Surfactant Selection With the Three-Parameter Diagram
- Maura C. Puerto (Exxon Production Research Co.) | Ronald L. Reed (Exxon Production Research Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1990
- Document Type
- Journal Paper
- 198 - 204
- 1990. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 4.3.1 Hydrates, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 4.1.2 Separation and Treating, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.2.1 Phase Behavior and PVT Measurements
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A procedure to select surfactants for microemulsion flooding is developedthat requires only nominal phase-behavior information, acquired in experimentswith pure oils and NaCl brines, provided that the field brine composition,crude oil molar volume, and crude oil hydrogen/carbon ratio are known. It isshown that, for alkylaryl sulfonates, microemulsion phase behavior correlateswith ion size and charge rather than with ionic phase behavior correlates withion size and charge rather than with ionic strength alone.
A reasonable inference from the well-established key role of microemulsionphase behavior in oil recovery is that a practical knowledge of phase behaviorcan be used to develop screening procedures to assist in surfactant selectionbefore extensive procedures to assist in surfactant selection before extensivecoreflooding. Hence, one objective of this work was to develop a procedure touse the three-parameter diagrams as a basis for surfactant screening andselection early in the design process. An additional consideration was theaccumulation of data in such a was that they could be used, not only for thecurrent project. but also for future applications. To accomplish this, we showhow phasebehavior data for crude oils and complex reservoir brines arephasebehavior data for crude oils and complex reservoir brines are related todata for pure oils and brines containing only NaCl.
Our objective is to make a sound initial selection of surfactants that willbe good candidates to recover oil from a given reservoir, recognizing, ofcourse, that subsequent steps in the total flooding-design process may affectthe final selection.
Posing the problem is difficult because of constraints engendered byavailability of commercial surfactants. Certainly the diversity in structure ofcommercial surfactants with potential for EOR is rapidly increasing, but theavailability of suitable surfactants is still a limiting factor for sometemperature/salinity combinations, particularly where both are high.particularly where both are high. First, the microemulsion to be used for oilrecovery is assumed to be formulated with a reservoir brine or its equivalentand the injection composition is assumed to lie on the optimal pseudoternarydiagram for the brine. A salinity different from optimum may be preferred insome cases, but this does not change the preliminary selection process. Next,if all other requirements are met, the preferred surfactant will have thelargest solubilization parameter preferred surfactant will have the largestsolubilization parameter measured at optimal salinity, because thesolubilization parameter and interfacial tension (IFT) are inverselyrelated.
With the reservoir oil, brine, and temperature specified, some would claimthat the question of which of two surfactants is better can be answered on thebasis of coreflooding. Results may not be acceptable, however, because thespecified environment probably will not be optimal for either surfactant or anyblend of them. If two or more surfactants can be selected judiciously so that ablend is optimal for the reservoir environment, then this blend can be comparedwith a blend of two or more other surfactants and a decision made as to whichblend is better. In this way, any set of sur-factants can be compared with anyother set, even if some of th surfactants are common to both sets.
This selection process uses the three-parameter diagram. Effective use ofthis diagram requires knowledge in three areas: directional effects oflipophile structure on the way the diagram changes; effects of the variousinorganic ions that make up reservoir brines, in particular the impact ofdivalent ions; and the relationship between particular the impact of divalentions; and the relationship between crude oil hydrogen/carbon ratio and optimalsalinity reduction.
Review of the Three-Parameter Diagram
The three-parameter diagrams is a graph of optimal salinity, C phi, vs. oilmolar volume, Vmo, with parametric curves drawn in for various values of thesolubilization parameter measured at optimal salinity, V/Vs. [At optimalsalinity, (Vo/Vs)C phi = (Vw/Vs)C phi = V/ Vs.] Fig. 1 is such a diagram forthe monoisomeric surfactant n-C 12 OXS (Sodium salt of dodecyl orthoxylenesulfonate) at 78 deg. F [26 deg. C], a WOR of one, and no alcohol. Each point(Vmo, C phi) rep-resents the optimal salinity obtained with an oil of molarvolumVmo selected from the n-alkanes, n-alkyl cyclohexanes, and n-alkylbenzenes. The dashed lines refer to constant values of the solubilizationparameter at optimal salinity, V/Vs, and were obtained by interpolation fromnumerous data that have been omitted for clarity. Instead, the numbers neareach datum represent the hydrogen/carbon ratio, FH/C, in wt/wt for that oil.Note that, for constant oil molar volume, FH/C increases as C phi increases, aswill be discussed later (see Crude Oils/Pure Oils).
We found this diagram very useful for understanding microemulsion phasebehavior and for selecting surfactants for microemulsion floods. Its utilityrests in that it is unique, preserves order, and reflects a measure ofhydrophile/lipophile balance (HLB). By preserves order, we mean that, for everysurfactant studied, the line for n-alkanes lies above that for the n-alklycyclo-hexanes, which in turn lies above that for the n-alkyl benzenes. Further,oils that provide points intermediate to these lines will always do so.
Optimal salinity can be decreased by appropriate branching of the surfactantlipophile (less lipophilic because Vo/Vs decreases) or by increasing lipophilemolecular weight (more lipophilic because Vo/Vs increases). Thus, on thethree-parameter diagram, the location of the n-alkane line depends onsurfactant HLB. In this regard, the view expressed in Ref. 11 that"increased branching" implies increased "oil solubility" isquestionable.
When only a few surfactants were commercially available (primarily petroleumsulfonates), screening all of them may have been petroleum sulfonates),screening all of them may have been possible, with reservoir brines used forall phase-behavior studies. possible, with reservoir brines used for allphase-behavior studies. Now that large numbers of surfactants are available,reducing the number of experiments required through an understanding of theeffects of specific ions in oilfield brines is valuable. Simply put, havingmeasured microemulsion phase behavior using multi-ion brines, one can find anNaCl-only brine that will reproduce oil solubilization at optimal salinity whenother variables are kept the same. This NaCl-only brine is called an eqivalentbrine by analogy to " equivalent oils" introduced earlier. A procedurefor calculating the composition of this brine is developed in this paper. Inpreparation for this, some basic considerations are discussed, data preparationfor this, some basic considerations are discussed, data illustrating the waysmicroemulsion phase behavior depends on specifiions and combinations of themare presented, and an equation is proposed that can he used for alkylarylsulfonates to calculate the proposed that can he used for alkylaryl sulfonatesto calculate the Na+ equivalent of Ca++ in mixed brines.
Hedges used the Shinoda diagram (fraction of phase volume vs. salinity) tointroduce the idea of an NaCl equivalency number. This number is the NaClsalinity where relative phase volumes for a field brine and a pure oil equalthose for an NaCl-only brine and the same pure oil. In contrast, the thrust ofthis paper is to avoid measuring phase behavior with field brines altogether.Further, Hedges uses alcohol in all formulations, whereas we prefer to avoidcosolvents. From a mechanistic view, cosolvents often obscure datainterpretation, and from a practical view, their use incurs unnecessaryexpense.
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