Rheological Yield Behavior of Qinghai Gelled Crude Oil Under Multiple Loading Modes: Creep, Ramp, Step, and Continuous Shear.

The rheological yield behavior of Qinghai gelled crude oil, a representative waxy crude oil, was investigated under multiple loading modes. Under constant stress, the flow of gelledwaxy crude oil exhibits a three-stage creep process: decelerating, steady-state, and accelerating creep. A nonlinear creep equation is developed by incorporating a damage variable and hardening function. This model accurately describes the complete creep behavior of various waxy crude oil samples without requiring piecewise treatment. Under linearly increasing shear stress, the yield stress increases linearly with the stress loading rate, as shown by a semilogarithmic plot, while the yield time decreases linearly with the loading rate on a double logarithmic plot. Under stepwise increasing shear stress with constant-stress increments, the yield stress decreases exponentially as the stage duration increases, while the yield time increases linearly, as shown by a double logarithmic plot. Conversely, with constant stage duration, the yield stress increases and stabilizes as the stress increment increases, while the yield time decreases exponentially. Under continuously increasing shear rates, stress fluctuations occur initially at low loading rates but diminish as the rate increases, while the yield stress increases.