2.1.2. Surfactant

The mass fraction of nonionic surfactant TX-10 is 99%, which formed by condensation of nonylphenol and ethylene oxide in the presence of a catalyst. Its molecular formula is C15 H24O. (C2H4O)n, where when n changes, the product has di fferent properties and di fferent applications. Its melting point is 44–46 ◦C, boiling point is 250 ◦C, density is 1.06 g/mL, and flash point is 535 ◦F. It does not exist in an ionic state in water and has good stability, which is not easy to be a ffected by strong electrolyte, acid and alkali. At the same time, good compatibility with other surfactants. The mass fraction of zwitterionic surfactant HSB1618 is 40% which has both ionic properties and excellent resistance to calcium and magnesium ions. Its free amine content is ≤2%, sodium chloride content is ≤7%, and organochlorine content is 0. The mass fraction of anionic surfactant penetrant T is 40% which is cheap and not resistant to strong acids and bases, reductants and metal salts. The pH value of 1% aqueous solution is 7.0–9.5. It works best when the temperature is less than 40 ◦C and the pH value is 5–10. Its active part tends to dissociate into negative ions in water, and there is a large organic anion that can interact with alkali to form salt.

#### *2.2. Solid-Free Drilling Fluid*

On the basis of previous research, the basic fluid of the solid-free drilling fluid was obtained as Table 1.


**Table 1.** The basic fluid composition.
