Surfactant multivalent cation precipitation

The adsorption densities ( r ) on minerals (C< CMC) of the salt type are in some cases higher because of precipitation of the ionic surfactant with multivalent cations in the bulk phase. Measurements were carried out to determine the fraction f the precipitated surfactant by divalent cations Ca and Ba " leading to a decrease in its equilibirum concentration. They showed a shift of the adsorption maximum towards lower values of r, even after a correction of the adsorption density due to the precipitation. On the other hand, a direct co-adsorption of the precipitated surfactant on a mineral surface cannot be excluded. 

The primary reaction of alkali with reservoir water is to reduce the activity of multivalent cations such as calcium and magnesium in oilfield brines. Upon contact of the alkali with these ions, precipitates of calcium and magnesium hydroxide, carbonate, or silicate may form, depending on pH, ion concentrations, temperature, and so on. If properly located, these precipitates can cause diversion of flow within the reservoir, leading to better contact of the injected fluid with the less-permeable and/or less-flooded flow channels. This then may contribute to improved recovery. Also, this reduction of reservoir brine cation activity will lead to more surfactant activity, resulting in lower IFT values (Mayer et al., 1983). 

In the course of attempts to determine adsorption isotherms of anionic surfactants on selected clays two other phenomena requiring separate investigation were noted, namely, salting-out of surfactants by NaCl, and surfactant precipitation as calcium or magnesium salts by multivalent cations displaced from clays. 

Precipitation of anionic surfactants by multivalent cations is well known and it has been studied intensively (10, 11, 12,13). Powney and Addison (10) found that the addition of small amounts of CaCl to dilute sodium dodecyl sulfate solutions caused precipitation of the calcium salt. The addition of small amounts of n-hexanol was reported to postpone precipitation to higher CaCl concentrations, and increasing the surfactant concentration to a value greater than the CMC was found by Pearson and Lawrence (13) to prevent precipitation of the calcium salt of dodecyl sulfate owing to fixation of calcium ions by the micelles. 

Where surfactants were used in these experiments they were present in relatively large concentration. Hill and Lake, for example, injected a solution containing 0.046 meq/ml of a surfactant mixture having an average equivalent weight of 410 thus, the surfactant concentration was about 1.9 wt.%. Since this concentration is far above the CMC for the surfactants involved, multivalent cations may be bound by the micelles with the result that calcium sulfonate precipitation does not occur. 

To evaluate the importance of multivalent cation precipitation of surfactants by multivalent cations present in the formation brine or resulting from cation exchange and dissolution of minerals such as limestone, dolomite, etc., a very limited study of the calcium tolerance of selected alkylbenzene sulfonates was undertaken. 

It is to be noted that precipitation and redissolution of the above type can lead to an abstraction (abstraction of surfactant from solution) maximum and, if precipitation is not totally isolated from adsorption, it can consequently lead to an apparent adsorption maximum. In the case of clays, a number of monovalent and multivalent cations such as those of Na, Ca, Al, etc. can be expected to be present in a supernatant of it and to produce precipitations. 

Surfactant loss due to phase trapping is minimized if only oil and water phases are present at all times during the flood. Surfactant loss is only related to adsorption onto the mineral surface provided that the surfactant tolerates multivalent cations, i.e. no precipitation. In the following sections we will discuss recent published work which is relevant in reaching this goal. 

A number of inorganic cations form salts with anionic surfactants which have sufficiently low solubility to be useful for gravimetric determination. Generally, multivalent cations are more useful than monovalent cations, allowing the solubility of particular anionics to be reduced below the millimolar range (31). For example, the calcium salt of dodecylsulfate can be separated with better than 99% recovery by precipitation from aqueous solution. 

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