Análise experimental da tensão mínima para escoamento de materiais viscoplásticos em tubos: carbopol e óleo parafínico gelificado

Abstract

The restart of paraffinic oil flow is a problem of extreme severity in the scope of flow assurance.After a period of production stoppage, the paraffins precipitate and the oil begins to behave asa non-newtonian material. In fact, it behaves as a viscoelastoplastic and thixotropic material.In order to resume flow, the viscoplastic aspect is the most important. The minimum pressurerequired to restart the flow must be predicted. For that purpose it is common to use the relation,∆P= 4τ0L/D, where L and D are respectively the length and diameter of the pipe andτ0is theyield stress. This work evaluates how realistic this expression is, since it does not consider, forexample, slip phenomenon at the wall. An experimental apparatus is used, consisting of a pistonthat pushes the oil inside a tube. The critical stress to start flow is compared to the yield stressby using the sliping factorSF= (τ0−τc)/τ0. For carbopol, the critical stress obtained on thebench was lower than the yield stress measured on the rheometer for the case of a smooth pipe.Nonetheless, using a rough inner surface, the SF reduces, being an indication that the slip effectis important to the carbopol flow restart. In the tests with paraffinic petroleum, two initial heatingtemperatures were evaluated50◦Cand60◦C. A good concordance was verified between thecritical tension obtained in the bench and yield stress. The results indicated that the mechanismof gelled structure breaking is the adhesive failure and, in this case, the above equation providesa good estimate of the pressure required to restart the flow