Low-temperature pH-regulable gel-breaking of galactomannan-based fracturing fluids by the mannanase from Bacillus aerius

摘要

Mannanase acts as an environmental-friendly breaker in the hydraulic fracturing to hydrolyze the galactomannan (GM) of fracturing fluids facilitating the fluids flow-back. In this work, an endo-beta-mannanase (Man3-2) was separated from a newly-isolated GM-degrading bacterium, Bacillus aerius (Strain3-2). The GM molecular could be broken by the hydrolysis of Man3-2 into low-molecular oligosaccharide fragments primarily, leading to the rapid decrease of GM solution viscosity, where the enzymatic breaking efficiency was over 90% in 2.0 h. Also, in the core-scale simulation, the permeability recovery of Man3-2 was 73.93%-94.18%, much higher than that of oxidative breaker ((NH4)(2)S2O8) of 29.18%-32.21%. Man3-2 was a growth-associated extracellular inducible enzyme and it could be obtained by the culture of Strain3-2 at 50 degrees C and pH of 6-7 in the GM-organic nitrogen medium. Man3-2 retained high-efficient enzymatic degradation ability at the temperature range of 30 degrees C-80 degrees C, pH range of 4.0-7.5 and high-concentration of brine (NaCl, CaCl2, BaCl2, artificial formation brine). The enzyme activity of Man3-2 is inert in the GM-borax gel-system due to the alkaline condition, but it can be reactivated as the pH turned to neutral. These findings suggested that Man3-2 is a new high-efficient lowtemperature and pH-regulable gel-breaker for GM-based fracturing fluids, and shows potentiality to control the gel-breaking of fracturing fluids.