Ureolytic activities of a urease-producing bacterium and purified urease enzyme in the anoxic condition: Implication for subseafloor sand production control by microbially induced carbonate precipitation (MICP)

Microbially induced carbonate precipitation (MICP) involves the hydrolysis of urea by indigenous or introduced urease-producing bacteria, which induces carbonate precipitation. By allowing this process to occur in the pores of unconsolidated sand, sand particles bond together, creating a sandstone like material. Although MICP has been explored recently for possible applications in civil and construction engineering, this study examines its application to sand production control during hydrate gas exploitation from subseafloor sediments. The major uncertainty is the ureolytic activities of bacteria and associated enzyme under the subseafloor condition. The main aim of this study was to quantify the ureolytic efficiency of a urease-producing bacterium and purified urease enzyme in the oxic and anoxic conditions. The purified urease enzyme and Bacillus megaterium were subject to bench shaking ureolyic activity tests in both conditions. Biochemical parameters including urea concentration, electric conductivity (EC), pH, and optical density at 600 nm (OD600) of the solution at different time intervals were measured. As a quality control procedure, dissolved oxygen concentration (DO) of the final solutions was also measured. Results show that the effect of oxygen availability on ureolytic efficiency of purified urease enzyme is marginal. However, anoxic ureolytic performance of B. megaterium is better than its oxic counterpart. It is also found that higher concentration of urease and multi-amendment of bacteria help raise ureolytic efficiency. In order to sustain ureolytic efficiency and facilitate its up-scaled field application, several practice measures can be implemented including growing bacteria aerobically to exponential stage before implemented into the subseafloor sites, injecting larger bacteria cell number, and repeatedly supplying fresh bacteria cells.