Study On Bubble Suspending And Migrating In Yield Stress Non-Newtonian Fluid

Drilling fluid used in oil and gas industry is usually a kind of yield-stress（Y-S）on-Newtonion fluid.The presence of yield stress and shear thinning behavior are two distinct features of Y-S fluid.The yield stress has a significant impact on the relative movement of gas bubbles dispersed in the fluid.The shear thinning effect can change the viscous resistance of the bubbles as they migrate in the fluid.Therefore,systematic research is carried out in this work to investigate the migration characteristics of gas bubbles in Y-S fluid.A non-spherical bubble expansion model is established to predict the geomertries of gas bubbles in Y-S fluid.The model takes into account the pressure difference,yield stress,surface tension and critical thickness of shear region（Rsc）.It indicates that the geometries of bubbles will change from sphere to the shape of an inverted tear drop with the increase of yield stress and Rsc and/or the decrease of surface tension.The surface tension is the dominant factor of the bubble geometry when sur₁（the ratio of surface tension to yield stress）is more than 3.On the basis of stress field around a single bubble,a model is developed to describe the geometries and volumes of suspending bubbles in Y-S fluid.The model takes into account the yield stress,surface tension and Rsc.A method is proposed to calculate the equivalent radius,length and the maximum width of critical suspending bubbles.Considering the distribution of bubbles and the coupling stress field in the fluid between them,a model is developed to calculate the critical distance of suspending bubbles.Based on the above theoretical and experimental studies,a model is proposed to predict the critical concentration of suspending bubbles.With the model,the effects of different parameters（yield stress,surface tension）on the limit concentration of suspending bubbles can be analyzed.As a result of bubble ascending experiment,we choose the P-G model corrected by Harmathy as the applicable velocity model since the minimum error method.Base on the suspending and migrating characters,we proposed a new method in avoiding typhoon by injecting a high yield stress slug.