Coated recombinant Escherichia coli for delayed release of β-mannanase in the water-based fracturing fluid

A low-temperature active glycosyl hydrolase family 26(GH26) β-mannanase (Man37) was identified in Bacillus velezensis BS-37 by gene mining. The β-mannanase Man37 contained a signal peptide at the N-terminus. The Man37 and Man37_△1−25 (without the signal peptide sequence) coding sequences were successfully expressed in Escherichia coli BL21(DE3) using the pET-28a(+) vector. The specific activity of the purified Man37 and Man37_△1−25 with guar gum as substrate was 98.12 and 150.34 U/mg, respectively. Man37_△1−25 was superior to Man37 due to its higher specific activity and soluble expression. The temperature and pH range of Man37_△1−25 was 20−55 °C and pH5.0–10.0, respectively. The enzyme could effectively reduce the viscosity of fracturing fluid from 6200 to 1000 mPa·s after 10 min at 25 °C, which demonstrates its application potential as a gel-breaker in low-temperature hydrocarbon reservoirs. Uncoated recombinant Man37_△1−25 cells could completely reduce the viscosity of water-based fracturing fluid within only 3 h, while was greatly prolonged to 18 h with coating polyethylenimine-glutaraldehyde (PEI-GA). Thus, this study provides an effective solution for premature degradation caused by the addition of water-based fracturing fluid together with a short-acting gel-breaker.