Mechanisms and Mitigation of Heavy Oil Invasion into Drilling Fluids in Carbonate Reservoirs

Drilling operations in carbonate rock heavy oil blocks (e.g., in the Tahe Oilfield) are challenged by the intrusion of high-viscosity, temperature-sensitive formation heavy oil into the drilling fluid. This phenomenon often results in wellbore blockage, reduced penetration rates, and compromised well control, thereby significantly limiting drilling efficiency and operational safety. To address this issue, this study conducts a comprehensive investigation into the mechanisms governing heavy oil invasion using a combination of laboratory experiments and field data analysis. Findings indicate that the reservoir exhibits strong heterogeneity and that the heavy oil possesses distinctive physical properties. The intrusion process is governed by multiple interrelated factors, including pressure differentials, pore structure, and the rheological behavior of the heavy oil. Experimental results reveal that the invasion of heavy oil occurs in distinct phases, with temperature playing a critical role in altering its viscosity. Specifically, as temperature increases, the apparent viscosity of the drilling fluid decreases; however, elevated pressures induce a nonlinear increase in viscosity. Furthermore, the compatibility between the drilling fluid and the intruding heavy oil declines markedly with increasing oil concentration, substantially raising the risk of wellbore obstruction. Simulation experiments further confirm that at temperatures exceeding 40°C and injection rates of ≥0.4 L/min, the likelihood of wellbore blockage significantly increases due to heavy oil infiltration. Based on these insights, a suite of targeted mitigation strategies is proposed. These include the formulation of specialized chemical additives, such as viscosity reducers, dispersants, and plugging removal agents, the real-time adjustment of drilling fluid density, and the implementation of advanced monitoring and early-warning systems.