Abstract

ASP (alkali, surfactant, polymer) flooding has obtained good results in the field. However, it has serious “scale” and “emulsion” and “chromatographic separation” problems, and because of the reaction of alkali with the formation, it cannot be used at high temperature conditions. A relatively pure surfactant that attains ultra-low Interfacial Tension (IFT) between crude oil and formation water without adding alkali into the system needs to be developed.

A mechanism (or condition) for surfactants to form ultra-low IFT at low concentrations is put forward. According to this mechanism, the chemical structure of 5 series of surfactants is designed, more than 30 specific surfactants were synthesized, all the above surfactants types and most of the specific surfactants attained ultra-low IFT (less than 9X10−3mN/m) with the crude oil and formation water systems. In the above surfactants, a series of “Betaine Amphoteric Surfactants” (a relatively pure surfactant with only one or two effective molecular structures) are the most promising. The surfactant was comprehensively evaluated. Besides the make-up water, it does not require additional “alkali”, “salts”, “co-surfactants”, “alcohols” or “solvents” into the system; the IFT between the driving fluid and crude oil of many reservoirs can reach an ultra-low IFT value at concentrations of only 10 ∼ 3000ppm; it is tolerant to salinities up to 229,000ppm, divalent ions 21,000ppm, temperatures 98°C. Test on cores shows that the recoveries of flooding by SP (surfactant, polymer) systems using this surfactant are higher than that by ASP systems using alkyl-benzene-sulphonates as the surfactant and NaOH as the alkali, its total recovery can reach above 70% OOIP. Micro-visual oil displacement experiments show that this surfactant can also change the pore surface wettability and further increase the recovery.

The above results show that this is a very promising surfactant for chemical flooding, it can be used in very challenging reservoir conditions (very few reservoirs have conditions harsher than those listed above); the mechanism to achieve ultra-low IFT, which is put forward, is useful to the development of new surfactants for EOR and to chemical EOR itself; Surfactants that do not require alkali, co-surfactants, alcohols, salts and solvents in the system to attain ultra-low IFT should promote better understanding of the function of surfactants in chemical flooding.

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