Experimental study on the selection of microbial strains for seawater-based drilling fluids

Abstract:

During the drilling and extraction of gas hydrates in marine areas， the problem of reservoir wellbore instability caused by the degree of sediment cementation and the stability of hydrates severely restricts the safety and efficiency of drilling. Developing a new type of seawater-based drilling fluid system that can enhance the cementation strength of the reservoir and inhibit the decomposition and regeneration of hydrates is the key to ensuring wellbore stability and achieving efficient drilling. The microbial-induced calcium carbonate precipitation （MICP） technology provides a new reference for solving this problem， and the type of microorganisms as a key factor in the MICP process， their growth conditions in seawater-based drilling fluids will directly affect the wellbore stability. Therefore， through experiments in this paper， the growth patterns of two types of microorganisms， Bacillus pasteurii and Bacillus megaterium， in seawater at temperatures of 30， 20， 10， and 4℃ were investigated. Microbial strains suitable for constructing a seawater-based microbial drilling fluid system were selected. The results show that in seawater， the urease activity produced by Bacillus pasteurii is much higher than that of Bacillus megaterium. The bacterial concentration （OD₆₀₀） within 48 hours is 0.583~0.926， and the urease activity can reach 0.77~0.88 mmol/min. The logarithmic growth period is shorter， the maximum growth rate is higher， and the stable period is longer. Therefore， Bacillus pasteurii is selected as the preferred strain for the seawater-based microbial drilling fluid system.