Thermo-Hydro-Mechanical Coupled Simulation of Dynamic Fracture Aperture Evolution Under Fluctuating Bottomhole Pressure

Pump start-up and shutdown, flow-rate adjustment, and tripping operations during drilling can induce bottomhole pressure fluctuations. These fluctuations may alter fracture aperture and change the development of lost-circulation pathways. To investigate the dynamic evolution of fracture aperture under fluctuating pressure, a thermo-hydro-mechanical (THM) coupled numerical model was established using ABAQUS. Bottomhole pressure fluctuations were induced by applying periodic perturbations to the inlet flow rate. The effects of fluctuation amplitude, fluctuation duration, and drilling-fluid temperature were then analyzed. The results indicate that fracture aperture exhibits a transient response before reaching a stable state. The fluctuation amplitude has a significant effect on the maximum transient fracture aperture. When the fluctuation amplitude increases to 30%, the maximum fracture aperture increases by 44%. In contrast, the fracture that has already formed may undergo reclosure during the low-pressure stage. The fluctuation duration mainly affects the persistence of the fracture opening and reclosure process, but has a relatively weak effect on the maximum fracture aperture. A decrease in drilling-fluid temperature promotes fracture opening and tip propagation. When the formation temperature is 100 °C, low-temperature drilling fluid increases the maximum fracture aperture by 3.06% and the fracture length by 13.89% compared with the isothermal reference case. These findings indicate that fracture aperture under fluctuating pressure cannot be characterized only by its stabilized value. The maximum transient fracture aperture, minimum fracture aperture, and temperature-induced changes in fracture morphology should also be considered. This study provides a numerical insight into the transient response of fracture aperture to bottomhole pressure fluctuations and drilling-fluid temperature during drilling in stress-sensitive fractured formations.