Abstract:

To improve the development efficiency of deep coalbed methane resources, a novel supramolecular surfactant SST-2 was prepared by using mixed long-chain fatty acids, N, N-dimethyl-1, 3-propanediamine, and chloropropene as raw materials. Then the surfactant was combined with KCl to develop a novel supramolecular clean fracturing fluid system which was suitable for deep coalbed methane wells. The comprehensive performance of the fracturing fluid system was evaluated in laboratory, and it was applied on site in deep coalbed methane wells. The results showed that this supramolecular clean fracturing fluid had strong temperature and salt resistance. When prepared with saline water with mineralization degree of 20×10⁴ mg/L, after shearing at 120 ℃ with a shear rate of 170 s^-1 for 120 minutes, the viscosity of the prepared fracturing fluid could be maintained above 50 mPa·s. The fracturing fluid system also had strong sand-carrying capacity, and when the experimental temperature was 90 ℃, the settling rate of ceramic particles was only 0.351 cm/min. This fracturing fluid had good gel breaking performance. When the volume fraction of kerosene was 10% and the gel-breaking temperature was 30 ℃ and 90 ℃, the gel-breaking time of the fracturing fluid could be shortened to within 60 minutes. All the performance parameters of the gel-breaking fluid met industry standards, and the damage of the gel-breaking fluid to the permeability of coal core was less than 10%, indicative of good reservoir protection effect. In addition, this supramolecular clean fracturing fluid had smaller impact on the desorption performance of coalbed methane than other conventional fracturing fluids. The on-site operational effect upon X-1 well using this supramolecular clean fracturing fluid was good. During the post-fracturing drainage stage, gas could be seen quickly. After 400 days of production, the daily gas production was basically stable at around 3 300 m³, and the effects of stable production and increased output were good.

Key words: deep coalbed methane, supramolecular surfactant, clean fracturing fluid, temperature and salt resistance, on-site application