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Miscible Gas Phase Behavior


     This section shows what the microvisual photographs in other sections mean.  The figure below is a combination of a PX (pressure composition) diagram for Wasson crude oil and multiple contact mixing ternary diagrams for the oil at the pressures shown in the description of miscible displacements.  A PX diagram is constructed by adding a gas to an oil sample and measuring the mixtures bubble or dew points, GOR, volume, density, viscosity and composition of the phases.  This is the basic data used in all compositional simulations.  At compositions below the critical point shown in the PX diagram in the figure the gas is released from the oil or lower liquid (bubble point) as the pressure decreases.  At compositions, above the critical point the lower liquid (oil) condenses from the gas (upper liquid).  All compositions above (to the left of) the bubble-dew point line are miscible.  Everything else is two or more phases. 


Pressure Composition (PX) and Ternary Diagrams for Wasson Crude Oil




PX Diagram

Ternary Diagrams



    At low pressure where the displacement is called immiscible, the displacement is technically not miscible even though twenty mole percent CO2 can dissolve in the gas.  For Wasson oil, this is a GOR of 240 m3/m3 because the original GOR is approximately 180 m3/m3.  Thus, even injection of CO2 at “immiscible” conditions is likely to increase oil recovery and often is indistinguishable from a truly miscible displacement.

     The microvisual photo of the low pressure miscible displacement shows that miscible displacement occurs when the middle liquid forms.  The compositions and pressures at which this occurs are shown in the PX diagram below by the line labeled middle liquid.  This is at 9,380 kPa (1,350 psig).  Ninety four percent of the oil was recovered in a slim tube MMP test at this pressure.  The recovery of the original oil by injection of reservoir gas may have been as high as 90 percent because the reservoir gas is mostly CO2 and C2 through C6 components, i.e., it is a rich gas similar to the miscible injectant used at Prudhoe Bay.

     The middle liquid line passes through a three phase region in the PX diagram.  As the figure shows the region becomes vanishingly small as the pressure increases to 10,300 kPa (1,500 psig).  The figures on the right side are ternary diagrams for multiple contact mixing (MCM) experiments.  Ternary diagrams can also be produced from the compositions measured at constant pressure in a PX diagram. These diagrams are plots of the composition with three axis, (Pure CO2, C1-C6, and C7+).   The ternary diagram for the middle liquid case starts at the original crude oil composition, 3.3% CO2, 28% C1-C6, and 68.7% C7+.  The mixing path then passes through the plait point  (where the tie lines between liquid and vapor compositions vanish) before passing into the multi phase region, and ending at approximately 95% CO2 with 5% C7+.

     The composition paths for the last three miscible displacement microvisual photos are shown for the developed miscible case.  Developed miscible means that enough light hydrocarbons have been extracted from the oil that the oils and injected gas are soluble for a longer period of time.  These light hydrocarbons are better solvents for the large hydrocarbon molecules than CO2.  Thus, when the light hydrocarbons concentration in the displacing fluid is low, the asphaltenes begin to precipitate and the composition path enters the two phase region of the developed miscible ternary diagram.

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