An Analysis of Miscible Flooding Chase-Fluid Strategies
- M.P. Walsh (Texas A and M U.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1991
- Document Type
- Journal Paper
- 437 - 444
- 1991. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 1.8 Formation Damage, 5.4.9 Miscible Methods, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.3.2 Multiphase Flow, 5.3.4 Reduction of Residual Oil Saturation, 5.2.1 Phase Behavior and PVT Measurements, 6.5.2 Water use, produced water discharge and disposal, 4.1.2 Separation and Treating, 5.5 Reservoir Simulation, 5.4.2 Gas Injection Methods, 4.6 Natural Gas
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This paper presents a fractional-flow-theory analysis of miscible floodingchase-fluid strategies. Graphical solution methods to analyze flood performancefor arbitrary solvent/water ratios, chase gas/water ratios (GWR's), and solventslug sizes are presented. The graphical solution techniques include methods topredict the effects of wave interference, an important precursor to oil-bankdestruction or loss in mobility control. An examination of interference leadsto a deeper understanding of the miscible slug process. The advantages anddisadvantages of chase-water, chase-gas, and simultaneous chase-gas/waterinjection are summarized. The analysis provides the theoretical basis for manychase-fluid design practices, including the idea of a minimum solvent slug sizeto ensure displacement integrity and the concept of an optimal chase GWR tominimize solvent requirements.
This paper presents a fractional-flow-theory analysis of chase-fluid designstrategies for the miscible slug process. The interrelationships betweensolvent/water ratio, chase-fluid selection, chase GWR, solvent slug size, andflood performance are explored. The results are based onmethod-of-characteristics solutions to the governing partial-differentialequations. The treatment is limited to 1D flows under vanishing fingering anddispersion. Admittedly, these assumptions limit the direct application of thiswork. Nonetheless, the analysis provides a strong theoretical basis for manydesign strategies, yields a simple graphical means to predict floodperformance, and provides a firm conceptual understanding of predict floodperformance, and provides a firm conceptual understanding of the miscibleflooding process that allows the effects of more complicated factors to beunderstood more readily. Previous investigators used fractional-flow theory toestimate slug-size requirements. These works, however, failed to consider thecomplicated effects of wave interference. Wave interference occurs in themiscible slug process whenever fast-moving waves invoked by chase-fluidinjection catch up to and collide with the slower-moving waves invoked bysolvent injection. Wave interference is important because it is a precursor tooil-bank destruction or loss in mobility control. One reason previous worksfailed to consider the consequences of wave interference is that this type ofwave response is complicated by states of local noncoherence. Problems subjectto interference and noncoherence are characteristically more difficult to solvemathematically than those without these effects. A unique feature of this workis that problems characterized by interference and noncoherence are treated andsimple graphical solution methods are illustrated. Moreover, the effects ofinterference on different chase-fluid strategies are illustrated. Three kindsof interference are noted: destructive, beneficial, and incidental. Destructiveinterference results in either loss of displacement efficiency or mobilitycontrol, beneficial interference is limited to improved displacementefficiency, and incidental interference includes all other types ofinterference. The conditions leading to each type of interference will bediscussed. This paper summarizes a set of companion papers by Walsh, and onlyselected paper summarizes a set of companion papers by Walsh, and only selectedresults are presented here. These papers, extend earlier work by Walsh andLake. Together, the works by Walsh and Walsh and Lake give a comprehensiveanalysis of the miscible slug process, and the reader is referred to thesepapers for a more complete understanding of the application of fractional-papers for a more complete understanding of the application of fractional- flowtheory to miscible flooding.
This analysis is limited by the usual fractional-flow-theory assumptions,most notably ID flow, no dispersion, and no fingering. Walsh itemized thecomplete assumptions.
Helfferich showed that first-contact-miscible displacements areindistinguishable from multiple-contact-miscible displacements in the limit ofno dissipation, Thus, this work is equally applicable to both types of miscibledisplacements.
Two of the most important phenomena affecting solvent-slug-size requirementsin ID flows are dispersive fluid mixing and immiscible fluid flow resultingfrom the effects of mobile water. Dispersion is neglected because its effectson miscible slug propagation are well-known. The effects of immiscible fluidflow on slug propagation, on the other hand, are not well-documented. This workaddresses this subject. Because dispersion is neglected, the slug-sizecalculations given represent lower-limit estimates of the actu-al solventrequirements.
The key parameters affecting flood performance are the injectedsolvent/water ratio, injected chase GWR, and solvent slug size; this paperexamines their effects on the displacement efficiency and en-suing mobilityratios. The goal of any design is to maximize oil recovery while minimizingsolvent requirements. Because this anal-ysis does not directly consider theeffects of fingering, the design presented in this paper also seek to minimizethe mobility ratio be-tween the driving solvent and the oil bank and betweenthe driving chase fluid and the solvent to ensure good volumetric sweepeffi-ciency. Mobility ratios reported in the examples are ratios of theupstream and downstream total fluid mobilities.
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