Existence of plug zones in viscoplastic fluid flow and their effect on drilling hydraulics in a concentric vertical annulus

Drilling fluid, an example of a viscoplastic fluid, is a type of non-Newtonian fluid characterized by the existence of a threshold stress, called yield stress, that must be overcome for fluids to flow. Drilling fluid is conventionally modeled using power law parameters and yield stress. Yielded and unyielded zones that exist in viscoplastic flow due to varying shear rate profile in an annulus are not considered while modeling flow. This thesis investigates the factors affecting the existence of plug zones and the effect of plug zones on a fluid’s cuttings carrying capacity and on annular pressure drop. A CFD model has been developed to carry out this investigation. The first half of this thesis presents the effects of flow rate, yield stress and drillpipe rotation on the size of plug zone. The results show that plug size decreases with an increase in flow rate until a minimum limit is achieved. Plug is nonexistent when yield stress is small and its size increases and reaches a maximum limit as yield stress increases. Plug zone ceases to exist when drillpipe is rotated at a speed greater than a critical speed. The critical speed is directly proportional to yield stress and inversely proportional to plug viscosity. Annular pressure drop increases with an increase in yield stress and plug viscosity though the rate of change of increase with respect to plug viscosity is comparatively small. The second half of this thesis investigates the effects of plug zones on cuttings carrying capacity and on annular pressure drop in presence of drill cuttings. The effect of plug zone on cuttings carrying capacity increases as the size and/or density of the particle increases. It can be concluded that a fluid with higher plug zone viscosity has better cuttings carrying capacity. As yield stress increases and/or flow rate decreases, the impact of plug viscosity on annular pressure drop increases significantly. Therefore, for accurate prediction of annular pressure drop and cuttings transport, the size and viscosity of plug zone should be considered and incorporated in the rheological models