Discrete Element Method for Investigating the Borehole Wall Rock Failure Process Based on the Enhancement Effect of Microbial Drilling Fluids

Borehole wall instability in unconsolidated formations is a critical challenge in drilling engineering and requires urgent attention. Microbially induced calcium carbonate precipitation (MICP) is a promising solution for reinforcing unstable formations. However, the microscopic mechanisms responsible for borehole wall re-failure after MICP treatment remain unclear. The discrete element method (DEM) model provides an effective means for investigating such processes. In this study, a DEM model was created using vaterite, a form of calcium carbonate, to simulate the compressive failure process under unconfined conditions. During the simulations, the stress, strain, discrete fracture networks (DFN), fragment count, coordination number, contact number, average normal contact force, and bond breakage of the specimens were recorded to understand the microscopic behavior of core failure after MICP enhancement by microbial drilling fluids. The simulation results showed that the proposed DEM model can effectively simulate the failure behavior of cores post-MICP enhancement using microbial drilling fluids. As the calcium carbonate content increased, the peak strength occurred at a larger strain. Furthermore, the peak strength itself increased, and the brittleness of the specimens became more pronounced. The reduction in specimen strength was primarily attributed to the development of DFN and breakage of vaterite–sand particle bonds. Moreover, more concentrated and frequent DFN corresponded to a more rapid decline in the strength of the specimen. The tensile stress at the vaterite–sand and vaterite–vaterite contacts played a crucial role in maintaining the strength of the core specimen, and specimens with a greater number of uniformly distributed contacts showed improved core strength. This study provides new insights into the failure mechanisms of microbial drilling of fluid-enhanced sand in unconsolidated formations.