Summary
Cuttings transport, in connection with drilling fluid rheology, has been extensively studied in the literature. Despite this, contradictory results continue to be reported regarding the effect of yield stress on cuttings transport. This study uses the concepts of static and dynamic yield stresses to investigate the effects of yield stress on cuttings transport. A modified form of an existing rheological function is proposed to model static and dynamic yield stresses while incorporating flow history. Flow equations are based on the mixture approach and are numerically solved using computational fluid dynamics (CFD) methodology. Assuming that the liquid phase is homogenous and drill cuttings are noncolloidal, it is shown that the distinction between static and dynamic yield stresses diminishes as volumetric cuttings concentration increases. The Herschel-Bulkley function predicts infinite viscosity at the limit of zero shear rate and, hence, improved cuttings transport with increasing dynamic yield stress, whereas in line with the majority of experimental studies, the proposed rheological model shows that high dynamic yield stress is detrimental for cuttings transport. Comparing fluids with the same dynamic yield stress, the fluid with a larger difference between static and dynamic yield stresses has better cuttings carrying capacity. However, these results are only valid for simple yield stress fluids in which yield stress is dependent on shear rate only.
