Study on the Drilling Fluid Aging Law and Sealing Formation Mechanism in an Unsealed Annulus

In order to clarify the aging law of drilling fluid and the formation mechanism of sealing properties, thereby providing a theoretical basis for the design of drilling fluid sealing evaluation methods and for determining the necessity of secondary cementing in uncemented intervals of oil wells, laboratory experiments were conducted in this study. The influencing factors of the coalescence stability and sedimentation stability of the drilling fluid, as well as the temporal and spatial evolution law of the macroscopic properties and microscopic morphology of the aging drilling fluid, were systematically analyzed in turn. The results show that a high temperature of 120 °C and a calcium and magnesium ion concentration of 1000 mg/L significantly weaken the coalescence stability of the drilling fluid. However, sedimentation stability can be effectively improved by increasing the viscosity and reducing the fluid loss, lowering the temperature, decreasing the concentration of calcium and magnesium ions, or increasing the amount of barite. With the extension of the standing time, the settlement of the system exhibits a three-stage evolution characteristic of “stability–stability deterioration–restability”. At 50 days, the density difference between the upper and lower layers reaches 2.27 g/cm3, and the gel strength steadily increases to a peak value of 124 Pa. The annular fluid forms a sealing layer due to the dense accumulation of a colloidal network and solid phases in the middle and lower layers.