In the vaporizing solvent drive process, compositional enhancement occurs by the injection solvent vaporizing intermediate-molecular-weight hydrocarbons from the oil and enriching the composition at the solvent front. In this way, solvent at the displacing front is progressively enriched to the critical composition P, which is first-contact miscible with the reservoir fluid. Solvent G2 then flows ahead and contacts fresh reservoir fluid, forming an overall mixture M3, which consists of dewpoint solvent G3 and bubblepoint liquid 元. Upon mixing, solvent G1 and the reservoir fluid form another overall mixture, M2, which consists of dewpoint solvent G2 and bubblepoint liquid L2. M1 is in the two-phase region of the diagram and consists of dewpoint solvent G1 and bubblepoint liquid L1.Īs more solvent is injected, solvent G1, formed in the first contact, is pushed ahead, where it contacts fresh reservoir fluid. When this solvent mixes with the reservoir fluid, an overall composition such as M1 may result. The injection solvent identified on the C 1–C 2–6 side of the triangle has a high methane content. 1 shows how compositions change in situ when a lean injection solvent displaces an oil represented by point A, whose composition lies just to the right of the limiting tie line. All mixtures outside the region bounded by the dewpoint and bubblepoint curves are single phase.įig.The tie line that just passes through the critical composition is called the critical tie line.The tie line that passes through a given mixture composition gives the equilibrium solvent and liquid compositions for this mixture where it intersects the dewpoint and bubblepoint curves.All mixtures inside the region bounded by the bubblepoint and dewpoint curves consist of two phases.The dashed lines, called tie lines, connect liquid and solvent compositions that are in equilibrium.These curves come together at a critical composition. There is a bubblepoint curve representing oil mixtures at their bubblepoint and a dewpoint curve representing solvent mixtures at their dewpoint.1 – The vaporizing-gas-drive process (after Young and Martin ). 1 is a traditional pseudoternary-diagram representation of phase behavior for the pseudo-components:įig. Even so, a pseudoternary diagram is still a useful way to represent some complex phase-behavior concepts that are not so easily visualized otherwise.Ī ternary diagram represents phase behavior at a constant temperature and pressure. Pseudoternary diagrams apply rigorously only to true ternary systems, and a strictly ternary analogy may give a somewhat misleading view of the mass-transfer mechanisms that result in compositional enhancement. another, and different groupings may give somewhat different insights into phase-behavior mechanisms. Different injection processes may be better represented by one type of grouping vs. There is no particularly "right" way to divide a fluid into three pseudocomponents. The higher-molecular-weight pseudocomponent in this scheme would be the leftover C 7+ fraction. The intermediate-molecular-weight pseudocomponent might include the C 2 –C 6 hydrocarbons and perhaps CO 2 if the CO 2 is a constituent of an otherwise hydrocarbon injection solvent. A fraction of the higher-molecular-weight materialsįor example, the low-molecular-weight fraction might include methane and nitrogen and perhaps CO 2 if CO 2 is the primary injection solvent.A fraction of intermediate-molecular-weight materials.A fraction of low-molecular-weight materials.Usually, the three pseudocomponents represent: This is done by representing multicomponent fluids or mixtures by three pseudocomponents and then plotting fluid compositions in the interior of an equilateral triangle with apexes that represent 100% of each pseudocomponent and where the side opposite an apex represents 0% of that pseudocomponent. Ternary diagrams and pseudoternary diagrams have been used for decades to visualize conceptually the phase behavior of injection-fluid/crude-oil systems. 1 Ternary and pseudoternary phase diagrams.
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