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A broken stratum is a complex stratum often encountered during drilling. Under erosion of the drilling fluid and disturbance of the drill pipe string, the rock in the well wall of the broken stratum is prone to collapsing and falling off, causing the well wall to lose its stability. Improving the cementing force between the broken blocks and forming a complete well wall are essential for overcoming this instability. The present study combined microbially induced calcite precipitation technology with solid-free drilling fluid technology for the first time to formulate a drilling fluid to overcome the instability of the well wall of a broken stratum. However, first and foremost, the growth of microorganisms in drilling fluids must be elucidated. To this end, experimental and theoretical analyses were performed to examine Bacillus pasteurii growth in drilling fluids composed of a single agent or combinations of various materials, such as a zwitterionic coating agent (FA367), a biopolymer (XC), a polyacrylate polymer (PAC-LV), and potassium polyacrylate (K-PAM). Experimental B. pasteurii growth data were then fitted using a modified Gompertz model. The mean square error indicated that the generated model had a reasonable degree of fit, and the bias and accuracy factors showed that the model could predict B. pasteurii growth. Among the different drilling fluid combinations used, suitable fluids for B. pasteurii growth were XC alone, XC, and PAC-LV in the two-material-based fluid and FA367, XC, and K-PAM in the three-material-based fluid. These results provide a solid foundation for the development of microbial drilling fluids to solve instability problems in broken geological formations.

Growth Kinetics of Bacillus pasteurii in Solid-Free Drilling Fluids