A semi-analytical model for predicting temporal invasion of microbial cementing slurry in deepwater gas hydrate reservoirs

Deepwater natural gas hydrate reservoirs are typically characterized by high porosity and low permeability, which exacerbate the risk of cementing slurry intrusion and pose a considerable challenge to wellbore integrity. Existing steady-state and thermo-mechanical models cannot adequately capture the dynamic invasion behavior of cement slurries modified by microbial-induced calcium carbonate precipitation (MICP). Therefore, this study develops a semi-analytical model that couples mass conservation and Darcy radial flow with an empirically fitted MICP inhibition function, enabling the prediction of the characteristic temporal evolution of the invasion distance of MICP-modified cement slurry, with model parameters calibrated to experimental conditions. The model is validated using an independently developed large-scale, high-pressure cementing simulation system, revealing a distinct self-limiting invasion behavior through MICP-induced pore plugging and a progressive increase in slurry viscosity. Parameter sensitivity analysis shows that the MICP characteristic time becomes the dominant controlling factor beyond approximately 150 min, surpassing injection pressure. Moreover, a three-stage segmented grouting strategy is proposed as a concept for dynamic injection control based on model-based sensitivity analysis, which is projected to reduce the invasion distance under equal-volume conditions, with the benefit ranging from 11.4% to 14.0%. This work provides a theoretical framework and experimental evidence for optimizing cementing practices in deepwater hydrate reservoirs, which extends the application of MICP technology from geotechnical reinforcement to deep-water energy engineering.