Water-soluble fluorochemical surfactants have been used in the oil field since the early 1970's as surface tension depressants in a variety of aqueous stimulation fluids for low-permeability oil and gas wells. This paper discusses a laboratory study of the behavior of water-soluble fluorochemical surfactants relative to their oil field use. Data are presented on surface tension depression, thermal stability, adsorption, fluid removal from sandpacks, flow rates, and emulsification tendencies.
Formation damage can result from retention of aqueous fracturing or acidizing fluids in formation capillaries with resulting water blockage because of capillary imbibition. To overcome this problem, various additives that lower the surface tension of the fluids have been found to give more rapid and complete stimulation fluid recovery. Originally, hydrocarbon surfactants and alcohols were used, but hydrocarbon surfactants are limited in their ability to reduce surface tension, and alcohols became short in supply and can react with HCl in acidizing fluids to form alkyl chlorides. It was the shortage of alcohols that led to the initial use of fluorochemical surfactants in the early 1970's as surface tension depressants for aqueous stimulation fluids, and they proved very effective. Now they are being used extensively in a variety of aqueous stimulation fluids, such as fracture fluids, acid, foamed fracture fluids, foamed acid, etc. for both oil and gas wells. Recently, organic soluble fluorochemical surfactants also have been used in the oil field to foam hydrocarbons such as diesel oil, kerosene, xylene, and condensate for a variety of applications in water-sensitive formations. This paper, however, discusses only aqueous fluids. The behavior of water-soluble cationic, nonionic, anionic, and amphoteric fluorochemical surfactants has been studied in the laboratory relative to their use in the oil field, and the results of these studies are the subject of this paper.
Fluorochemical surfactants were logical candidates for use as surface tension depressants in aqueous stimulation fluids because they typically produce the lowest surface tension values obtainable in aqueous systems-as low as 15 to 16 mN/m in some cases. To demonstrate their effectiveness compared with other additives, surface tension values were measured for aqueous solutions containing methanol, isopropanol, representative cationic, nonionic, and anionic conventional hydrocarbon surfactants of types commonly used in the oil field to depress surface tension, and four fluorochemical surfactants: FC-I (cationic), FC-II (nonionic), FC-III (predominantly anionic) and FC-IV (amphoteric). (See Table 1 for generic description of surfactants.) Surface tension values were determined with a duNouy tensiometer on solutions that had been allowed to equilibrate for 16 hours at room temperature. This procedure was used for all surface tension measurements discussed in this paper. The results are summarized in Table 1. It is evident that, compared with the fluorochemical surfactants, the alcohols are not very effective at depressing surface tension. For example, even with 100% methanol or isopropanol, surface tension values are reduced only to 22 to 23 mN/m. The conventional anionic hydrocarbon surfactant depressed surface tension to only slightly below 30 mN/m at best, and the nonionic and cationic hydrocarbon surfactants to about 35 mN/m.
JPT
P. 1565^
