Reduced Adsorption And Separation Of Blended Surfactants On Sand And Clay
Authors: Dawe B. , Oswald T. 
Published: March 2010

Abstract
Recent successful field trials of foam floods using surfactants containing disulfonated alkyldiphenyloxides have stimulated interest in the use of these surfactant in other EOR projects. Adsorption and separation of such surfactants is still a concern among some members of the petroleum industry. This paper will briefly discuss some of the advantages of using blended surfactants, notably enhanced mobility control and improved hardness tolerance. The main emphasis will be on the adsorption and separation of these surfactants when in contact with sand and clay. It has been found that adsorption of surfactants from blends is significantly lower than from "pure" surfactants and that separation of the blends is much less of a concern than is commonly believed. Adsorption and separation studies were done using water containing different concentrations of inorganic salts, and sand and sand/montmorillonite adsorbents.

Introduction
Recent successes in Alberta oil fields, notably by Signalta Resources of Calgary and Dome/Amoco, also of Calgary, have stimulated interest in the use of blended surfactants for enhanced oil recovery by foam flood. There are several reasons for considering blended surfactants for EOR application. These include enhanced mobility control and improved solubility. Experiments carried out at the Petroleum Recovery Institute (PRI) in Calgary using high pressure core tests, and in the Oil and Gas Lab at Dow using low pressure equipment have indicated a synergistic effect in mobility control as a result of blending surfactants. These results were presented at the 7th International Symposium on Surfactants in Solution(1), held in Ottawa in October 1988.
Alpha olefin sulfonates (AOS's) are relatively inexpensive and available, produce good foams and are thermally stable, all important considerations for a good foam flooding surfactant. The biggest impediment to the use of AOS's in enhanced oil recovery is the very poor solubility of these surfactants in water containing divalent captions such as calcium and magnesium. Many reservoirs contain enough of these metal ions to preclude the use of pure AOS. This obstacle can be overcome by blending AOS with another type of surfactant.
There are some lingering concerns about the wisdom of using blends, primarily because of anticipated separation of the components of the blends as a result of preferential adsorption.

Results

Solubility
One of the products which has been used in the field is a blend of AOS and DOWFAX* disulfonated alklydiphenyloxide (DPOS) produced by Dow Chemical. The structure of this disulfonate surfactant is shown in Figure l. Calcium and magnesium salts of these disulfonates are water soluble, and remain so even in very concentrated solutions of inorganic salts of these metals.
The solubility of AOS in sodium chloride solutions is increased by the presence of the disulfonated diphenyloxide. The first set of results (Fig. 2) shows the effects of temperature and blend composition on the solubility of the surfactant in sodium chloride solution. The concentration of AOS was held constant at 1.0 g AOS (- 50% active) in 100 mL of water. Varying amounts of the diphenyloxide surfactant (- 50% active) were added to the AOS solutions to give AOS/DPOS weight/weight ratios as shown.