Deep water drilling is normally associated to narrow operational windows where gains and losses are frequent. drilling fluids are designed to gel when it is not submitted to shear stress. This is necessary to avoid cuttings to settle during circulation stops. When circulation is resumed, the pump pressure rises above the circulation pressure in order overcome the gel strength. Due to its thyxotropic effect, the gel viscosity remains high for a while after the circulation restart. The gelation may have significant importance, specially, in deep waters where high pressures and low temperatures take place. Gel effects can both generate excessive pressures (inducing losses) and cause delays in pressure transmission which can directly affect gain detection devices. The current work presents a compressible transient flow model of the restart of drilling fluid circulation, in order to predict pressures at the borehole. The model comprises the conservation equations of mass and momentum which are solved by the finite volume method. A constitutive equation is employed to model the time dependent rheology of gel breaking. Case studies are conducted to evaluate the effect of fluid properties, well geometry and pumping pressure on the bottomhole pressures. Major effects on pressure propagations delay in deepwater drilling are highlighted.

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