An Experimental and Modeling Study of Miscibility Relationship and Displacement Behavior for a Rich-Gas/Crude-Oil System
- John Mansoori (Amoco Production Co.) | G.L. Haag (Amoco Production Co.) | D.F. Bergman (Amoco Production Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- August 1993
- Document Type
- Journal Paper
- 189 - 194
- 1993. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 1.8 Formation Damage, 5.2 Reservoir Fluid Dynamics, 4.3.3 Aspaltenes, 4.6 Natural Gas, 5.2.2 Fluid Modeling, Equations of State, 5.4.2 Gas Injection Methods, 5.2.1 Phase Behavior and PVT Measurements, 5.5 Reservoir Simulation, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex)
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This paper presents an experimental study that uses many slim-tubedisplacements to characterize the effect of hydrocarbon solvent composition onmiscibility development. It was found that the minimum enrichment requirementcan be estimated reliably with a previously proposed pseudocriticaltemperature, T , concept. previously proposed pseudocritical temperature, T ,concept. Tuning equation-of-state (EOS) parameters to match these resultsrequired incorporating rate and recoveries from selected slim-tube displacementtests in addition to conventional PVT data.
The operation of an enriched-gas injection project often involves blendingone or more natural gas liquid (NGL) streams with dry gases to obtain anoptimum solvent. The composition of such NGL's (ethane, propane, and somehigher-carbon-number components) is subject to frequent change over the life ofthe flood because of source and supply availability. Therefore, a procedure isneeded to estimate miscibility conditions between the potential injectionsolvents and the reservoir fluid. Benham et al. 1 developed a graphicaltechnique where the minimum enrichment requirement can be estimated from thetemperature, pressure, average molecular weight of intermediate hydrocarbons inthe displacing fluid, and C5+ molecular weight of the reservoir oil. Thisapproach, however, often results in minimum miscibility requirements that aretoo conservative. Rutherford conducted a series of displacements in sandpacksand found that miscibility development with a reservoir oil at constantpressure was a function only of the injected gas T . Rutherford studied alimited range of NGL fluids, consisting primarily of ethane and propanemixtures. His results were primarily of ethane and propane mixtures. Hisresults were extended by Jacobson to systems containing CO2 and H2S.Rutherford's TPI method is used in this study to develop a miscibilitycorrelation over a broad range of hydrocarbon species and enrichment levels.Accurate EOS oil recovery predictions require a fluid description withparameters tuned to match certain laboratory data. Typically, these experimentsinclude such routine measurements as differential vaporization analysis (DVA)for black-oil properties and static PVT tests on selected mixtures. In someapplications, the resulting fluid description may be adequate for predictingthe performance of dynamic slim-tube and coreflood tests. Kremesec andperformance of dynamic slim-tube and coreflood tests. Kremesec and Sebastian,however, showed that fluid descriptions based on static PVT data alone wereinadequate for predicting the results of PVT data alone were inadequate forpredicting the results of dynamic tests for a vaporizing CO2/Oil system. Theyconcluded that matching the multiple-contact minimum miscibility pressure (MMP)can improve the accuracy of such predictions significantly. Negahban andKremesec observed that this EOS tuning strategy can correctly identify miscibleand immiscible regimes and slim-tube rates and recoveries over a wide range ofdisplacement pressures. One objective of this study is to extend theseinvestigators' findings to rich-gas condensing drives by investigating whetherfluid descriptions based on only static PVT tests can predict multiple-contactdisplacement behavior in these systems. A further objective is to determine theadditional information required to improve fluid descriptions if the PVT-baseddescriptions are inadequate.
Experimental Studies and Simulation Requirements
To develop a miscibility correlation based on the T concept for thereservoir oil in this study, we conducted 25 slim-tube tests at 135F and 2,000psi using an apparatus previously described. Table 1 shows reservoir oilproperties and NGL mixture compositions with as few as one hydrocarboncomponent (NGL 1) and with multiple hydrocarbon components and additionalspecies (NGL's 2 through 5). These NGL's were diluted with methane to giveinjection solvents with enrichment levels ranging from 35 to 51 mol % C21. Thehydrocarbon components in these solvents ranged from two to seven, and somecontained up to 5 mol% CO2 or nitrogen. Sight-glass observations were recordedin all slim-tube tests to provide supporting information for the displacementmechanism. Slimtube tests to measure MMP also were conducted with Solvent A(Table 1) over a 1,650- to 3,000-psi pressure range. For fluid descriptiondevelopment, DVA, saturation pressure, and p-x diagram measurements andconstant-composition-expansion (CCE) tests were conducted with the reservoiroil and Solvent A. In addition, we compared actual and predicted displacementmechanisms using Tiffin et al.'s results from a coreflood experiment conductedwith the same reservoir oil and Solvent A to verify the fluid descriptionfurther. Amoco's version of the Redlich-Kwong EOS was used for fluid propertyand phase-equilibrium calculations. Slim-tube and property andphase-equilibrium calculations. Slim-tube and coreflood simulations wereperformed with a previously described generalized compositional model. Otherdetails of the slim-tube and coreflood displacement modeling were also reportedpreviously. previously. Results and Discussion
Miscibility Correlation. Using Rutherford's method, we plotted slim-tube oilrecovery measured at 1.2 HCPV of solvent injection vs. solvent T for all NGLstreams (Fig. 1). T is defined as the mole-weighted average of criticaltemperature T of all drive gas components corrected to CO2 and nitrogen T asdiscussed later. The general behavior in these experiments suggested that thedata could be fitted reasonably well with two straight lines intersecting at T- 464R. We observed that displacement efficiency initially increased sharplywith increasing T , followed by a gradual leveling off at higher enrichmentlevels. For solvents with T greater than 464R, slim-tube recoveries were highand sight-glass observations confirmed a miscible condensing-gas-drivemechanism. These tests were characterized by an early gas breakthrough,followed by a gradual color change in the two-phase transition zone. Forsolvents with T less than 464R, injected PV at gas breakthrough decreased andthe transition zone became broader, resulting in lower oil recoveries. Thesedisplacements showed immiscible behavior despite a prolonged oil productionperiod after 1.2 HCPV of solvent injection. Fig. 1 also shows data from allother supporting slim-tube experiments conducted with the same recombinedreservoir oil. The results further substantiate the choice of TPC as acorrelating parameter.
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