Growth Kinetic Models of Sporosarcina pasteurii in PAC Seawater-Based Drilling Fluid Under Low-Temperature Conditions

Marine natural gas hydrates are considered a highly promising green energy resource. However, their reservoirs are susceptible to wellbore instability during drilling and extraction due to their occurrence forms and the weakly cemented characteristics of the reservoir. Enhancing reservoir cementation and improving the strength of the formation surrounding the wellbore are critical to overcoming this instability challenge. Microbially induced calcium carbonate precipitation (MICP) technology offers a novel approach to addressing this issue; however, elucidating the growth patterns of microorganisms in low-temperature seawater-based drilling fluids is a prerequisite for applying MICP to improve the wellbore stability of marine natural gas hydrate reservoirs. In this study, the growth of Sporosarcina pasteurii was monitored in polyanionic cellulose (PAC) seawater-based drilling fluid at 4, 6, 8 and 10 °C, and the growth data were fitted to construct primary growth models. The results indicate that both the seawater environment and low temperatures inhibit the growth of S. pasteurii, prolonging its lag phase, and the bacterial concentration decreases with decreasing temperature. Notably, at temperatures ranging from 4 to 10 °C, the growth of S. pasteurii in PAC seawater-based drilling fluid was superior to that in natural seawater. Based on model evaluation metrics, the modified Logistic model exhibited a relatively high R2 (0.99530–0.99908) and low RMSE (0.00009–0.01487) and AIC (−93.87389 to −66.60236), indicating that it was more suitable as a primary growth model for S. pasteurii in the low-temperature PAC seawater-based drilling fluid system, whereas the square-root model exhibited an R2 of 0.90131, an Af value of 1.03306, and a Bf value of 1.00092, with all three evaluation metrics being closer to 1, indicating that it was more appropriate as a secondary growth kinetic model. These findings provide a critical theoretical foundation for future investigations into the relationship between the growth characteristics of S. pasteurii and wellbore strengthening by PAC seawater-based microbial drilling fluid, as well as for the further development of MICP-based approaches to mitigate wellbore instability during marine natural gas hydrate drilling.