Fracture plugging failure under oil-based drilling fluids condition in deep geoenergy formations: A mesoscopic mechanical structure evolution perspective

The structural integrity of the particle plugging zone directly affects the success of drilling operations in oil, gas, and geothermal reservoirs through its ability to withstand tectonic pressures. The impact of oil - based drilling fluids on the structural stability of the fracture plugging zone is crucial for preventing leakage during the drilling process. This study employs photoelastic experiments to investigate force chain evolution in rigid particle plugging zones under dry, water-based, and oil-based conditions. Results show distinct behaviors: dry conditions exhibit linear pressure increases followed by decline, water-based conditions show fluctuating trends, and oil-based conditions stabilize after multiple fluctuations with the lowest pressure-bearing capacity. Weak force chains dominate numerically. Under load, strong and medium force chains increase. The key point is that under oil-based conditions, the strong force chain network is relatively sparse, with the dominant orientation being 60°; orientations along 0° and 240° are less prevalent. This makes it difficult to form a fishbone-like high-pressure-bearing structure, and the reduced topological complexity results in poor stability. The evolution of force chain energy is influenced by fluid properties: the decrease in friction leads to particle displacement and structural damage, which compromise the integrity of the force chain structure, induce local fracture failure, and consequently trigger macroscopic failure. Modifying particle properties and integrating tackifiers can enhance force chain networks, improving plugging zone stability and shear strength. These findings offer practical strategies for optimizing drilling fluid systems in deep fractured reservoirs.