and asphaltene content (measured as pentane insolubles) from 3 to 17 weight percent of the C7+ fraction. An extensive laboratory program was undertaken to provide phase behavior data for designing and operating the EOR project. The laboratory program identified a third hydrocarbon phase when miscible injectant was mixed with reservoir oil. The three-phase equilibria observed with the Kuparuk oils and enriched gases differ from those reported in the literature for other enriched gas/crude oil mixtures and CO2/hydrocarbon mixtures. In the systems reported in the literature, three phases exist in a relatively narrow temperature-pressure composition window and consist of a liquid, a vapor, and a second liquid lighter than the first and rich in components comprising the injectant. In some systems, asphaltenic flocculation and deposition may also occur.

The compositional equation of state formulations by Coats (C), Nghiem, Fong, and Aziz (NFA), and Young and Stephenson (YS) are compared for both immiscible (IM) and multiple contact miscible (MCM) problems. Coats formulation has the largest computer memory requirements and computation times. Nghiem, Fong, and Aziz's formulation requires the least memory, and Young and Stephenson's formulation is generally the fastest. Introduction Beginning in the late 1950's, several mathematical models were developed to study the effects of compositional changes on oil recovery. Jacoby and Berry developed a tank-type model which used vapor-liquid equilibrium (VLE) and composition-dependent densities and viscosities. Constant pressure, linear, one-dimensional, (1-D) models were then developed by Welge, et al. and Attra to study the displacement of oil by gas injection. Pressure effects were included in a radial, 1-D model by Kniazeff and Naville; introduced in linear models by McFarlane, et al.; and improved upon by Price and Donohue. Roebuck, et al. added water as a flowing phase and Price and Donohue. Roebuck, et al. added water as a flowing phase and presented the first formulation applicable to higher dimensions. presented the first formulation applicable to higher dimensions. Formulations by Culham, et al., Gondouin et al., Van Quy, et. al., Tsutsumi, MacDonald, Nolen, and Kazemi et al. also contributed various items such as dispersion or more rigorous methods for computing VLE. In all of the preceding formulations, the VLE, densities, and viscosities were computed from independent correlations. Many times these correlations would not predict consistent critical points and, hence, oscillatory behavior was sometimes observed, in the critical region. Fussell and Fussell overcame this difficulty by using a cubic equation of state (EOS) to determine both the VLE and the densities which were then used to compute the viscosities. Since 1980, three more EOS formulations have been presented: Coats developed a fully implicit formulation; Nghiem, Fong, and Aziz modified the Kazemi, et al formulation to include an EOS; and Young and Stephenson presented a formulation similar to, but more general and efficient than presented a formulation similar to, but more general and efficient than Fussell and Fussell's. The C, NFA, and YS formulations represent state-of-the-art, VLE, compositional simulators. The primary objective of this study is to compare the last three EOS formulations. The comparisons are analogous to those made for black oil simulators - except we focus here on run characteristics rather than characteristics of the solution. A secondary purpose of this work is to provide detailed, standard, run-time data for other programmers. provide detailed, standard, run-time data for other programmers. We compare the formulations in terms of computer storage, accuracy, number of iterations, and computation time, for both immiscible and multiple contact miscible cases. Great care was taken to make the comparisons on a consistent basis. The C formulation was implemented only in 1-D; the YS and NFA formulations were implemented in both 1-D and two-dimensional (2-D) areal geometries. EQUATIONS The formulations model the flow of two hydrocarbon phases: oil and gas; and an aqueous phase. Each hydrocarbon phase is composed of Nc hydrocarbon components (which may include non-hydrocarbons such as CO2, N2, or H2S, but not water) and the aqueous phase is entirely water. p. 123